Unusual variations on two-stroke head design.

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Further notice:

The engine looks fairly OK, but the ring is a bit scored.
Ring gap is still around 0.3mm, so I gave it another run.
I discovered that the carb pulse tube had a hole in it, so that could be causing lean running, more seize-up heating, and hard starting.
That also seems to cause the rpm racing effect.

Minimum idle seems to be about 2300 rpm. It used to be lower before any mods.
I will run it at that for a while and see if the ring improves.
I honed the bore, and it is looking OK.

I chamfered the bottom of the ring groove about 15 degrees each side of the land damage.
I must have dropped a tool tip in there. It only goes halfway into the groove.
this stops any blowby being channelled to a specific spot.

I have flooded it now, so I pulled the plugs and and letting it dry out a bit.
More problems- J-B weld getting into the crankcase is a bit like grinding paste.- probably the source of rapid piston and ring wear.
All my rings are too gappy now, and won't start.

I will order some more rings and some pistons. They are fairly cheap.

The wrist pin, thrust washers, needle rollers seem to be doing ok.
They take a while to get here ex Singapore.
I ran a couple of tissues around the crank cavity, and one removed heaps of fine powder.
I will give it a rinse out before more running.
I think barrel tolerances will still be OK , despite several hones.

That J_B weld on the back of the piston was put on with too much haste.
Possible sandblasting or some kind of surface strip should be used.
I will look for a micro jet sandblaster??
It looks like Torque-pro is not supplying parts anymore, so I have bought a chainsaw top end that is also 45mm bore, 52cc.
This looks like it could be adapted to my crankcase.

The TP60 will have slightly more stroke, so a base spacer will be needed.

Not to bad a price- ex Canada. This should arrive about 10 April.
This also gets around TP-60 dodgy piston cutouts.

I also get two rings, so it is not so sensitive to ring wear.
Here are the buy details.
the stud centres are similar.

This is a flat-top piston, rather than my dome piston, so the head swap may be a problem.
There is plenty of material I can cut out of the head.
I will need longer studs.

This looks like a desirable direction to move. There are Husqvarna models that are similar, so plenty of parts.
I can also switch to a chainsaw bottom end if I want to, to get a double-ended crank.

I was thinking about moving to 125cc motorbike engines, but an old bike would be $4000 nzd, plus I would need a complete road bike as a donor.-
rather expensive.
Engines by themselves are only on ebay uk or us, which has high freight- probably $500 nzd shipping cost.
for something that size and weight. - likely 15-20 KGs??

Chainsaw parts are a lot cheaper, but I need a clutch and gearbox.


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Sometime you need to start off a blank page... could be the good one.
As the PTFE and JB-weld insert I think they could have contributed to overheating because of the very poor heat conductivity compared to aluminium, as the piston get mostly cooled by its contact with the cylinder wall (lacking the full cycle of exhaust and intake of 4 strokes)
This looks like it could be a little tricky, as the chainsaw piston appear to have different wrist pin boss details:
11mm diameter, not 10mm, and bottom-located rod, not top-located rod.

Some messing about is needed to locate the top pin and space out the piston.
possibly I can bore the pin bosses out a bit more and machine up some inserts.

Also, stud holes are at 47mm spacing, not 45mm, so hole ovalling is needed.

Stroke goes from 32.7(52cc) to 36.5 (60cc), so there is a bit of fingers crossed that there is enough upper bore length for exhaust timing.
I want to keep just below 180 degrees.
Around 178 degrees seems to be normal for this bore size and capacity.
Normal street motorcycle engines go up to 180 degrees for 80-100 cc capacity.

The lower transfer pockets need some changes, and the barrel extension is likely too big in diameter as well.
- full dismantle and machine of cases may be needed.

Possibly a 5mm spacer and some exhaust port adjustment may do.

The piston has side holes, so a similar setup to the TP60 engine may work, where transfers are fully in the barrel section.
In any case, the side transfer pockets are limited to the depth of the main base flange on the crankcase.

This barrel seems to be a lower tune than the TP-60 one, so some mods may be needed.
I can run it fairly standard as a benchmark.
Probably will turn more like 5000 rpm rather than 6000 rpm with my test prop.


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Sometime you need to start off a blank page... could be the good one.
As the PTFE and JB-weld insert I think they could have contributed to overheating because of the very poor heat conductivity compared to aluminium, as the piston get mostly cooled by its contact with the cylinder wall (lacking the full cycle of exhaust and intake of 4 strokes)
It seemed to be doing OK on the exhaust side.
Gritty bits in the oil possibly increases heating by blowby and poor ring seating.
This may have contributed to more piston and barrel wear on the cool side, even when it wasn't seizing up.

A totally new piston and barrel ex the chainsaw plus a good rinse-out should help.

The actual brand of the chainsaw that the parts are for is a bit of a mystery.- generic Chinese, with several brand names.
It looks to be as good as the Husqvarna and Stihl rebuild kits, though.

Engine Builders don't like putting 2 rings on race engines, but Hus and Stihl only use one ring, to be cheap.

If you have one ring, it shouldn't be 1.5mm or 2mm wide, high spring tension.

1.2mm rings are adequate.

The 2-ring piston probably uses 1.2mm rings.

Carburettor problems didn't help.

I don't think the pulse pumping was working at all, and it was running on the low end jet only.

Me blowing a bit of fuel through the carb with a syringe as a prime got all the air out of the carb passages.

This meant the engine was chronically lean at any significant revs.
Not good for an air cooled two-stroke.
The insulating effect could be in play with the rear wall insert. It is fairly isolated from the rest of the cylinder by J-B Weld.

This is normally two long slots, and not an area of conductive cooling.

It could have been mixture-cooled as for most of the piston underside.

The holes at the sides of the piston help with this.

With me filling in the piston slots and rear wall slots, more heat could be coming from the piston skirt, and making the area too hot for good lubrication- hence the severe wear. There is no external finning at that spot.

Quite a few engines have "full" skirts, and they rely more on conduction through the skirt to the cylinder wall for cooling.

Other engines have a "Skeleton skirt" for the bottom half of the rear skirt.

The top half acts as the thrust face above the transfer ports, and the bottom is cooled by incoming carburettor mixture.

The skeleton extension helps counter piston rocking, and helps maintain the exhaust-side minimal clearance.

This clearance is not too critical, as the exhaust side is the thrust side on compression.

The engine can become more noisy with noticeable piston slap, if general clearances are over about 0.3 mm.
0.1 to 0.2mm is a better range.
It is more of a metallic tapping sound.

That won't stop it running. The piston ring seal and ring gap are more important.

You do want some actual suction at the carburettor.
It the engine is not so good , it may only start with fast electric cranking.

Mine has been like that for the last few starts, and now the ring has "gapped up" to the point even that does not work.
"New Ring Time". :)
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Possible schemes for reworking the piston pin bore:
1) I don't have tooling to face off the bosses inside the piston, accurately.
2) any bore should be reamed to size. I think I have 10mm-12mm adjustable reamers.
I can do up to 15mm if needed.

3) what side thrust is to be expected from top rod location?

4) scheme 1:
Shouldered bushes pushed in from outside, with separate cap plates, and 2 sets 2x m3 retaining screws.
The cap plate can also locate the piston pin.

The retaining screws can be locked with J-B weld. That should take the heat.
I am a bit iffy about high temperature locking fluids.

The separate caps allow some lateral clearance in the main bush without passing loading on to the screws.

There looks to be enough material for the screw threads.
Then I can put the side shims back in the usual place.
These just take up wear from the needle roller cage.
I can hand-fit the side bushes for a good fit on the rod eye.

I think I can turn the bushes on my micro-lathe.

The chuck is a bit miserable, though. It doesn't have a lot of grip.
I will need to make up rod stock from larger rectangular pieces, and turn both ends to get it to grip properly.
The chuck is not all that central, so turn, bore, part have to be in the same set.

Getting a nice bore on the piston pin hole is a bit trickier.

I need to make up an adjustable fixture I can bolt to the existing lathe chuck.
I have done similar face plate fixtures before, but this one needs to be fully adjustable and really square.
maybe 3-way radial flats and clamp bars.
The radial flex in the micro-lathe should be small enough for fairly good results.
The thing is a bit springy, but nowhere near as bad as my drill press-that is pretty chronic!
Topic variation:
Building a 125cc watercolled twostroke engine.
1) I can cut the gearbox off a 125 honda 4-stroke engine.
*******I need to get a tig set that can handle aluminium.****
Engine cases can be tig welded, if they are lined up correctly.
Mr Millyard has demonstrated this.

2) I can probably line up a gear primary drive well enough to tig weld it.

It is best if case parts can be lined up against and existing built-up crankshaft with large ballraces,.

3) A chainsaw engine case set could be used, and the chainsaw engine crank can be adapted to the existing crank gear, which
I think is not machined as part of the crank.
Entry point of the carburettor can be a bit odd on some chainsaws, which affects how I can mount the engine.

4) can eutectic alumium be tig welded?

5) what is a good material to make the barrel and waterjacket from?
It does not need to be high silicon eutectic, and uses Nikasil coating as a bearing surface.

6) I need to get my lathe/mill combo first, for barrel boring and other machining.

8) big blocks of aluminium are expensive, and I would have to pay $50+ freight to get them.

A 150 x 100 x 150 block is 2.25 x 10^-3 cu mt, den = 2.7, mass = 6.075 kg.
I have found that freight coasts about $50nzd per kg, material $10 nzd/kg, adds up to $364.50 nzd.
This may be a lot cheaper rough-cast from ingot in nz, and freight will be a lot lower.

9) I don't want to get into "lost wax casting", as it needs a lot of equipment, and you make a lot of bad castings learning how to do it right.
Patterns can be 3d-printed, if you cook the mould hot enough to get all the plastic out.- maybe 350 deg C??

10) could the exhaust passage, barrel, and water jacket be welded together from individual parts?
The would need to be machined as "split" and joined.

11) I can get rough-cast machinable blocks done using sand casting, and machine them out.
They are likely to be a bit porous. Is that important?

12) I can design my own water-cooled head, "puck" and barrel set, but I needed a real example to copy.
Maybe buy an old engine off ebay? - freight is a problem.
There are lots of fine details that need to be measured and examined.

13) I don't really need an exhaust power valve, as this is not for track racing.
If I make the exhaust with two bridges, and timed at 190 degrees ref the stroke bottom, it doesn't really need a power valve.

14) I can copy parts of the "2Stroke-Stuffing" 50cc barrel as a design source. The exhaust bridges can be very narrow if they are recut after adding the nikasil coating and boring. This has some good ideas on exhaust and transfers.
in particular, the front transfers are angled at the top and quite low, to allow more exhaust area.

His initial drawing of the rear transfers is not quite right. both sides of the port should be parallel as they enter the cylinder.

"He" has not fully tested and developed the wide-port barrel yet, so there will be lessons to learn.

I will need a chain-driven dyno-engine load. An eddy current system sounds good, but 50-60 hp raping means it would be quite large,
and costly.

15) it is probably best to use a piston with a full skirt, but with external piston pin holes.
This limits the exhaust a little, but the fancy screw-together pistons are only being made by Mark Atkinson, and they are quite experimental.

Under-piston cooling could also be rather experimental, as many race engines use flow-through pistons for cooling in this area.

I suppose I can start shopping for required equipment.
On a 250 cc MZ motorcycle , that I in a 'tuned' non- standard condition, there was a pair of very narrow rings high on the piston, and a 3rd transfer port through a hole high on the back of the piston. This gave a jet of transfer gas from the back to cause more turbulence in the combustion chamber - added to the transfer draughts from the symmetrical side transfer ports. It was also suppose to aid piston cooling.
On a 250 cc MZ motorcycle , that I in a 'tuned' non- standard condition, there was a pair of very narrow rings high on the piston, and a 3rd transfer port through a hole high on the back of the piston. This gave a jet of transfer gas from the back to cause more turbulence in the combustion chamber - added to the transfer draughts from the symmetrical side transfer ports. It was also suppose to aid piston cooling.
Race-car rings seem to be getting down to 0.8mm.
I wonder if they are spring steel with chrome facing? It seems very thin for cast iron.
Thinner rings are better, as ring surface force and friction depends on the ring width.

With a reed valve engine, the boost port can run from the reed chamber, and the hole in the piston is not needed.

Rotary valve engines can run a circular transfer gallery around the back side of the cylinder.
The boost port jet seems to be desirable.

With simulations, it seems to get a long way towards the cylinder head centre, and beats the main scavenge loop jets.
Have I posted the article with the simulations and AVL Fire yet?

I watched a YouTube article that found no difference in power output between one ring and two, when the rings are new.

I like the idea of two rings, as the period between ring replacements is longer, as the bottom ring wears slower than the top ring.

A downside of two rings is that if the top ring breaks, you are more likely to wreck the barrel surface, and mark up the head puck.

Broken rings do not seem to be too common, though.
If you have a race-prepped or motocross two-stroke engine, frequent stripping and inspection would be a good idea.

I have collected a vast number of You Tube videos to watch on diagnosing piston carbon patterns, 2-stroke wear and faults,
and general building up oddball engines from parts- mixing 2 and 4 stroke parts, making multi-cylinder engine from single-cylinder parts,
Regarding TIG, 300A ac seems to be a good target size, to do up to 8mm aluminium.
I suppose you turn it right down to do thin stainless.

I see I haven't had any replies on the slip rollers. I will search more for roller diameter specs. That may be my next purchase.
HI Owen.
Back in the late 80s when I worked in Engine Design, we had a couple of engines with cast iron rings, but the "new" engines had steel rings - made from wire, drawn and rolled to "ring" profile, then coiled into long "springs" - slit - ends re-shaped - Sized and gapped then Nitrided. The steel was NOT really hard like spring steel (120 ton), more like an 80-ton silver steel than a mild (40 ton). BUT I don't remember the spec. Try the "Perfect Circle" website for information... they are USA expert ring makers.
The outer face of the rings were slightly curved, so a point contact on the bore as new. The ends (last 30 degrees or so?) were slightly bent off round, as the intention was to maintain a constant radial pressure on the bore.... But I can't remember if this is a slight outward bend, or inward bend. I discussed the maths (algorithm) with the engineer... but my brain has forgotten some stuff in 40+ years.... I think a "square" rule...? Without this change of curvature, the spring radial pressure on the end of the rings drops-off and blow-by occurs.... I think? But get it wrong and the ends wear rapidly with the "end-of-ring" pressure being too high? => SO BOUGHT STEEL RINGS from a "Proper" ring manufacturer, achieve something you cannot replicate in the workshop. They are more efficient, unlikely to break, nitrided for hardness, and have curved faces which improves the ring lubrication and gives a smaller contact point. And the tan load (radial pressure) is more uniform around the rings, especially at the ring ends (and lower) than is traditional for cast iron rings. (Ring=> bore pressure and friction is significantly lower... of the order of "percentages"...).
Hope this helps?
1) Unlikely that I will use them for now, as I am using standard pistons designed for wider rings.
The next engine may use some Yamaha YZ parts, so I will see what they have.
The YZ125 crank looks fairly well thought out and proven.

2) I heard back from the low volume vehicle people, and a 2-stroke engine is OK in a low volume motorbike.

3) I have ordered a slip roll set ex UK - freight costs about the same as the price, but not to bad at $490nz in total.

4) This current engine layout will keep me going for a while, but eventually I want to get a suitable used top end and barrel unit to measure up.
I need to find the correct base stud pattern and what kinds of top-end gaskets are used.

they seem to use about 4x m8 at the cylinder base, and 6x m6 at the top, I think.
There is quite a step out to the base stud ring.

If copper ring gaskets are used, a lot of bolt pressure is required.
I would have that an inner "C" ring and an outer o-ring would work, with o-rings on each stud.
The centre sparkplug tower need one or more o-rings to seal the water jacket.
The "c" ring doesn't need as much pressure.

This is a big step up from the model airplane engines, which use 4x m5 on the barrels, 45mm bore diameter.
I am using exhaust cement on the top head joint, to avoid continually lapping in the head and barrel, and losing barrel height.
Motorcycle engines - for normal road use - need low speed torque to pull-away, and mid-range torque for cruising speed. Although the adrenalin pushes us to ride as fast as we can, we cannot do so for traffic... etc. So the engines have lots of scope for tuning, but un-tuned, the low speed torque may be useful for you? In fact, semi -off- road bikes have better low speed tuning than "regular" road bikes...
Chainsaw and strimmer engines etc. on the other hand, are aimed at running at full speed, when the power is being used. So perhaps not so suitable for the variable power and torque running you need?
Hope this helps?
Motorcycle engines - for normal road use - need low speed torque to pull-away, and mid-range torque for cruising speed. Although the adrenalin pushes us to ride as fast as we can, we cannot do so for traffic... etc. So the engines have lots of scope for tuning, but un-tuned, the low speed torque may be useful for you? In fact, semi -off- road bikes have better low speed tuning than "regular" road bikes...

Chainsaw and strimmer engines etc. on the other hand, are aimed at running at full speed, when the power is being used. So perhaps not so suitable for the variable power and torque running you need?
Hope this helps?
Interesting idea-open a sector of the converging chamber cone at lower engine revs, plus a larger secondary chamber.
This should reduce the blocking pulse. This is instead of having a power valve.
I think I read that Honda used a similar scheme?.
The errant blocking pulse tends to make them splutter and drop in power.

Good use of model airplane servos, arduino processor to monitor rpms and send out a pwm signal for the servos.
(pulse width modulated)
-do the servo and other line need opto-isolators?
One of those grunty 10kg-cm servos may do the job?
what kind of rpm sensing can I use? Possibly if I buy a small honda, all the dashboard stuff will need replacing as well if I have a different ignition module.
-I have all that arduino and servo stuff here- I haven't learned how to use it all yet, though.

Mr 2Stroke-Stuffing thinks that his engines are too peaky to use standard gearboxes- though they seem to pull from 8000 through to 17500 rpms ok-
a bit of clutch slipping needed.
- that was my estimate just listening to the engine on the motorbike dyno, with the vario rubber belt drive.
A normal 5 speed box would work OK if quite a bit of clutch slipping was used. A wet clutch would stand up to that.
I don't know why racers like dry clutches- more precise slip characteristics? They overheat very quickly.
Also, higher coefficient of friction- less plates- shorter engagement range??

You could stack a centrifugal clutch in front of the standard clutch, or just slam-shift at speed.- needs a solid dog-box.
Those small honda-style "flat" engines work like that.- up to 110cc.

A regular road engine will pull away 2500-3000 rpms fairly well. I had a hot cam in a 750 honda which needed these kind of revs to get going.
It wasn't all that noticeable.

The engine never really performed any better. It needed high-comp pistons as well to benefit from the cam.
Discussion of adjustable geometry expansion chamber:

I want to investigate closable hatches in the expansion chamber.

1) Are there noticeable pressure pulses in the chamber?
2) Is a good chamber seal important? What percentage of seal can be sacrificed and still get a good reflecting wave?
I would guess a maximum of 2-3 sq mm could be tolerated- maybe a bit more.
A sound wave depends on a substantial pressure fluctuation over a very short time.
This wave is quite directional, so radial pressure leaks don't make a big difference.

3) What temperature are the parts subjected to? - this could vary from 130 degrees C up to 350 degrees C intermittent.
4) Cross-section geometry is not that important. Parts of the pipe could have flats on them. Would a full square section chamber still work?
5) Would round butterfly valves in short tubular sections be adequate? angled butterfly valves give a fairly good seal in a machined bore.
Half-circle overlap plates could also be considered.
The bearings could be stainless steel in bored PTFE bushes. Stainless is not that stainless if heated and subjected to exhaust gasses.
It does make good car exhausts, though, so it maintains fair oxide stability.
6) How much of the surface area needs to move? does this directly relate to the chamber maximum cross-sectional area?
A 125 engine has a nominal 5.1 inch chamber maximum diameter, but it could go up to 6 inches or more.
Would opening up 1/4 of the perimeter in a rectangular section, or a round area, be enough to substantially disrupt the blocking pulse?
Would a smaller opening do the disruption as well?
7) Where should the opening be? a) in the straight section? b) half way down the end cone?
9) what is the effect of half the valve protruding into the chamber? This will have a wave and flow diverting effect.

I did some Google searches, and only found one article from Thumpertalk, about having a secondary inner cone.
I would have thought that more people would have discussed this online.

Regarding the exhaust power valve, this would only have a minimal effect of the rpm when the blocking pulse would have its effect.
If you drop the timing from 195 degrees to 180 degrees, at 13,000 rpm, this is about 8% difference, or 1040 rpm.
Over what range is the blocking pulse normally effective? I have heard that power bands can be as narrow as 200 rpm for extreme tunes.
Opening the exhaust removes the blocking pulse, but also stops it interfering with the transfer process, so may eliminate the characteristic
"choking misfire" that these engines get. You get a similar effect with long overlap 4-stroke engines.

I will try a few more different google queries. Maybe the reference I saw was on 4-stroke exhaust tuning valves, but was unclear.
B-port top profiles.

This is normally called the secondary transfer port, but on some engines, they are larger than the A-port.
Here is a diagram. The right-hand side is the one under discussion.
I would have thought converging the sides of the port to this extent would be bad, leading to unstable flow.
Note: the top of the port may be angle up to 25 degrees as well. This may be relative to a domed piston, about 8 degrees start of dome.
My idea is to have no more than 8 degrees total convergence for both sides in the plan view, and the cold-side wall of the port should be twisted to suit.
What is the general opinion on this?
There seem to be many ideas on all the top view (plan) angles for the A and B ports.

2Stroke-Stuffing seems to direct the leading edge of the b-port almost straight across the piston, and the a-port angles are parallel, and a lot flatter than those shown.
The leading edge of the A-port crosses just before the piston centre.
The objective is that all port flow converges a fair bit above the piston, and is biased towards the rear of the cylinder.
I would have thought that a sweep at a fairly shallow angle to the piston, from the A-port, would be good, but is does not seem to be necessary.
If the top angle of the transfer duct is too shallow, the rising flow is very unstable.
the B-port form will make the stream fan out and get thinner.

We are spending energy on the fanning process, and losing it from velocity.

This means that the stream direction of interest is the average of the two port sides.

As drawn, this seems to intersect the line from the front of the A-transfer port.

The fanning action may be of advantage to:
1) thin out the vertical stream;
2) filling in to the sides and bottom of the chamber.
These shapes are the results of simulation and testing, so there must be a positive benefit.

Contrary to the A-port shapes shown, most other examples have a rounded top leading edge, and I see the
Aprilia port A is slightly lower than the other ports.

The Aprilia top view seems to be slightly biased more forward, as well.

I will go over a few port maps I have here, and compare timing, using a 2:1 rod to stroke ratio.
I would think that 2 exhaust bridges would give the straightest exhaust top edge, if we are going to cover about 160 degrees
with the exhaust ports.
The outer ports would be t-shaped.
I will post the last Aprilia image as well.
26mm out of 54.5mm for exhaust opening.
I will check my diagram.
at 1.88 rod ratio, this is 206 degrees exhaust. this is a fair bit.
inlet is 142 degrees.
The port bottom shows at 56mm, and I would expect 54.5mm, so the top of the cylinder may be higher than the top of the piston.

Bore is shown as 53.8 rather than 54mm, so I am not sure whether this reflects that stroke will be more than 54.5mm.
anyhow, this gives me a rough idea.

2Stroke-Stuffing(2SS) is only using 190 degrees exhaust, but the difference in volume may need more timing.- he is 49ccs vs 125ccs.

Anyhow, Aprilia timing is only a rough guide. Similar output may be gained at slightly different timing.

I want to see how 2SS gets on with his wide-exhaust barrel.
His secondary barrel does 20 hp at 17,500 rpm, which corresponds to 40hp at 13,000 rpm for 125cc on pump gas.
Aprilia claim 55, so he has some way to go.

He is working on the supercharged 50cc for now, so it may be quite a while before he gets back to the other engine.

I would think turbocharging would be better and easier for a two-stroke, as the supercharger is robbing a lot of power.
If you tune for 12,000 rpm on the turbo, you may not get a big power band.
Finding a very small turbo could be a problem. The IHI ones go down to 600 cc- quite a lot larger!.

The back-pressure of the turbo helps the engine run as normal, but at higher pressure.
Boost and power are fairly unlimited with a turbo, but strictly limited with a roots-type blower.
- limitations on intercool , detonation, heat, strength of engine.


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Cost contributions and solutions for a high-rpm 125 two-stroke:

1) As a crate engine, these are really expensive! - maybe $3500 usd?? Also, USA sources seem to have unusually high freight costs.

2) crank, con-rod, piston pin seem to be up-rated a lot from equivalent chainsaw parts to handle higher rpm loads.
They still only pull 25m/s avg at 14,000 rpm, though.
These parts should be bought ready made, as they are proven components.

the piston can be bought with a skirt coating included, which is good.
Quite a bit pricier than a chainsaw piston.

3) a gearbox off a 125 honda 4-stroke could be used, if the two-stroke crankcase was modified to suit.
A 125 race 2ST engine has much more torque than a 4-stroke road engine- so the standard clutch will not do.
They are probably both rated at around 13,000 rpm.
the 4-stroke has a shorter stroke.

the 4-stroke is 10 hp.- this seems a bit low for a fast-spinning engine? I would have expected 15 hp.
Maybe they have de-rated these a bit since earlier versions.
Possibly emissions is a problem if you go for higher power, plus low end pull is not good.

A 250 4-stroke race engine is good for around 45 hp at maybe 16,000 rpm.
They used to do 18,000rpm, but needed a lot more servicing.

The modified crankcase does seem possible with tig welding, and still preserve the ballrace housings.
Possibly the barrel mating surface should be skimmed.
The whole thing could be machined from solid with relatively simple equipment,
but horrendous material costs- $600 plus if using imported slab aluminium.

The difficulty is getting existing cheap cases that fit a proper crankshaft.

4) Aftermarket aluminium radiators are not very expensive.

5) Purchase of a complete set of second-hand parts is desirable to get a close look at finer construction details.
Some parts could be tig welded and re-machined and redone in Nikasil.
- freight looks like a problem, though.

ex TradeMe are quite expensive. I have heard of them changing hands "privately" for under $900 nz.

6) Carburettors seem to be very expensive new.
What would be a suitable size? 36mm? 38mm? 40mm?- possibly seek some advice, and look for cheaper options.

7) I need to get one of those pricey right-angled porting tools.
2 Stroke Porting and Polishing Kit (ccspecialtytoolstore.com)
This one seems to be best, but pricy at over $1000 USD.
are there cheaper ones which are just as good?

1/8" 90 Deg. Right Angle Pencil Die Grinder : Amazon.ca: Tools & Home Improvement
this is $389.95 USD. Is it any good? what does it fit onto?
2 stroke porting kit for smaller 2 stroke engines (ccspecialtytoolstore.com)

Here is an $871 USD version.
it seems to have a compact head size.

Dremel 575 Right Angle Attachment - Power Rotary Tool Attachments - Amazon.com
The Dremel one is cheap, but is not compact.

Dremel 3000-2/30 130W Rotary Tool Kit - Bunnings New Zealand
Full Dremel kit- I will get one of these- more durable that the Ozito equivalent.

I have 3 Ozito sets, and the flex shafts die quickly, then the speed control gets dodgy- maybe worn brushes?
The extendable hanger is no good, either.

IC-300 | 90° Angle Attachment (NAKANISHI) | NAKANISHI | MISUMI (misumi-ec.com)
body unit temporarily not available.
Mold Shop Tools - TELESCOPING RIGHT ANGLE ROTARY HANDPIECE , $660 handpiece only,
Mold Shop Tools - FOREDOM FLEXIBLE SHAFT MOTORS & KITS , $239 USD for flex shaft tool.
These look to be 110V units, with no 220v option.
Were are 230V, 50Hz here.

Re: motorbike layout. - possibly redo the frame in 15mm,20mm, 12.5mm ms tube. They have a basically steel frame.
The engine needs to be mounted higher to clear the big fat exhaust pipe.

This means more swingarm down-angle, possibly totally replace the swingarm structure, rear shocks, longer swingarm.

I also like a fairly high seat. Can this be done with the standard forks, fuel tank shell?

I have fairly long legs, and I should be just able to "flat-foot" it when stopped.

The Honda has a racy-looking fuel tank.
The original shocks are nowhere near race-grade, but I don't want to race it.

It has to be safe when cornering at 90+ mph, though.
- the original is flat out at 75mph, if you duck your head a bit.

I hear the "LC" 350 Yamahas were a bit dodgy at speed, and they are bigger and heavier.

This all sounds incredibly expensive, so it is on the Long Term plan.

It the meantime, I can amuse myself grafting the chainsaw and model aeroplane engines together, and modifying that.
There is plenty of scope for upgrading.
This is more in the spirit of MODEL engines! :)

I had a thought of adding extra transfer channels opposite the exhaust. This means adding on to the crankcase,
and creating new mating faces.
Some chainsaws are built like this. The main barrel studs can be sleeved, and pass through the secondary transfer channels.
Possible alternative:

Chinese water-cooled 49cc 2-stroke motorcycle engine, including carb, plus separate set of radiators.
The header pipe can be parted-off from an aftermarket cheap expansion chamber exhaust??
- images to follow-
I can then build a water-brake dyno, and play around with modifying the engine.
It looks like it has quite a small carb. Maybe a 35mm carb is better for a hotted-up version.
I can try out the intake-side power valve, major transfer port mods, wide exhaust ports, total cylinder porting rework.
This one looks like it is rated for about 13,000 rpm, and 15 hp. - that can be taken up to 17-18,000, I would suppose, and 20 hp.
That normally would require a disc valve, but may be possible with the resonant power valve- possibly a separate resonant chamber-stub
(helmholtz-style) to open at the same time as the bypass to the reed valve.
The reed block can be opened up to wrap around a centre tract.

Very likely a 250+ amp ac tig and a lathe-mill combo would be prerequisites to doing a proper job.

Again, this is getting away from the usual model-engines concept and into roadable motorcycle powerplants, albeit quite small ones.

- maybe doing somewhat similar to mr 2Stroke-stuffing, but stick to pump gas!

I don't think I would be getting so much into automated milling or aluminium casting, though.

I would be quite happy if I can match his 20 hp on pump gas.

-hone dyno, fume extractor, good silencing system!!

Regarding carbs- possibly an SU-style 1.5 inch would be suitable.
It doesn't have to have a snappy pickup.
Are these still made? Can I find plans for a home-made version?
A good Lectron carb would be maybe $500 by itself!.

Selling this style engine on AliExpress seems to be something new. Maybe they will go to proper 125 motocross engines eventually??
Keihin PWK 35, 36 seem to be vey cheap.

This looks to be a somewhat illegal knockoff, though.
The Chinese seem to have a loose attitude to brand marks.

- would customs seize these on entry into the country??

A 38 (1.5") is the size for the 125 yamaha YZ engine, so likely way too big for a 50 cc.
21mm seems to be a common scooter hot-up size.

SU carbs can be got second-hand, but very expensive for "as new" condition.
maybe the Keihin is the way to go...

At least they are very common, and there are lots of spare jets available.
The Lectron ones are $750 + USD!!!


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The rebuild of the 60cc two-stroke is under way.

It has a problem with the exhaust side piston skirt of the chainsaw parts being too short.
I have raised the piston about 2 mm to give me a later exhaust opening, and there is an equivalent gap under the piston in the exhaust port.
I will screw an aluminium "blocker plate" to the floor of the exhaust.
there is very little ring load at this point, so plain 6061 aluminium should be OK.
It has to be shaped to match the cylinder shape, but does not have to be super accurate.

I think I can screw in from inside the cylinder at an angle. I will have a go.

The exhaust port is a lot narrower that the TP60 barrel.- about 65 degrees in rotation around the bore instead of 75 degrees, and has a square top.
the transfers also have a square shape. They could have an angled top insert up to 25 degrees.

I have ordered another kit for a different engine, a Husqvarna 154, which seems to have a longer piston skirt.
That has the 45mm bore, but 34mm stroke instead of my 36.5mm.

I wonder if I could build up the piston skirt width so that I can use a wider exhaust port?

The J-B Weld could hold extra skirt pieces in shear, if the surfaces are clean and sanded.
I have also ordered a spare piston.
That extra part would get well crunched if it dropped into the cases.

I have had a chunk of piston drop off before in another engine, and the engine seemed to survive it OK. The piston parts tend to fracture rather than
jam in place, and don't get kicked out through the side of the case.
The engine does make a rather odd jangling noise.

All the nominally 45mm pistons vary a bit in base size. the TP60 piston will not fit in the new barrel, which is a fraction of a mm smaller.

The next job is to make adapter bushes for the pistons.
I think I can take the piston pin bore out from 11 mm to 13 mm.
This is better than 12mm, as I am more able to make a 13-10 bush with a small shoulder.
I should bore before doing the outer size, as the material may be more stable while cutting.
A good job for tonight!

I will see if it can be done on my micro-lathe. The chuck does not seem to have much grip.

If that works, then I can make a fixture for the piston, and do the piston pin hole machining.
I can bolt a plate fixture to the chuck, and have done this before for the head recess machining.
Looking for 250 or so amp tig for welding aluminium.
I think a can run 250A out for 230V ac, 10A in??
300A would be better, though.

I need to find out how aluminium welding works.
Do I need inverter rectification, then high-low pulsing over that??

This gear would feed back to my two-stroke barrel project plans.

I think even "2Stroke-Stuffing" gave up on aluminium casting, and got one 3d metal printed.
Casting intricate "lost-wax" 2-stroke barrels takes lots of practice and knowhow.

Welding and heat treating could be a bit of a learning curve, too, especially if parts are made with high-silicon alloys.

This needs a certain level of heat treatment to work with head studs, otherwise the barrel tends to go out-of-round.
- what do they call those honing rigs you bolt on the barrel-?-might need one of those.
Do I need a ceramics-style furnace for heat treatment?

I need to find out about removing nikasil. Then I can welds, heat treat, and send the barrel and piston away to be redone in Nikasil.
I presume I will be shifting exhaust port bridges around.

Two bridges looks good for the straightest exhaust top over 120 degrees of exhaust.
*** correction- 180-20 = 160 degrees exhaust port. This should clear the piston pin.
Full-skirt pistons are a bit experimental.
I think 65 degrees is about maximum for square-top ports.

Is a full straight port even the most desirable? most have a slight curve, or step in the middle, to go with pipe block-supercharge.

This chainsaw barrel I am working on now looks suspiciously like eutectic aluminium-silicon. It is fairly brittle.

It also seems to have a Nikasil coating as well- I have accidently flaked a few bits off- fortunately not in critical areas.

I am keeping this in engine discussion, as it is not really about hobby engines that I have built or am building,
but tends to get more wide-ranging.

A left a message on KiwiBiker, but this kind of "Mad Engine Science" doesn't seem to be that popular there.

I suppose it is a bit far-fetched even for Bucket Bike builders.

I wonder if two-stroke hotup is a bit of a dying art- there doesn't seem to be a lot on-line at the top end , like
"record" engine level, or even mucking about with old Grand Prix bike engines.
I suppose those are "collectible" and " restored" nowdays.

There is plenty of action with "bicycle engines"- but you don't want to get over 25 mph with those things-they vibrate badly when wound up,
and the bikes are too flexible and under-braked.

These things are still used for Motocross, Bonneville-type record racing, and drag bikes.

I suppose some people have their "Trade Secrets", still.

The hotted-up model RC engines tend to be smaller.
Even 15cc is quite rare as a hotted-up engine nowdays.

I think those would run better if converted to spark, but still run nitro glo fuel.

On a "Plus", my slip-roll set arrived today, from the UK.- maybe a pipe for the "chainsaw" engine, when I get it going.
I am slowly getting the piston bushes sorted out.

Regarding full-skirt pistons:
I am not sure screwing the skirt on to the rest of the piston is the right way to go.

I would prefer recessing hex socket screws into the piston head. Actual steel nuts would be good, too, but there may not be enough room.
for a 40mm dia piston, 4x m5 grade 8 screws would do, with 12mm thread engaged.
Screw head dimensions are 8mm wide and 5mm tall, approx.
Exhaust cement would make a good locking compound. It is not very strong, but keys into the surfaces quite well.

A slightly bumpy piston top makes no difference to the engine output power.

Should the split be at the ring groove, for single ring pistons, or offset?

2Stroke-stuffing was talking about stepped rings for wider ports, but I don't know if he will carry on with this.
vertical pin-lugs in rings doesn't seem to work.
I don't think the HP penalty of using ring bridges and losing exhaust width is as high as he was saying.

Bridges can be put in as airfoil webs, with quite a narrow leading edge, which reduces port obstruction.
He did tests with airfoil fins in the exhaust flow, and difference was minimal compared with no fins.

- that was with water at much lower velocity, and for a different reason, but it is a good indication.

He also runs an elliptical exhaust channel out quite a distance. I think the two side-branches can be merged into a circle over a much shorter distance,
at maybe 8-10 + degrees convergence at the merge. I will draw it up.

Some car collectors can come in at up to 30 degrees merge angle, but they are not all firing at once.
I was thinking 30mm exhaust port exit diameter.

The lower-speed engines are 22 mm, and a fast 125 single is around 36mm, with 38mm carb.
I shall compare these cross-section areas with the actual port area at the cylinder.
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