Engine sucks but will not blow

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minh-thanh

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Many ringed engines will have near zero compression (initially) when wound over by hand.
See how it responds with the same arrangement only with a drop of oil in the bore and spun with a drill at a hundred revs or so.
That's not how I do my engine - and I would never try
My way : 1- make cylinder . 2- make piston .3- make ring
Try testing pistons without rings with cylinder : they must have compression.
Assemble the rings on the pistons and check with the cylinders: they should have compression but more than the ringless pistons
That testing easy just by hand.
All engines I made: they have to have good compression from the start - and I think that's the right way to make an engine - no matter how big or small the engine is.
 

awake

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Hmm ... do you typically make the piston and cylinder from the same material? I have always read that with differing materials, such as an aluminum piston in a cast iron cylinder, the piston has to be a few thou smaller; otherwise the difference in the rate of heat expansion will cause binding. Note however that this is only hearsay for me, so I cannot attest to whether it really works that way, especially in the small scale of a model engine!
 

minh-thanh

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Hi Andy !
I have always read that with differing materials, such as an aluminum piston in a cast iron cylinder, the piston has to be a few thou smaller; otherwise the difference in the rate of heat expansion will cause binding. Note however that this is only hearsay for me, so I cannot attest to whether it really works that way, especially in the small scale of a model engine!
It's not a hearsay ! It is the truth
Happened to me 3 or 4 times
If you make the cylinders and pistons good enough, with "enough" tolerances you will always have compression and it won't jam when it heats up.
Piston : Aluminium , Cylinder : steel or cast iron
 

bluejets

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There is simply no tolerance in miniature engines for poor workmanship.
There would be a perfectly simple explanation if one could actually have one of these non-running engines in hand but all one can do is rely on vague explanations and assurances that everything is done correctly.
I'd like 2 bob for every time someone has said, valves are fine, nothing wrong with them when in fact it usually amounts to 99% of problems.
Suck but won't blow doesn't make any sense what-so-ever.
If I didn't know better I'd say it was a troll posting.
 

minh-thanh

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bluejets !​

Read it again
With the head removed when I put my hand over the bore it has great suction and will even hold the vacuum for a long time but almost no compression. It obviously is not valve leakage or head gasket because this is with the head removed and just my hand sealing the bore. Also there are two cylinders and both act the same way. It would seem that if a piston works in one direction it should work in both directions. What am I missing?

the head removed and just my hand sealing the bore .

There is simply no tolerance in miniature engines for poor workmanship.
There would be a perfectly simple explanation if one could actually have one of these non-running engines in hand but all one can do is rely on vague explanations and assurances that everything is done correctly.
I'd like 2 bob for every time someone has said, valves are fine, nothing wrong with them when in fact it usually amounts to 99% of problems.
Suck but won't blow doesn't make any sense what-so-ever.
If I didn't know better I'd say it was a troll posting.
Very simple : If you have a cylinder small at TDC and a large at BDC - taper cylinder , you change BDC -> TDC , TDC -> BDC and try compression - suction
You will see, the larger the taper, the more compression you lose
even, no more compression near the end of piston stroke
 

Vietti

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Agree with Bluejets. I would take a hard look at the valves. Perhaps apply air pressure to one port and see if the others leaks, soapy water? Then switch to the other port. I've gone so far as to make a plate that seals against the bottom of the head and see if the valves are behaving.

I hope you will let us all know what the solution ultimately is.
 

Asm109

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All, please read and comprehend what the maker of the engine has said.
He takes the cylinder head (with the valves in it) OFF of the engine.
With is thumb plugging the cylinder, he spins the crank. He feels suction on his thumb when the piston is descending.
When the piston is rising there is no pressure trying to blow his thumb off.

Valves can have zero, zip, zilch, nada to do with this issue.

I have zero idea what could be wrong. To find out, will require tearing the motor down and putting your eyeballs on all the parts and features and connect those eyeballs with your brain to figure out what is the culprit.
 
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Is it possible that the grooving tool for the piston ring grooves is the problem? If the tool has a ‘defect’, slight taper, etc, on the side that is cutting the lower face of the groove, but the upper face is ok, then as the piston descends, (suction), and ‘seals’ against the, (good), upper face of the groove, a partial vacuum may be felt? As the piston rises and the ring moves and attempts to seal against the lower, (imperfect?) face, any compression would be lost. If the same tool is used for all of the pistons, and machining setups are the same, then chances are the results would be the same?
 

Gordon

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OK Present status on this. I have the engine firing on LH side and not the RH side. I was getting fair compression on the left but not on the right. I swapped the heads and I was getting fair compression on both sides but now I am only getting compression on the left and not the right. I am presently in the process of making two new heads. I could have just made one but I wanted to make another design change so I am making two heads. Original heads were cast iron and the new ones are 12L14. I suspect that my problem was initially in the ring groove. Groove too wide or too deep. Once I have the new heads made I will probably go back and perhaps make new pistons if it still does not run. I still get much better suction than pressure. I have run the engine with an electric motor for a while so things are seated.

The original bore was drilled out, bored undersize to insure that it was actually round, reamed with a 1" reamer and lapped with an Accro Lap. The rings are purchased from Dave Reed after I was not having any luck with my home made rings. End gap on the rings is .002. Rings were made per Trimble method which I have used successfully before. Cylinder is cast iron and pistons are aluminum. I sure am glad that I am not trying to make my living with this hobby.
 

kuhncw

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Gordon, if you make new pistons, you might use your existing cylinders for gages to determine final piston diameter. You've probably thought of this, but I'll mention it anyway.

I recently made new CI pistons and rings for a twin cylinder Titan and checked piston diameter with my existing CI cylinders. Once I had a very close fit, I measured the pistons. I found that my original liner measurements were undersize. Based on my original measurements the first set of pistons and rings were enough undersize to cause poor compression.

Good luck. You'll get there.

Chuck
 

minh-thanh

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Gordon !
Although I'm pretty comfortable with the causes of loss of compression, rings, pistons, cylinders....but I'm pretty vague about what you're describing.
I hope you will find the real cause
 

Gordon

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Gordon, if you make new pistons, you might use your existing cylinders for gages to determine final piston diameter. You've probably thought of this, but I'll mention it anyway.

I recently made new CI pistons and rings for a twin cylinder Titan and checked piston diameter with my existing CI cylinders. Once I had a very close fit, I measured the pistons. I found that my original liner measurements were undersize. Based on my original measurements the first set of pistons and rings were enough undersize to cause poor compression.

Good luck. You'll get there.

Chuck
I have been under the impression that the OD of the piston was not super critical. The compression is developed at the ring. The piston is generally .002 to .005 undersize to allow for expansion and that allows a lot of air to bypass the piston. The only time that the OD of the piston would be critical is when you are trying to run without rings. Am I wrong?
 

kuhncw

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Gordon,

You are definitely right. It's the rings that seal. In feel, keeping piston clearance as low as possible, depending on piston material, will reduce piston cocking which should help ring sealing.

With aluminum pistons 0.0015 to 0.002 diametral clearance per inch of bore has worked well for me. Cast iron pistons can run tighter, of course.

Regards,

Chuck
 

CaneSelvatico

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Piston OD have a big influence on compression and also ring gaps, while ring groove depth is not as important. Working on old two stroke motorbikes, usually there's about 1mm of radial play behind the rings on a 50mm bore, but tenths on the ring gap and piston clearance can make the difference between a runner and a non runner. Sometimes it can feel as it have compression but could be not enough for running (compression ratio also in play) and a good trick to test that is to squirt some oil down the spark plug hole, so it will help sealing the rings
 

minh-thanh

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Simple : make the piston ringless and test it with the cylinder head removed
If the engine has no compression: 99.9 % is due to the cylinder
If the engine has compression : is due to rings or grooves
 

Donrecardo

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I know its not a helpful reply but every time I read the title " Engine Sucks " I can help but think
most of the IC engines I build " Suck" which is why I usually only build steam or compressed air
engines, give em ehough pressure and they always run. but then I guess it wasnt in the same context
that I was thinking when you said " it sucks " ;)
 

Badhippie

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Why not just put o-rings in place
Of the piston rings and see what you get then turning the engine over by hand. Won’t need to fit new pistons or anything. It would be a real simple test to do. Would tell you there is a problem with the rings or ring lands on the piston. Of course would need to get o-rings to fit what you got or make them takes about 2 min to make a o-ring
 
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