270 Offy

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stragenmitsuko

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Great Terry , another runner . And what a beauty it is !

My suggestion for a next build would be the snow engine .

Pat
 

sbdtasos

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Piece of art
congrats Terry for this masterpiece
 

prophub

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Great Terry , another runner . And what a beauty it is !

My suggestion for a next build would be the snow engine .

Pat
I would love to see Terry do the snow engine! I'm currently gathering materials for that one.
 

oldCB

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The cam gear covers were gasket'd with Oracal vinyl before being installed, but the cam box covers were only temporarily set in place for now. There's an oil flow adjustment screw on the front starboard side of the engine which will be finally set while watching the top-end oil delivery with the engine running.

The block and crankcase side covers were installed next with each being sealed with a Teflon gasket. Since it would have been easy to drop one of the 0-80 block cover fasteners inside the crankcase, the crankcase covers were installed first.

The magneto (which is actually a distributor) was then mounted on the front cover bracket. A pair of timing adjustments involving the phasing's of the rotor and trigger magnets completed its installation. Since the ECM was needed for these adjustments, it was time to move the engine to its display/running stand.

With the crankshaft set to TDC in the number one cylinder's compression stroke, the rotor was aligned with the number one high voltage tower. Since the shaft driving the magneto spins at the same speed as the crankshaft, a 2:1 gear reduction inside the magneto drops the rotor speed down to match that of the camshaft. An alignment mark, visible on the top inside surface of the magneto, will be inline with the grub screw located on the rear of the rotor when it's properly positioned. This grub screw locks the rotor in place.

A collar containing the pressed-in trigger magnets was slipped onto the distributor driveshaft during the magneto's installation. Three grub screws lock it to the driveshaft so it spins at the same speed as the crankshaft. The pair of diametrically opposite magnets generate the four trigger pulses required for every two crankshaft revolutions. Before tightening the grub screws, the collar was aligned with the magneto so the CDI is triggered at TDC in the number one cylinder's compression stroke. A lever on the side of the magneto can manually advance the ignition timing up to thirty degrees.

Finally, the carburetor and exhaust subassemblies were added. Each was sealed to the head using one of the nineteen custom Teflon gaskets made earlier for this engine.

A set of plug wires was made up using repurposed automotive vacuum fittings (Dorman 47411) for plug boots that nicely fit the VR2L spark plugs. Before installing the plugs, they were temporarily fixture'd outside the engine to verify the ignition one last time. After they were installed, the engine had plenty of compression.

A little oil and gas were added and the engine cranked for a few seconds to verify it at least wanted to run before adding the radiator and (messy) coolant. With the four carb screws opened 3/4 turn, the little engine started right up. I shut it down after a few seconds since I really didn't want to run it without coolant.

There's just a few loose ends including the addition of the already completed radiator to finish this build. With no experience with multi-carb set-ups, nor carb design for that matter, tuning promises to be a learning experience. It seems I've elected to put four unknown and untested carburetors on the same engine. - Terry

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Awesome build dude!
 

stihl1master

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The front-end machining on the crankcase includes a bore for the front crankshaft bearing and a pocket for a gear case containing bearing recesses for a pair of driven gears. One of these gears will eventually connect the crankshaft to the gear tower, and the other will drive the water and oil pumps. The Offy uses a dry sump oiling system, and its pressure and scavenger pumps will eventually be located inside the gear case.

The crankcase was set up in the mill vise and indicated for access to its front end. The location of the bore for the crankshaft bearing was picked up from the test rod running through the three bronze bearings. The rear bearing in the rear housing and the starter bearing in the not-yet-machined front cover each have a bit of 'wiggle' room since they're bolt-ons to the crankcase. The front bearing, though, will be hard fixed to the crankcase and must be on the same axis as the main bearings.

I was concerned about attempting an interference fit for the front bearing since I was unsure of just how much interference could be tolerated while drawing the crankcase halves closed. If the flat surfaces of the crankcase halves don't completely close against each other, not only will there be an oil leak, but the axis of the front bearing may end up displaced from the axis of the main bearings and create a bind in the crankshaft. On the other hand, too loose of a fit will allow the bearing to spin in its bore and damage it.

Boring the perfect fit would require more luck than I was willing to risk and couldn't be tested without removing the crankcase from its setup. So, I opted for a one thousandth oversize bore that would provide a clearance that I could later shim out - something that's possible with a split bore. Aluminum foil as thin as .0005" is used by some candy manufacturers on their chocolate treats and can be a sweet source shim stock.

The remainder of the gear case machining was completed including bores for the two driven gear bearings which were also bored a thousandth over. Although I'd have preferred zero interference fits for these bearings, the front cover will eventually contain a matching pair of bearings, and there's no way to align bore them in pairs. The front cover's wiggle room will likely be taken up by the starter bearing, and unavoidable errors in locating the positions of the two bearing bores will make the cover difficult to assemble unless the bearings have some wiggle room of their own. When assembled, there will most likely be enough friction created by positioning errors to prevent the gear bearings from spinning in their bores.

A trial assembly of the crankcase halves around the front bearing was successful. I found that adding a thousandth shim around only the upper half of the bore allowed the crankcase to close tightly around the bearing and allow the thousandth-under test rod to freely rotate inside all five bearings.

One of the things that attracted me to the quarter scale Offy is its faithful adherence to the original engine's great looking appearance. The painstaking detail in many of the model's individual parts will provide a number of interesting mini-projects with their own short term satisfactions that'll help keep me interested in such a long term project.

The first of these parts is the front cover. It encloses the gear case and contains the starter bearing which will be the sixth crankshaft bearing. Other than rearranging its mounting bolt pattern to accommodate the crankcase split, I duplicated Ron's design. It's finished periphery will provide a template for the later machining the crankcase's lower sloping sides. The magneto mounting bracket was an integral part of the casting for this part in the original engine. Ron attached a separate bracket to the cover with hidden screws and blended the seams with fillets of metal-filled epoxy. I used my Tormach to machine the cover and bracket as a single part. There isn't room for a front shaft seal, but Ron included a groove for a 12 mm x 1mm CS o-ring around the bearing's i.d. that should be effective against oil leaks.

My first serious mishap in this project occurred while removing the overhanging excess stock from the rear of the cover in preparation for its rear face machining. The large multi-insert facing cutter that I was power feeding in my manual mill grabbed the overhanging lip and pulled the end of the part partially out of the vise. This stalled the cutter until I was able kill power to the spindle. There was no damage to the already machined top surface, but there was a deep gouge on the part's back surface. With all the effort invested so far, I felt there was nothing to lose by trying to salvage the part with a tig-welded repair. The welding created some of its own damage to the topside surface, but the final result was much better than expected with no visible trace of the repair.

As it turned out, my biggest concern was with the mill itself since I had to hammer the R8 cutter out of the spindle using a long drift in place of the draw bar. The cutter had spun inside the spindle bore and was jammed against what remained of the collet key pin. Fortunately, TIR checks on the spindle bore showed there was no apparent damage, and re-tramming the mill seemed to return things to where they were before the accident.

With the cover installed on the crankcase, there's still no sign of binding of the test rod, but the friction of the cover's o-ring has added significant drag. - Terry

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This is art worked. is it all CNC, or hand work ??
 

stihl1master

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Much of last week was spent modeling the complex gear tower which includes a pair of front and rear halves, end caps, and a tightly integrated take-off block for the magneto. After a lot of frustration, I still don't have an assembly that I trust to begin machining. My wife would say it's because I can't follow instructions, and to an extent she'd be right. I've found a few online screen shots from those that have gone before me to be invaluable.

I took a break from the tower and returned to removing chips from the crankcase since its modeling had been completed. With the foundational machining done, I felt it was safe to finish up the external profiling that will finally give the crankcase its distinctive shape. Other than several o-ring grooves that are still planned, its bottom and both sides were finish machined. The bottom was milled using a tiny ball cutter in order to create an array of cooling fins with rounded tips and filleted roots.

The exhaust fan on my bead blasting cabinet is currently out of service, and so the photos show the machined surfaces straight off the mill. Some cleanup will be required, but that will become more obvious after the surfaces are bead blasted for the first time. - Terry


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The front-end machining on the crankcase includes a bore for the front crankshaft bearing and a pocket for a gear case containing bearing recesses for a pair of driven gears. One of these gears will eventually connect the crankshaft to the gear tower, and the other will drive the water and oil pumps. The Offy uses a dry sump oiling system, and its pressure and scavenger pumps will eventually be located inside the gear case.

The crankcase was set up in the mill vise and indicated for access to its front end. The location of the bore for the crankshaft bearing was picked up from the test rod running through the three bronze bearings. The rear bearing in the rear housing and the starter bearing in the not-yet-machined front cover each have a bit of 'wiggle' room since they're bolt-ons to the crankcase. The front bearing, though, will be hard fixed to the crankcase and must be on the same axis as the main bearings.

I was concerned about attempting an interference fit for the front bearing since I was unsure of just how much interference could be tolerated while drawing the crankcase halves closed. If the flat surfaces of the crankcase halves don't completely close against each other, not only will there be an oil leak, but the axis of the front bearing may end up displaced from the axis of the main bearings and create a bind in the crankshaft. On the other hand, too loose of a fit will allow the bearing to spin in its bore and damage it.

Boring the perfect fit would require more luck than I was willing to risk and couldn't be tested without removing the crankcase from its setup. So, I opted for a one thousandth oversize bore that would provide a clearance that I could later shim out - something that's possible with a split bore. Aluminum foil as thin as .0005" is used by some candy manufacturers on their chocolate treats and can be a sweet source shim stock.

The remainder of the gear case machining was completed including bores for the two driven gear bearings which were also bored a thousandth over. Although I'd have preferred zero interference fits for these bearings, the front cover will eventually contain a matching pair of bearings, and there's no way to align bore them in pairs. The front cover's wiggle room will likely be taken up by the starter bearing, and unavoidable errors in locating the positions of the two bearing bores will make the cover difficult to assemble unless the bearings have some wiggle room of their own. When assembled, there will most likely be enough friction created by positioning errors to prevent the gear bearings from spinning in their bores.

A trial assembly of the crankcase halves around the front bearing was successful. I found that adding a thousandth shim around only the upper half of the bore allowed the crankcase to close tightly around the bearing and allow the thousandth-under test rod to freely rotate inside all five bearings.

One of the things that attracted me to the quarter scale Offy is its faithful adherence to the original engine's great looking appearance. The painstaking detail in many of the model's individual parts will provide a number of interesting mini-projects with their own short term satisfactions that'll help keep me interested in such a long term project.

The first of these parts is the front cover. It encloses the gear case and contains the starter bearing which will be the sixth crankshaft bearing. Other than rearranging its mounting bolt pattern to accommodate the crankcase split, I duplicated Ron's design. It's finished periphery will provide a template for the later machining the crankcase's lower sloping sides. The magneto mounting bracket was an integral part of the casting for this part in the original engine. Ron attached a separate bracket to the cover with hidden screws and blended the seams with fillets of metal-filled epoxy. I used my Tormach to machine the cover and bracket as a single part. There isn't room for a front shaft seal, but Ron included a groove for a 12 mm x 1mm CS o-ring around the bearing's i.d. that should be effective against oil leaks.

My first serious mishap in this project occurred while removing the overhanging excess stock from the rear of the cover in preparation for its rear face machining. The large multi-insert facing cutter that I was power feeding in my manual mill grabbed the overhanging lip and pulled the end of the part partially out of the vise. This stalled the cutter until I was able kill power to the spindle. There was no damage to the already machined top surface, but there was a deep gouge on the part's back surface. With all the effort invested so far, I felt there was nothing to lose by trying to salvage the part with a tig-welded repair. The welding created some of its own damage to the topside surface, but the final result was much better than expected with no visible trace of the repair.

As it turned out, my biggest concern was with the mill itself since I had to hammer the R8 cutter out of the spindle using a long drift in place of the draw bar. The cutter had spun inside the spindle bore and was jammed against what remained of the collet key pin. Fortunately, TIR checks on the spindle bore showed there was no apparent damage, and re-tramming the mill seemed to return things to where they were before the accident.

With the cover installed on the crankcase, there's still no sign of binding of the test rod, but the friction of the cover's o-ring has added significant drag. - Terry

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How long does it take to run the program on a CNC mill ?? great work
 

mayhugh1

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Thanks everyone for your kind comments. They're very much appreciated.

Stihl1master:
I'd say the percentage of CNC was around 60%. Often a part like the crankcase or head received both types of machining. To be honest, my experience with CNC (or the way I use it) doesn't always result in a time savings over manual if I'm making just one piece of a particular part. For safety, I tend to take smaller cuts on my Tormach compared with making the same part on my Bridgeport clone. And so, the total machining time is often the same on many parts either way. CNC does let me make 3d cuts that would otherwise have to be accomplished with lots of filing, and that is where a major time savings can be had. Hope that helps. - Terry
 

srobovak

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Congrats! Excellent workmanship. And thanks for taking so much time to share all the details with us.
cheers,
Branislav
 

johwen

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An excellent project, a great engine model and an engine note to complete the package.
Well done John
 

mayhugh1

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Even though I hadn't planned another Offy post, I thought I'd share a few things I've learned working with it during these past several weeks. Since I usually enjoy making engines more than running them, mine often end up on a display shelf with only an hour or so running time. It seems that once they're running reliably, I'm on to the next project. So far, even after running a liter of gasoline through it, I still enjoying playing with the Offy.

Being obsessive about oil leaks, the Offy was almost my first leak-free engine. There's some seepage around the forward end of the crankshaft due to an o-ring I omitted, but a proper fix would require a new front cover machined for a lip seal. Since it's relatively hidden by the front bracket and doesn't drip, I've decided to live with it. Fortunately, some of this oil seems to be wicking into the starter clutch and lubing it.

I've had doubts about the grease-packed seal inside the water pump I designed, and it started leaking right after a long hard run. In addition to leaking onto the display stand, coolant was also finding its way into the oil. Adding an o-ring shaft seal just ahead of the pump's rear bearing turned out to be easier than expected and eliminated the problem, but several oil changes were required to completely flush out the coolant.

Many cold garage starts and short runs soon caused water (a by-product of combustion) to begin accumulating in the oil. Although the head heats up quickly while running, the crankcase doesn't get hot enough to drive out moisture. Over time, its chocolate color warned that the oil was becoming a corrosive mixture of water and crankcase gasses. I expected to see puke oil inside the clear PCV hoses that had been routed to the oil tank, but they never contained anything but clean condensed water. It didn't make sense to continue piping this into the oil tank, and so I eventually vented the PCV hoses to the atmosphere.

The most frustrating issue I run into was trying to control the oil smoke in the exhaust. The oil holes in the piston groove just below the scraper ring don't seem to be enough to keep oil out of the Offy's combustion chambers. This same oil control scheme has worked well in deep sump engines, but seemed to be overwhelmed by the oil whipped up by the connecting rods inside the Offy's confined crankcase compartments. Testing showed the problem to be worse with straight 30W than with 10W-30, and so 5W-20 might be a worth try. If I were re-making the pistons, I'd mill oil return slots rather than drill holes.

My 'easy fix' to the smoke problem was initially to limit the amount of oil in the crankcase drains by maintaining a minimum amount of oil in the tank. Since I'd originally placed the tank's outlet near the bottom of the tank, this allowed only 10 mL of reserve oil - an amount determined by removing oil from the tank (while maintaining a flow in the pressure pump hose) until the smoke went away. Even with this minimum level, the plugs still showed signs of oil. A new problem created by such a small reserve, though, would be its increased vulnerability to crankcase vapors. To get around this, a new outlet was installed higher up on the tank which allowed me to increase the reserve oil nearly 10x.

During a two minute 3000 rpm run, the upper half of the radiator becomes noticeably warm since about half the gets moved through the engine. A thermometer showed the coolant in the upper part of the radiator to be just over 100F.
The actual flow was measured by temporarily diverting the output of the upper radiator hose into an external container. Coolant flows through the engine at all running speeds, and the flow rates roughly agreed with the measurements made during the water pump development.

However, worrisome air bubbles show up in the coolant at high rpms. There's no oil leaking into the coolant, and the bubbles haven't gotten any worse since the engine's very first run. I've noticed these same bubbles, which are likely due to a head gasket leak, in the Youtube videos of two other Offy builders. And so, before tearing into the engine, I decided to run a compression test to see if I had a major cylinder problem.

Compression tests on model engines are complicated by the small volumes of their combustion chambers. While running a test, the volume of the tester is effectively added to the volume of the combustion chamber which reduces the pressure readings. An additional issue with the Offy is the limited access to its spark plug holes created by the water outlet pipe.

I converted an inexpensive mechanical tire pressure gage purchased from Amazon into a compression tester. The meter and valve body were reused, but the Schrader end was replaced with a custom adapter specially machined for the Offy's 1/4-32 spark plug holes. The tester's internal volume was also determined in order to come up with a correction factor for the Offy's pressure readings. The huge 41% correction factor is an indication of the measurement problem.

My pistons produce a theoretical static compression ratio of 7.85 which by design is less than Ron's high compression pistons. This reduced c.r. should theoretically produce readings of 115 psi. Although I was prepared for one or two dramatically lower readings, all four were within a generally accepted 10% variation. My corrected readings were: #1 = 99 psi, #2 = 107 psi, #3 = 113 psi, and #4 = 99 psi. Although the two outside cylinders are a bit low, I'm enough satisfied with the results that I have no plans for a teardown. To be honest, I'm not sure how I'd improve on the current head gasket.

The engine continues to start easily and after a brief warm-up idles reliably at 1100rpm. The air bleed adjustments had more effect on the idle performance than I expected. There are no hot-start problems and, even without a choke, starting in a 55F garage hasn't been a problem. The stainless steel exhaust gets incredibly hot, and its previously polished chrome-like finish has become a golden yellow. With 25 degrees advance the engine will rev to its top-end of just over 5k rpm. The #1 and #4 carbs seem to be well matched to each other and so are the #2 and #3. There seems to be only an eighth turn adjustment screw difference between the two pairs. The plug insulator colors aren't the ideal tan color that I've been able to achieve on my other engines using TruFuel 4 cycle gasoline, but that might be related to the still excessive oil.

The radiator fan turned out to be more effective than I initially thought. At the end of a run, the engine cools down twice as fast with the fan running, and so the air it's blowing over the head is having more effect on it than on the radiator.

Lastly, our pond fish and turtles survived our infamous week long Texas freeze and power blackout, but it will take months to replace all the landscaping we lost. - Terry

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Vietti

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I too made a compression tester and was shocked by the low numbers. Wonder how you calculated a correction factor? I still use it but consider the numbers relative.

I have a two cylinder engine that has oil control issues. A well respected builder and collector suggested I try heavy (60-70) weight oil, the old Harleys used it. I tried it and not sure it helped much but I'd be interested in your opinion re oil weight and oil consumption in model engines.

One of the best things I did to help control the oil getting past the rings was to go to 0 rings but we won"t go there. At one time I had 4 ci rings per piston and oil drain holes around the bottom ring with no joy. Tried three sets of pistons with different ring arrangements, drain holes etc. Probably, like you, a very low oil level seems the best answer, though is is scary to consider such a low oil level.

Your engine is wonderful so I looked up your previous engines, equally outstanding. What you do shows what is possible and encourages me to pick up my game.

John
 

The_reach

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Oil choice for these smaller motors isn't something I had previously considered to make a huge difference, once again I'm surprised 👍
 

stragenmitsuko

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Commercial compression testers work with a shrader valve from a bicycle tire .
Valves from a car tyre have a higher opening pressure .

I found out when I made an adaptor for my tester and used a car tyre valve .
Compression was abt 1.2 bar lower then with the tester's original valve .
This was consistent and repeatable .

It was only then that I found out that there are two types of shrader valves .
The bicycle tyre one has a much weaker spring .

Pat
 

mayhugh1

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I too made a compression tester and was shocked by the low numbers. Wonder how you calculated a correction factor? I still use it but consider the numbers relative.
John
John,
The pressure of a gas is inversely proportional to its volume. So, calculate or measure the volume of your combustion chamber and the internal volume of your tester. Then to correct your reading multiply it by (VOLhead + VOLtester)/(VOLhead). - Terry
 

Vietti

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Terry,

Thanks, I ordered one! Still unclear how to measure the volume of the gauge fill with water somehow?

Are you going to try some 60 weight oil?

John
 

mayhugh1

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Terry,

Thanks, I ordered one! Still unclear how to measure the volume of the gauge fill with water somehow?

Are you going to try some 60 weight oil?

John
John,
I measured the volume of the meter by droppering in alcohol. The rest of the gage I was able to calculate. I also calculated the head volume. Right now I'm not planning to try any more oils.

By the way, I enjoyed your magneto article in Model Engineer Builder several year ago. - Terry
 
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The_reach

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Terry,

I have just about collected all the material needed to make a 1/2 scale model of this engine. In your opinion what would you change if you were able to do so due to making it on a larger scale? Any suggestions would be appreciated

Jimmy.
 

johwen

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When testing compression it is important to have a full open throttle to allow the full compression reading to take place if you are not aware of this. You will not get a top reading if this is the case. John
 

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