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What to do about blow-by?
Old 12-22-2004 03:35 PM
Smokstak Smokstak is offline
 
Views: 34,861
On my Jaeger, the cylinder was honed and the piston was put back in with the old rings (6 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 get for an engine, unless of course, you're working on something with a huge bore. 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 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. Some engines are funny when it comes to blow-by. I have a Fairbanks headless "Z" that has no blow-by sometimes and other times you'd wonder if it had rings in it at all! I think there is a multitude of things going on in my engine, such as, worn piston, worn ring grooves, the cylinder probably has taper and is out of round. I did just recently renew the wrist pin and it seems to have helped it a lot by possibly keeping the piston a little more true in the cylinder instead of letting do its own thing due to the sloppy wrist pin and bushing. These are my experiences so take them for what they're worth. -- Mike

There might be a lot of end gap on the rings and once and a while the end gap on all the rings get lined up with one another and the compression just blows right through the end gap. -- Chuck

The above post is probably spot on. Worn rings in a honed cylinder wear even more and faster and the end gaps are probably aligning now and then. Is oil or air blowing back in to the oiler? Is oil getting to the piston - 5 to 6 drops a minute? Is the piston wet with oil when the engine is running, or does it appear to be dry? Is oil being blown off the rear of the piston? If you pull the engine over smartly, does it blow by? Or does it only blow-by at slow speed? So many questions! If the piston is wet, and the blow-by is not noticeable at running speeds, don't worry about it. If it is noticeable at running speed, and/or the piston is running dry, you will have to remedy it, and soon, if you want to save the piston and bore. Running dry and hot leads to extreme wear and fast! Check your piston to cylinder clearances - if the piston is too sloppy in the bore, the rings will not be able to seal correctly. -- Andrew

With six rings the gaps lining up seems a bit unlikely to me (my opinion not worth much, only had a H&M for a year now). 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 info 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 days time. By introducing this much wear you most likely wore the rings much past the point of working "acceptably". Had you not honed you could have probably gotten away with it. In any case there has been a past compression problem with your engine. Having 6 rings in it tells me that 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 1.5 thou of clearance due to using the doubled rings. With 2 per groove you should have 3 thou when new, and 6.5 thou is the max. Seven or more and you need machine work. The top groove always wears the most. You need a set of three rings 3 1/4" x 1/4" and probably three .030" 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. 95% 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. The two big problem areas today are: #1. The thing that is way overdone to engines is honing. If you want to save yourself a lot of headaches skip this step. If the cylinder is not rusty or is not a newly bored cylinder you are better off not to hone. #2. NOT checking the side clearance in the ring grooves. 5 thou is the max acceptable. Any more and compression problems are coming to your neighborhood shortly. Good luck in your quest for compression. Give me a call or email and I will be glad to help. Dave

If the cylinder/rings/piston is 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. Does the oiler have a Check Ball in it? 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 FMZ has a massive amount of compression, but at t.d.c. it has a bit of blow-by. My Novo is the same way. Granted, the FMZ only has about 2 hrs on it since honing and I put new rings in it. I think the rings were $5 each and the engine uses three. For the amount of trouble they can cause, it is really 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 that 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. I like using them! 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 can't see the rings traveling that much that fast. Also if the rings aren't pinned and you reused them you are fighting an uphill battle as they were worn to the cylinder and now they are probably in a different place. I would suspect you are washing the oil off the piston/rings either when starting or by feeding too much fuel. Try feeding more oil and see if that helps. The engine on my saw rig will lose compression (it has lots miles on it) if I don't feed enough oil but with enough oil feed it runs just fine. This engine doesn't just sit there and PUT. It gets worked hard about 20 to 30 hours a year. Yes, one of these days I will have to clean up the ring grooves and get some new rings. -- 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 a while of that, vary speeds up and down with load and then back to highest RPM. -- BW
Loose ring lands will let rings line up the end gaps in less than a hour of run time. Bob
I have never had the problem of the ring gaps lining up. I do know that the reason they pinned the early rings is because they were made much thicker on the side away from the gap and gravity would actually cause the heavy sides to end up on the bottom and cause the gaps to line up. But modern rings are no longer made that way. How many pinned rings have you ever seen in a 2 cylinder John Deere and they were certainly successful. I have removed lots of ring pins and have had no problems. Any compression problems I have ever had have been traceable to wear or lubrication issues. -- Ken

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