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E: Engine Piston ring gap



chrislaun

Well Known and Active Website User
VOC Member
#1
I have a small query here, has anyone had trouble with the ring gap on their club piston rings.

I've recently got a +20 piston and had a barrel bored and unfortunately it was done slightly oversize, it has 7 thou clearance, when I tried the rings there was a 30 thou gap, which I thought was excessive and as the club was out of stock of + 20 rings contacted Omega who sent me another set, exactly the same.

Now my maths suggests that these rings have a 10 thou gap at ZERO running clearance so if you introduce say a 3 thou running clearance you will already have a 20 thou piston ring gap. Does this seem right.

By the way the person I spoke to and explained my predicament said that with running a 7 thou clearance I ran a chance of there being so much piston slap it could shatter the skirt and I ought to have it bored to +40.

This piston by the way is not running in a Vincent but a JAP V-twin with an iron barrel, about 4500 rpm max and total loss 50w oil so I don't expect that much piston slap and really don't want to bore it again.
 

timetraveller

Well Known and Active Website User
VOC Member
#2
I agree with your sums Chris. Circumference is Pi x Diameter. If the diameter goes up by, say, three thou then the circumference is up by 9.4 thou If that three thou is all the way round then diameter is up by six thou and the circumference is up by 18.8 thou.
 

Bill Thomas

Well Known and Active Website User
VOC Member
#4
Racing clearance on " The Piston " with a Vin' Was 9 or 10 thou, I never broke any rings, And I ran 12 to one on petrol, I trimmed the valve cutout on the outside so there was only a Nats of piston left to the top ring, Thought it might be bad, But never had trouble. Cheers Bill.
P.S. I was always told , Ring gap, 3 to 4 thou per inch of bore.
 

chrislaun

Well Known and Active Website User
VOC Member
#5
Yep I was told the same, but the rings supplied by Omega were too big a gap even without the oversize bore, anyway I've stuck it together and I'll see what it's like come spring.
I tried some +40 rings with the intention of filing them down to give the right gap but I felt they needed too much pressure to get them in the +20 bore.
 
Last edited:

Bill Thomas

Well Known and Active Website User
VOC Member
#6
Strange, A while ago I read that Pistons with rings, Don't know what make, are being sold with too little gap.
And that we should check them !.
Yours will be fine, Cheers Bill.
 


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