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Low Compression

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Old 04-26-2003, 07:04:40 PM
Larry Kastens
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Default Low Compression

Today, I replaced the piston rings on a 1 1/2 hp John Deere E. It had good compression before, but it was using oil. It always started on the 2nd pull. Now, I can only get it started with an auxillary motor. Is this because the rings aren't seated or are there other causes? I ran the engine for over an hour but it still has low compression. I didn't change any other settings.
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Old 04-26-2003, 08:50:45 PM
Mark T
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Default Re: Low Compression


Sometimes the bore gets worn out of round. This happens more often with horizontal cylinders like your John Deere.

Your old rings probably wore into the egg shape of the cylinder and quit rotating.

Your new rings will also need to wear into the egg shape to match your cylinder. Sometimes, if they won't seat, you may have to pin them to keep from rotating around the piston.

But first, I would put the engine under a hard steady load. Seldom will rings seat when an engine is running freely, no matter how much you run it.

This will probably cause some argument, but I would not use an abrasive such as "Bon Ami" to seat the rings.

A lot of people do this, but throwing an abrasive material into a perfectly good engine is just asking for trouble, in my opinion.

Again, put a heavy load on it for quite a while (hours) and you should be happy with the results.

Take care and good luck.

-Mark Thompson
Old 04-26-2003, 11:03:22 PM
Posts: n/a
Default Re: Low Compression

Don't rule out the possibility that the piston ring gaps my have lined themselves up with one another during installation... not probable but possible. Too I got to agree with Mark about adding an abrasive. I replaced some rings not long ago and the engine smoked pretty bad even with the new rings. I let it run & run & run... slow but sure they finally seated and no more smoke. Compression is at least an improvement and given more time should restore. To buy some time running them in, try running with a load and at the same time run the engine on the hot side to speed up the seating process.
Old 04-27-2003, 07:33:30 AM
Elden DuRand
Posts: n/a
Default Re: Low Compression

I agree about not using any kind of abrasive to seat rings. It will get into other parts of the engine and cause a lot of unnecessary wear.

Did you hone the cylinder (or carefully sandpaper it) to remove the glaze before putting it back together? If you don't do this, it will always take longer to seat new rings.

Running the engine at operating temperature (water boiling) and at varying loads will speed the process and sometimes is the only way to break new rings in with a not-so-perfect bore.

Take care - Elden
Old 04-27-2003, 10:29:58 AM
Al Hettich
Posts: n/a
Default Re: Low Compression

I had the same thing on a FM-ZA a while back. It was the ring lands (grooves) were worn to a taper. As the rings old wore thinner they were tapered and wore the piston to a taper also. The new rings only were seating on a small area near the bottom of the groove. I machined some of those .030 spacers into the piston, and it healed up after only several hours running. It is getting better all the time. At this point I believe the side clearance is just as important as the end gap. Al
Old 04-27-2003, 05:06:58 PM
Larry Kastens
Posts: n/a
Default Re: Low Compression

Thanks to all who have given me their suggestions. I honed the cylinder and placed the rings equidistant. I have a good notion to replace the rings with the old ones, ha.

On a side note, in this particular cylinder, the "liner" is a smaller diameter than the front end of the cylinder. The "liner" bevels down to the diameter of the cylinder on the front end. Was the peculiar to John Deere E's, or do I possibly have a replacement sleeve?

Thanks, Harry, for this site. It is the best there is.
Old 04-27-2003, 05:57:55 PM
Mark T
Posts: n/a
Default Re: Low Compression

I agree!

Dave Reed from Otto's Gas Engine Works (site sponsor) told me the ring to piston side clearance is the most important part of fitting a ring.

If the ring groove in the piston is worn like you said "on a taper" the rings constantly rock back and forth. This causes the rings to be radiused on the edge against the cylinder wall. This created a very tiny bearing area. They will never seal.

If I remember correctly, I think optimum side clearance is .003" of an inch. I could be off a thousanths or two, but I am close.

Most of the time, the rings have .003-.004" clearance in the bottom of the piston ring groove, but taper out to .007" or more at the top.

The piston is nothing more than a carrier for the rings. The better a piston can do that, the better the compression.

I don't think this engine has such a problem, if it had good compression to begin with, but stranger things have happened.

However, if this is a problem , Dave Reed can make wider rings for you. Not cheap, but his experience and help on the phone far outweighs the additional cost for custom made rings.

As a side note, Dave Reed sells standard rings also. He is a great "go to guy" for standard or specials.

He helped me before I bought rings for a Rumely. After some talking and measurements, it was decided to make some wider rings.

If I were to have just ordered rings from someone that didn't care, I would have wasted my money on a set of rings that wouldn't do the job.

Maybe this sounds like an adverstisement for Otto's Gas Engine Works. All I can say is that I have been very satisfied and my dad has bought rings from Dave for years.

The point I am making, is that sometimes before we order parts, it is a good idea to talk to someone with experience and the patience to help work through a probelm.

Most of the sponsors of this site run into more situations by accident in a day than most of us run into in a lifetime.

Nuff' said. Hopefully the rings sealed by loading the engine.

-Mark Thompson
Old 04-27-2003, 10:59:22 PM
Al Hettich
Posts: n/a
Default Re: Low Compression

Mark, I could have written the same note you did as I have exactly the same experience with Dave. I needed the .030 spacers because of excessive wear both top and bottom, and yep I was in a big hurry. Al
Old 04-29-2003, 11:15:36 AM
Posts: n/a
Default Re: Low Compression

You may want to check my piston ring fitting tips page below. Get your feeler gauges out and check ring side clearance. 0.005" and less is good. 0.006" and more means machine work is necessary for good results. If you pin the rings in the groove you can get away with about twice the groove wear as normally and maybe use the old rings if end gap is not excessive. Another thing to try with used rings is to lightly sandpaper the rings AROUND on the outside to put light scratches perpindicular to the direction of ring travel. This will help an old ring seat in better. Use 400 or at least a fine grit paper. Depending on your situation it may be as easy to fix it right. Good luck with the JD and thanks guys for the kind words. Dave Reed

Piston ring fitting tips page


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