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Blow-by Solutions

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SmokStak

• On my Jaeger, the cylinder was honed and the piston was put back in with the old rings (six of them), and the engine ran well after the rings took a seat. After a few months of running occasionally, there is a noticeable amount of blow-by. I see no reason for this to start now, any ideas why? What can I do? - Mac

• I have never had any luck reusing old piston rings. I know a lot of people that do it, though. If I don't have to pull the piston out of an engine, I don't. If I have to, I replace the rings. The rings are probably one of the cheapest parts you can buy for an engine. If I were you, I would get a new set of rings and lightly hone the cylinder to break the glaze, if there is any, since you said you honed it not too long ago. You should also check the side clearance of the rings in the grooves. If that is excessive, compression can leak around the back side of the rings. These are my experiences so take them for what they're worth. - Mike

• Every now and then the oiler will have air blow back. The piston is getting about 15 drops per minute and at 10 or so the bottom of the cylinder had no oil. Is too much oil possible, gumming up the rings? The engine never does any work because I have nothing for it to run. Oil is not blown off the rear of the piston in any great quantity, just a puff of exhaust from the top of the cylinder/piston when it fires. Any more information needed to evaluate the situation? - Mac

• When you honed the cylinder you forced the old rings to get about 20 years of accelerated wear in a day's time. By introducing this much wear, you most likely wore the rings much past the point of working "acceptably." Had you not honed, you probably could have gotten away with it. In any case, there has been a past compression problem with your engine.

"If you have good compression when turning it one way and much less when turning it the other way, I would expect to find excessive wear." 

Having six rings in it tells me some past owner was trying to get better compression than he had by doubling up. Usually doubling up doesn't work any better or worse than single rings and is only a desirable solution where the proper width single ring is not readily available. Check your ring side clearances. This is likely your problem area. Expect to see an extra 0.0015 of clearance due to using the doubled rings. With two per groove you should have three thousandths when new, and 0.0065 is the maximum. Seven or more and you need machine work. The top groove always wears the most.

You need a set of three rings measuring 3-1/4-by-1/4-inch and probably three .030-inch spacers to go on the top side of the rings. Measuring with feeler gauges will tell the story. When the rings are not held tightly enough in the ring grooves due to wear, the rings can turn in the grooves. The force of gravity makes the gaps all line up at the top in a horizontal engine much like a marble finding the bottom of a punch bowl when released. Maybe not quite this fast, but faster than you might imagine. In any case, the compression doesn't escape past the lined-up gaps.

Ninety-five percent of the leakage is by the sides of the rings where they don't lay flat against the ring land. End gap means very little in the business of sealing compression. It can, however, indicate a ring that is not the proper size for the cylinder and tip you off that the ring may not be round for the desired cylinder diameter. Good luck in your quest for compression.
- Dave

• If the cylinder, rings or piston is/are worn at all, it will depend on oil to help seal the rings. If you flood the engine when starting it once, oil will be washed off, and once it starts to blow-by, it will keep the oil from getting to the proper place. This is sort-of a no-win situation. Sometimes blow-by will keep the oil from going down the pipe to where it is supposed to be. If the piston is worn too much or the cylinder is tapered, the piston will cock in the bore and the rings will not seat.

If you have good compression when turning it one way and much less when turning it the other way, I would expect to find excessive wear. - Ken

• My Fairbanks-Morse Z has a massive amount of compression, but at TDC it has a bit of blow-by. My Novo is the same way. Granted, the FM Z only has about two hours on it since honing and new rings. I think the rings were $5 each and the engine uses three. For the amount of trouble they can cause, it really is easier to just put on a new set of rings. If you do this, then honing to break the glaze is a good idea. I run this engine with the oiler up as high as I can without smoke coming out the exhaust. Not surprisingly, it is just a few drops per minute over what Fairbanks recommended. - Serf

• All replies are excellent. The only thing I would use instead of a regular hone is a bottle brush type of hone - you know, the one with the little balls. They break the glaze but do not change the size and never chatter. Also, they will follow the taper or out-of-roundness of the cylinder. - Wisc

• I still see no reason for the rings to seat so well and then have blow-by a short while later, other than the ring gap lining up. It seems to have compression when I turn it over backwards, and very little blow-by. - Mac

• I would suspect you are washing the oil off the piston/rings by feeding it too much fuel. Try feeding more oil and see if that helps. The engine on my saw rig will lose compression if I don't feed enough oil, but with enough oil feed it runs just fine. This engine doesn't just sit there and putt. It gets worked hard about 20 to 30 hours a year. - Ken

• Running at idle or very low rpm can cause rings to unsettle. It's possible that carbon is interfering also. Try running at the highest speed allowed for a while. Put at least some load on it, and vary it while running high rpm. After some of that, vary speeds up and down with a load and then back to highest rpm. - B.W.

SmokStak (www.smokstak.com) now has nearly 19,000 threads like this with over 85,000 posts on file. Over 1,200 users have registered with more joining in daily. SmokStak is an engine conversation bulletin board and is part of the Old Engine series of websites that started in 1995 as "Harry's Old Engine."

Harry Matthews is a retired electronic engineer and gas engine collector from Oswego, N.Y., now residing in Sarasota, Fla.