Engine bore size versus piston ring size etc.

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Jul 16, 2007
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The purpose of this thread is to answer some questions that have come up this past week.
A fellow contacted me asking for my help to get his hit and miss engine running. I have had conversations with this gentleman in the past and have given him some ideas that have helped. One being the valve seat cutter to cut the seats by hand and thereby simplifying that process.
His current problem is a lack of compression with a 2.00 bore engine. He told me the valves seal great but when he turns the engine over there is no noticeable compression. The cylinder was bored and then finished to size with a Sunnen hone and then measured with a bore gauge at several different points in the bore. The bore is 2.00 and basically perfect. The piston is iron and has .002 clearance and is machined for 2 piston rings.
As we talked to try and solve what seems like a mystery he mentioned that he had purchased 6 piston rings from the same supplier, all for a 2.00 bore. He said that the end gaps ranged from .012 to .022 and hadn't been filed. I told him that with everything concentric and a light coat of oil the engine should have compression even without the rings. I told him to do another check on the rings. Put them in the bore, square them up and shine a light from the back side to make sure that they were concentrically made. He did and they were.
Now it's
getting more mysterious!
A day later he called and said that when he did the first compression check the bore was dry so this time he oiled the cylinder before turning the engine over. He said that the engine would now bump up against the compression and kick the crank backward.
Now were getting somewhere!
He then asked my opinion on the ring gaps. I told him that even the small gap was way too large and that he should probably see about getting some rings from another supplier. I gave him a name and he contacted them. In the course of the conversation with this suppler he mentioned the problem he was having with the previously purchased piston rings, (the end gaps).
This supplier then asked what he bored his cylinder to. His response was 2.00, exactly what the print called for. The ring man then said, "that's where your problem is!" "You should have bored it .005 undersize and then the ring gaps would be right.
Now hold on to your horses!!!!
I am fast approaching 67 years. I got started with internal combustion engines like many fellows did, playing with old Briggs and Stratton engines and trying to build home-made go carts and mini bikes. From there I moved up to real engines and did many an overhaul with the fitting of new pistons and rings.
Upon my discharge from the Army I got into motorcycles, both the street and dirt variety. I can't tell you how many times I had my dirt bike cylinders bored and fitted new pistons and rings
I'm here to tell you that when a bore dimension is called for whether it be in an American car engine or an English or Japanese motorcycle they don't tell you to bore it .005 undersize so that the ring end gap will fit.
I don't mean to step on any toes here that is why I have left everything anonymous but having heard these remarks I just had to present this story.
Since joining this board and several others I have had many phone calls and requests for help. I will spend hours trying to help out fellow modelers and builders up to the limit of my knowledge. Beyond that if I don't know something or have never had experience with a particular situation I just say so.
George D. Britnell


John Rudd

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Oct 14, 2009
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I'm 53, have spent many years with cars and bike engines....
I don't ever recall a bore being undersize to accomodate rings....However, I do recall checking ring gaps in bores to ensure the rings have the correct gap..I'm sure there's a rule of thumb....4 thou per inch of bore?


Ex Bogstandard
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Dec 26, 2008
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Crewe, Cheshire, UK

You should have bored it .005 undersize and then the ring gaps would be right
That is the biggest load of twaddle and bullsh I've ever heard.

I have been through the same sorts of learning processes and stages as yourself, and you are quite correct of course.

It seems like his suppliers reply is trying to get around the fact he has had the rings made incorrectly, and if he said it to someone who didn't know any better, could well mean the customer would go away and make a new cylinder, or even buy new castings, just because of his mistake.

People like that need to be brought to the attention of all, just so that he can be avoided, as he surely doesn't know anything about the products he is selling.



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Aug 30, 2010
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Never heard of it either. How can you make a bore to suit a ring? That's like going for new tyres and being told you should have bought a car with bigger wheels.


Aug 26, 2009
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I have bored and fitted many engines for friends while working in the shop and not once have I ever ran across such a thing just as you say if it calls for a 2.0 inch bore that is what you go to you do not hold the bore U/S what a bunch of bull. I started this trade 38 years ago and before that I worked as a diesel mechanic for awhile until I decided that was not for me. And this is the first time I have ever heard this.

Dan Rowe

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Feb 12, 2010
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I have never built a model IC engine however my avatar photo is my younger self in the bore of a Sulzer RLB 90. I have a 4" angle grinder in my hands and I am about to grind the ridge at the top of the cylinder liner so we can pull the piston.

If the bore was intended to be 1.995 ID then the print should have indicated that. We all know that the cylinder liner will wear which is why I have a grinder in my hands. This is why rings are made to fit the cylinder not the other way around.



Well-Known Member
Sep 12, 2008
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Adjust the bore to fit the rings. ??? ??? ??? ??? ??? ??? ???

What a load of hot snapping effervescent duck doo.

Best Regards


Apr 9, 2011
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An Experience I had years ago

When I was rebuilding a J.A.P. 500 cc speedway racing engine I was caught by famous English rider Ken McKinlay checking rings in the bore, then reaching for an ignition point file to file the gap a little. "What are you doing"? So I told him I was aiming at .003" gap. "Is the bore good?" "Yes" I replied "just back from a machinist" who did most if the re-bores in Adelaide for racing motorcycles.

"Give it here" Ken said and he went through the rings I had and eventually came across one that was really tight. "use that one" . "But, but there is no gap". "First time you fire it up it will have a gap". " You cannot win races with ring blow by " was his statement. Much to my relief when I ran the motor that night, it didn't seize up.

I no longer build racing engines and haven't dome so for nearly 40 years , but
if the bore is good and round there is no need for ring gaps. But I still liked to have a couple of though just in case it is not as round I had hoped.


Dave G

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Jan 15, 2011
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Normal practice in full size racing engines is to order your rings .005" oversize so the gaps can be filed to size. For an example the rings for a 4.000" bore would be ordered as 4.005". The placement of the rings in relation to the dome is to be considered also. Todays hypereutectic pistons which are stronger than cast pistons can be run with tighter clearances than a forged piston to control noise and the rings are placed closer to the dome. For this reason the ring gaps are larger than generally used. The ring running closer to the dome will run hotter so it will grow more needing more gap. Some gaps on automotive pistons rings are now .028-.030" on the top ring. I have never heard of anyone sizing a bore to fit a ring. Dave

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