Engine sucks but will not blow

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KennyMcCormick315

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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.
Eh, rotating them by hand and not feeling compression isn't the biggest of deals with small engines like this. I've got a couple of Magnum 52 4-stroke RC airplane engines in active flight rotation(IE on aircraft I fly regularly). One, an engine I'm the only one to ever put fuel and fire to, has so much compression at hand flip speeds that I will cut my fingers on the prop when priming it if I'm not careful. The other one has so little compression at hand flip speeds it's as if the glow plug isn't even installed.

Both engines run flawlessly. The softer of the two is a complete starter queen, I'll grant, but they both run fine and both fly their respective models with grace and authority.
It can be incredibly satisfying to feel that 'snap', but as far as how the engine runs is concerned, it's not as critical as many think.
 

minh-thanh

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Eh, rotating them by hand and not feeling compression isn't the biggest of deals with small engines like this

There are many reasons for the engine to have no compression:
Cylinder Liner
Pistons
Rings
Groove of the rings
Piston - Cylinder Liner
Rings - Cylinder Liner
Cylinder Liner - Block
Valve and valve seat
Valve guide and cylinder head
Cylinder head with engine block
....
And sometimes compression loss through bolts of cylinder head..
So : why not test everything before assembling !?? The test measures are simple and do not require specialized tools.
For me when I make internal combustion engines : Compression is required - And that's the principle of the internal combustion engine !!!
I will never assemble and try to run the engine without any compression. And I would never advise newbies run the engine without any compression ,
Rotating them by hand No compression is felt: It depends on the friction of the engine parts. If the force of friction is equal to or greater than the compression of the engine, you will probably never feel the compression. You only feel the compression of the engine through the spark plug hole .
 

KennyMcCormick315

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There are many reasons for the engine to have no compression:
Cylinder Liner
Pistons
Rings
Groove of the rings
Piston - Cylinder Liner
Rings - Cylinder Liner
Cylinder Liner - Block
Valve and valve seat
Valve guide and cylinder head
Cylinder head with engine block
....
And sometimes compression loss through bolts of cylinder head..
So : why not test everything before assembling !?? The test measures are simple and do not require specialized tools.
For me when I make internal combustion engines : Compression is required - And that's the principle of the internal combustion engine !!!
I will never assemble and try to run the engine without any compression. And I would never advise newbies run the engine without any compression ,
Rotating them by hand No compression is felt: It depends on the friction of the engine parts. If the force of friction is equal to or greater than the compression of the engine, you will probably never feel the compression. You only feel the compression of the engine through the spark plug hole .
There was a day I would have agreed, but then I got my hands on that Magnum 52. It's seriously one of the best running engines I own of any kind. It runs better than the engine in my '14 Challenger! I could spend hours of time with it all strewn about across my desk trying to find the half a gnat's nard of excess clearance that makes it feel so soft at hand flip speeds, needlessly rebuild an engine that doesn't need a rebuild...or I could just kiss it with the electric starter and fly it anyway.


That engine has a far heavier task laid out for it than most engines on this site do, too. It isn't tasked with simply spinning its own flywheel once or twice a month. Oh no. It's tasked with flying an aerobatic RC aircraft most of the time. When it's not doing that, it's tasked with flying that same aircraft in trainer service. It actually has to do meaningful work and a flame-out is a far FAR bigger deal.

Also worth noting that piston rings can be a bit fussy when they're not seated. The other Magnum 52, which has super strong compression even at hand flip speeds and is the one only I've ever put fuel to? Yeah that thing was soft at first too. It didn't really build any meaningful compression when turned by hand until it had an hour or so of runtime on it. Getting the engine started and running it will often drastically increase the felt compression as the ring beds into the cylinder wall and vice versa.

Long and short of it? I would not be overly concerned about a low felt compression on an engine, especially if that engine is fresh off the lathe. If it runs well I would not question it. I would instead run it, let it bed in, see how things change over time. No sense creating a lot of work that just doesn't need to be done.
 

minh-thanh

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KennyMcCormick315 !​

When you and I make an engine, we must follow its principles.
And with the internal combustion engine, the important principle: There must be compression !
With the homemade engine, we have many options for compression: 10-1, 9-1, 7-1, 5-1, 4-1 but must have compression.
You can't feel the compression when turning the engine by hand, but you will feel the compression through the spark plug hole
Don't advise me : the engine will have compression when spinning it really fast 500, 800, 1000...rpm .
 
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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?
Fit and finish of the sides of the ring groove will make the seal to get compression. The seal of the ring to the bottom crankcase side of the groove seals the compression. The seal to the top side makes vacuum or suction for the intake stroke. Normally 0.0015" side clearance is built into a ring. I have had many customers cut ring grooves in pistons with a one piece plunge cutter and I never felt that I had the whiz bang skill to do this. I always use 2 tools one right and one left. It takes longer but makes grooves with parallel sides. If a one piece tool does not go in straight exactly 90 degrees it will cut a taper on one side as Ranger has pointed out. Any possibility here??
Next is a rough V8 car cross hatched hone finish on the cylinder. Not needed for cast iron rings in cast sleeve. Chrome oil rings need the cross hatch and we are not using any of that in our stuff. This thing is not a car. Smooth finished cylinders will seal better than rough cobby cylinders. Honing is hot these days. Total Seal sent me an email today touting proper honed cylinder surfaces that will affect ring seating and sealing making more horsepower at the crankshaft available with attention paid to hone surfaces. They were pushing a webinar or something to sell a gizmo to measure honed cylinder roughness. Take a look at this page for information closer to what we work on: Piston Ring Tips . It is geared towards those with old worn out engines but I have seen some model engines that ran so much they too actually did wear out.
Also if too much side clearance is next to the ring in the groove it can be brand new and be made with wide out of spec clearances. Lots of things can cause the rings to not seal. This one is a head scratcher....
Thanks,
Dave Reed
Otto Gas Engine Works
2167 Blue Ball Road
Elkton MD 21921-3330 USA
phone 410-398-7340
http://www.ringspacers.com
 

Gordon

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Fit and finish of the sides of the ring groove will make the seal to get compression. The seal of the ring to the bottom crankcase side of the groove seals the compression. The seal to the top side makes vacuum or suction for the intake stroke. Normally 0.0015" side clearance is built into a ring. I have had many customers cut ring grooves in pistons with a one piece plunge cutter and I never felt that I had the whiz bang skill to do this. I always use 2 tools one right and one left. It takes longer but makes grooves with parallel sides. If a one piece tool does not go in straight exactly 90 degrees it will cut a taper on one side as Ranger has pointed out. Any possibility here??
Next is a rough V8 car cross hatched hone finish on the cylinder. Not needed for cast iron rings in cast sleeve. Chrome oil rings need the cross hatch and we are not using any of that in our stuff. This thing is not a car. Smooth finished cylinders will seal better than rough cobby cylinders. Honing is hot these days. Total Seal sent me an email today touting proper honed cylinder surfaces that will affect ring seating and sealing making more horsepower at the crankshaft available with attention paid to hone surfaces. They were pushing a webinar or something to sell a gizmo to measure honed cylinder roughness. Take a look at this page for information closer to what we work on: Piston Ring Tips . It is geared towards those with old worn out engines but I have seen some model engines that ran so much they too actually did wear out.
Also if too much side clearance is next to the ring in the groove it can be brand new and be made with wide out of spec clearances. Lots of things can cause the rings to not seal. This one is a head scratcher....
Thanks,
Dave Reed
Otto Gas Engine Works
2167 Blue Ball Road
Elkton MD 21921-3330 USA
phone 410-398-7340
http://www.ringspacers.com
From what you are saying here is that compression is determined by the lower (crankshaft end) so in my case the lower end is more important than the the other end since I have plenty of vacuum but low compression. I am going to try remaking the pistons and I will be more aware of the side finish.

I have been away from this project for a while because of other projects and a brief sojourn into the Michigan wilderness. Woodlands can make you forget things like engines.
 
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From what you are saying here is that compression is determined by the lower (crankshaft end) so in my case the lower end is more important than the the other end since I have plenty of vacuum but low compression. I am going to try remaking the pistons and I will be more aware of the side finish.

I have been away from this project for a while because of other projects and a brief sojourn into the Michigan wilderness. Woodlands can make you forget things like engines.
Hi Gordon-
I am not a good teacher but will try to explain how a compression ring works. If I get confusing please ask me for clarification. When a piston is moving towards the cylinder head it pushes the ring(s) up the cylinder. The air and fuel mixture is squeezed into a smaller volume when the piston is moving up. As the pressure increases it tries to get out. If the valves are sealed and the piston rings are sealed the pressure increases. The pressure goes down the outside of the piston to the top outside corner of the top ring which is sealing against the cylinder. The pressure then crosses laterally across the top of the ring to get behind the ring at the bottom of the ring groove. The pressure pushes the ring out against the cylinder. The pressure continues down to the bottom flat side of the ring and tries to get laterally back out to the cylinder wall so it can bypass the ring and escape to the crankcase. This is called "blowby". For a ring to work properly it must seal against the lower side of the ring groove that is the side closest to the crankshaft. This surface must be square and very smooth. Roughness, mis alignment from not having the ring 90 degrees to the cylinder wall and able to seal all the way across the side of the ring will cause leakage. Wear in the ring groove or incorrect clearances will allow the rings to jump around and not seal all the way across. Modern rings are double disk ground which is a very smooth very square super accurate ground surface. Homemade rings almost always do not have this accurate ground side surface and will not seal as good as factory made rings. The lower side of the ring groove must be as smooth and square as you can get it if you want a ring to seal. If the top side of the ring groove is not as pretty as the lower side, don't sweat it. It will still work OK. Remember the piston changes direction lots of times per second so the ring is slamming back and forth in the groove. Too much clearance will break the seal and cause compression failure. That is why 0.0015" of side clearance is built inti each ring. Make the groove dead on the size. Normally 0.005" or more of side clearance means a worn out piston. Maybe I didn't cover everything here but we got enough out to get you thinking. 150 years ago in the steam engine days designers thought that the ring would seal all the way across the outside edge of the ring but really it seals only on the top outside corner towards the pressure. Unfortunately these pioneer steam designers were wrong about this in those early days. Modern cars do not use wide rings like 5/16" and 3/8" width because they do not work as well as narrow rings. You can make a new piston or make the groove wider and use a wider ring or 2 rings per groove to correct a ring groove problem. Whatever is easier will work.
Thanks,
Dave Reed
Otto Gas Engine Works
2167 Blue Ball Road
Elkton MD 21921-3330 USA
phone 410-398-7340
http://www.ringspacers.com
 
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Thanks Dave, a very clear explanation. I worked with a car maker, getting rings from Hepworth and Grandage - UK (1980s), just at the time when rolled steel rings were being introduced. Some (older designs) still took cast iron rings, but the "new" engine took steel rings, that were "oval" on the outer profile (or so I remember?). But were still based on sealing the bottom surface and a line around the bore. I also think it was the steel rings that were nitrided in an oven burning Ammonia? - for longevity.
I also remember the cast iron rings being pre-lapped - in a machine that used recyclable cylinders, and stacks of rings, oscillating axially - while the cylinder also rotated in steps - in order to "Barrel lap" the rings. This eliminated the "running-in" lapping in the engine. - for longevity of rings and bore.
The contact pressure of the steel rings was a bit quirky, as the ends were not in the same circle as the majority of the bend. But I can't remember if they were curved inwards or outwards! That affected the initial (spring) contact pressure, which had the gas pressure acting on the ring as well (to make the Tan load). I followed the calculations, but didn't remember them! (30 years and a lot to forget!).
I enjoyed your explanation.
K2
 

Gordon

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I think that I have not been paying enough attention to the sides of the ring groove. I have been just taking a plunge cut with a cut off blade of the same width as the ring. If the ring was 3/32 wide I used at 3/32 cut off blade. I think that I will have to do do some work on a tool bit and give it side clearance on both sides and on a cutter narrower than the ring. Then take two cuts to cut on both sides of the groove separately.
 

bluejets

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Most don't "cut" the ring at all.
Most simply snap it and file ends to required gap which, on air cooled is around 2 thou per inch of bore size.
So fit the ring in the finished bore using piston to get it square, measure gap and refile as necessary.
 

GreenTwin

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Perhaps the old steam engine rings were very wide to give a lot of surface wear area.
I see many 100 (+) year old steam engines that still run well, of course at low rpms, such as perhaps 70-100 rpm.

I use a cut-off blade and make a plunge cut for piston ring groves.
The saddle on my lathe can move a bit, so I hold the handle to make sure the saddle does not move when I am plunging.

I make one cut per groove with one tool.

For the rings, I use gray iron, and turn the rings a few thousands oversized.
Parting off rings generally will leave a bit of flash, and so I put the ring on a piece of fine emory cloth on a piece of glass, and move it in figure 8's to clean it up, cleaning both sides.

Then I try the ring in the groove, to check the fit, and I try the ring in at least four places around its diameter, since sometimes the ring is not perfectly flat.

I take off slight amounts from the ring with the emory cloth, until I get a nice sliding fit in the piston groove.
I rotate the ring while I am checking it in the groove, to make sure there is a uniform sliding fit all the way around the ring.

It takes a few minutes to hand-fit each ring to its groove (my model steam engines only have one ring).
I have not had any ring problems or compression problems using this method.

The trick is to work slowly, and don't remove too much material from the side of the ring at any one time.
You can also use a micrometer in various places around the ring, to make sure it is uniform in thickness.

.
 

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