Mery Engine Piston Rings Won't Seat

[carlw]

I'm chasing various problems with my Mery Explosive Vapor Engine. It ran upon completion but poorly and I have been chasing various problems. The big one is the piston rings have not seated. I motored the engine for about 4 hours with lots of oil hoping that would do the job but a pressure test shows a substantial leak. At 50 psi I can feel a breeze coming from the open end of the cylinder.

A bit of measurement:
Piston diameter = 1.498"
Cylinder bore = 1.500" +/- 0.0002" (by telescoping gage)
Ring width = 0.0615"
Groove width = 0.064"
Ring depth = 0.064"
Groove depth = 0.070"
Ring end gap = 0.004"

Is the ring gap too large? These are commercial rings for a Maytag engine as specified by Roland Morrison. Two rings are installed and the gaps are staggered.

The bottom space is 0.006": is the ring groove too shallow?

I checked the cylinder bore with a Sheffield air gage for roundness and parallelism. It shows that both are within 0.0001". The bore was finished with a Sunnen hone. There is some cross-hatch but it is possible that the finish is too smooth. I do not have a finish comparison gage so cannot estimate the finish.

What do you guys think: is it time for Comet cleanser or TimeSaver lapping compound and many hours of motorized (electric) running? Deepen the ring grooves?

Thanks in advance
CarlW

 

[Jeff L]

Try some Automatic transmission fluid on the rings, that should let them break in .Jeff

 

[Cogsy]

The ring gap is definitely not too large. 4 thou per inch of bore is common in full size auto engines. Realistically, you have only 1 thou clearance each 'side' of the piston in the bore, so you have a .001 x 0.004 arc for the compression to leak through. I haven't done the math but it's a really small area so I wouldn't be concerned about it. In practice I have run a 7/8" piston with an 0.008" end gap with no problems.

When you say you 'motored' the engine for 4 hours, do you mean ran under it's own power or turned by an electric motor (or similar)? I have found with cast iron rings that the rings seal substantially better after half an hour to an hour of running under their own power. I think the theory is the combustion pressures bed the rings in properly. I don't think simply spinning it over with an external power souce would have the desired effect. If it won't run entirely under it's own power then I would spin it with an electric motor but make sure you are getting some combustion as well, eventually the rings should bed in and it will run on it's own.

 

[gbritnell]

Could you be a little more specific about how you tested the engine. You say at 50 p.s.i. there is a substantial leak coming from the open end of the cylinder. Are you applying pressure to the crankcase?
With the numbers you have presented us even if the rings aren't seated the leakage should be minimal. I really wouldn't want to apply any type of lapping compound to the cylinder with a very small piston/bore clearance because along with lapping in the rings you'd be wearing down the piston.
I'm not familiar with the Merry engine, mechanically, but have seen a few of them running. Does it have poppet valves like other I.C. engines? If so I would start my checking there. Over the years I have helped quite a few fellows with their engines and the major leak area is the valve to seat.
gbritnell

 

[Jasonb]

Were your measurements taken after shrink fitting and final machining to the outer cylinder?

As George says its usually the valves where people have a problem or porely made homemade rings.

 

[Rustkolector]

I'm with George. Valves are often the culprit. Find a good way to check them. Also, I try to keep the ring land to ring side clearance to a bare minimum. .0005" to .001" is what I try to maintain on a new piston. The ring still has to be free to move in the groove so a good ring lapping to fit and groove cleanup is required. These clearances were recommended to me by Otto Gas Engine works in my earlier days restoring antique engines. Too much ring side clearance degrades compression. If the engine has any bounce on compression, I would try to run it. Running under power will seat rings much faster. The ring wall pressure will be much higher than when motoring it. Also, non detergent oil will help seat rings faster than detergent oil. The friction and wear additives in detergent oil aren't working in your favor. I would refrain from using any abrasives....period! Find the problem and fix it.
Jeff

 

[carlw]

First, to clear up some things that I did not make clear: yes I attempted the seat the rings while running with an electric motor. The engine would not start at all and I was trying to sort things out. The intake and exhaust chambers are off the cylinder and the ports are blind flanged except the rear exhaust which is connected to 50 psi air.

The rings are commercial and should be OK. I tried to check their contact with the cylinder wall by marking them with a red Sharpie but that was inconclusive.

The bore was honed after assembling the inner and outer castings and the measurements taken after the gland casting was pressed into the cylinder.

The valves have been checked separately and are not perfectly air tight - I get a blub-blub at about .5 - 1 second interval which will have to be good enough.

Finding the problem: I will try Prussian Blue next to check the wall contact. Then assemble the engine with either ATF or non-detergent oil and check for compression bounce. If I have that I will try to run the engine and report back. It will be awhile because the new ignition system will not be available until late next month.

Thanks again. I completely missed the importance of seating the rings under operating conditions with compression pressure under the ring forcing it into contact with the wall. And no abrasives! But one question remains: could the cylinder bore surface finish be too fine? I could give it a quick lick with a coarse brush type hone to get more cross-hatch.

CarlW

 

[idahoan]

Hi Carl

I have seen commercially made (or at least sold) that were so far out of round that it would have taken a very long time for them to seal. You have motored the engine enough I would think with careful observation you could see the contact areas. Also a bright light from behind sometimes helps to see if light is passing by the ring in any areas. It will be interesting to see what you find.

Once the bugs are working out; the Mery is sure a sweet running engine.

Dave

 

[Rustkolector]

I doubt your cylinder is too smooth. Usually after finishing with a Sunnen hone rebuilders will run an oiled Flex hone through the cylinder a dozen strokes or so to take off any sharp peaks left by the Sunnen hone finish (plateau honing).

Like Dave said, it is a good idea when you are gapping rings (home made or by others) to square up the gapped ring in the cylinder. Using a strong back light, check the ring circumference for complete sealing against the wall. No light showing at the cylinder wall whatsoever.
Jeff

 

[Mechanicboy]

But one question remains: could the cylinder bore surface finish be too fine? I could give it a quick lick with a coarse brush type hone to get more cross-hatch.

CarlW

Never blank as mirror in surface of cylinder! Helical honing reduce cylinder wear 40% less than conventional plateau honing.
The honing angle should be above 20°, generally 40° to 50°.

It will remains oil in valleys in surface of cylinder to lubricate piston ring and the engine will last long time.

The ring must be full contact with cylinderwall. If not, check circularity in cylinder with cylinder bore gauge, if all is measure is ok. Make new piston ring ---> http://www.modelenginenews.org/techniques/piston_rings.html

 

[gbritnell]

I would also check the circularity of the rings with a bright backlight. This will show the slightest gap. I purchased custom made rings one time and they were'nt the proper O.D.
I check all my valve seating by making an adapter to go in the ports and then sucking on it. If there is the slightest leak you will know right away.
gbritnell

 

[Mechanicboy]

Here is difference pictures where i photographed from old Russian model engine book..

The last question: Is the ring groove smoothly without unsightly marks after turning tools? Loss of compression pressure will be still if piston ring is not tight against groove. I had problem in my early model engine where compression was bad, learned out the groove was not smooth due unsharp lathe tool. Turned clean and smooth and maked new piston ring with right size. The engine started easy.

It is a bad idea to make piston ring bigger than bore then part off. If the ring was mounted in bore, you can see there are not full contact between piston ring and bore.

The ring formula: Bore/30 or Bore/25= ring tickness, Ring tickness = ring height, Tickness x 4 = ring gap

1. Turn to tupe are 0.1-0.15 mm bigger than bore.
2. lap the tupe to the tupe fit very very light press fit in bore. (Break-in the engine will make pistonring fit to bore and keep tight.)
3. Part off the rings where the thickness are a bit bigger than ring groove.
4. Break off the ring and add the spacer between ring gap who are measured ring thickness x 4 = ring gap.
5. Hold the ring between the two disc who are bigger than ring and thicker than 6 mm with bolt and nut in mid of disc.
6. Heat up the disc with ring inside to 500-600 degree celsius.
7. Let it cool down and dismandle disc and ring gap spacer. The ring will be elastic.
8. Grind the rings to fit the groove with plain fine hone stone + oil. Do not grind in one way or rotate, grind as in "8". The ring must rotate free in groove without play.
9. Adjust the ring gap with needle file.
10. The ring are ready to use. Mount piston ring careful in ring groove. The ring are brittle.


Ring can be made of cam shaft or brake disc, both are made of cast iron.

 

[carlw]

I'm a little rushed for time today and this will be short. This thread has turned into a regular compression troubleshooting guide and will be invaluable to me and to future readers. The idea of back illumination of the ring to bore interface is interesting and reminds me that I read someplace that gaps as small as 0.0002" can be seen. Because the Mery cylinder is closed at the crank end it is not a simple matter of shining a light inside. Maybe I can do something with the head from a high intensity LED flashlight. If I don't post for a day or two I haven't forgotten you, I expect to be even busier in the first part of the week.
Carl

 

[Longboy]

....I'm looking at the piston diameter and ring groove depth and width listed Carl. That's a common O-ring size. Replace and test again. Then in short measure you can confirm bad piston or rings....or support that your engine sealing issues are elsewhere.

 

[carlw]

So I dropped a LED flashlight into the cylinder bore, put a ring in square, and saw the light. One gap was about 5/8" long - no easy way to put a number on the width but it was far bigger than I expected. Looks like I will be checking the bore for roundness again and making new rings. I will report back when I know more. Thanks for all the help.
Carl

 

[Capt,n John]

Here is a link to see more about this engine in this thread.

http://www.martinmodel.com/MMPengines-subfiles/engines-mery.html

[ame]https://vimeo.com/2489850[/ame]