Piston ring gaps
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  1. #1
    Official RnS Addict zephyr's Avatar
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    Piston ring gaps

    Bought some Summit piston rings for an engine I'm building and checked the gaps. Top ring has .020" and the second ring has .016" out of the box.

    With a 4" bore I'd have thought it would be the other way around: .016" on the top (.004" per inch of bore) and .020" for the second ring (1.25 x the top ring gap).

    I definitely haven't got the rings mixed up as the upper ring has a moly face and the second ring is plane iron. The details on the Summit website, don't help much beyond this, but they do say "File fit: No", which I guess means you don't have to file them to get a fit.

    Anyone know what's going on here?
    The Butty Run - Warwickshire breakfast meet, 1st Sunday of every month
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  3. #2
    JC.
    V8 addict JC.'s Avatar
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    How are you measuring the gap?

    Best way is to index them to the bore / piston they are going to be used on.
    Pop them into the cylinder and use a piston to push them down into the honed area so they are nice and square to the cylinder wall. You can then use a feeler guage to measure the ring gap.
    Remove the ring, adjust the gap with a file, blast off any swarf with compressed air and then reinsert the ring into the bore in the same way you did originally and check the gap again. Repeat until you've set the required clearance depending on your application e.g N/A, nitrous use etc...
    -JC.

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    Official RnS Addict BlueGenePop's Avatar
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    There should be some paper work in the box detailing the clearances for various applications, normaly aspirated, supercharged, nitrous etc. This is normaly a factor that you multiply by the bore size to come up with the required clearance. The best tool I have found for this is the ring grinding tools that fit int a vice and a hand crank to operate them.
    Please whatever you do do not fit the rings with insufficent clearance, the ends will but, probably breaking the rings, certainly scoring the bores very badly, and you will be needing a rebore and pistons as a minimum!

    Never fit rings without checking clearances, and while I get a lot of parts from Summit I find the website can have errors as manufactureers change things and they don't get updated etc.

    But why would you want to put "non file to fit" rings in anyway, after all if they fit a new engine they will have big gaps on a worn but serviceable engine

    I have just had a look around but I cannot find the details from my last engine refresh, but I have a recollection of 23 thou for heavy nitrous use on our 4.125" bores on "gapless top rings"
    Last edited by BlueGenePop; 19-09-2011 at 15:31.
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  6. #4
    Official RnS Addict zephyr's Avatar
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    I'm pushing each ring a couple of inches down a bore with a piston and using a feeler gauge. I'm not running NOS or any form of forced injection. The engine is newly rebored and the rings are correctly sized for the rebored cylinders.

    I'm actually assembling two engines at present so I had a look at gaps on the other engine (also rebored). Both sets are specific to the rebored size, pre-gapped and neither set of rings measure up to the old formula of .004" per inch of bore. The gaps on the other engine are much closer in size .018" (top) and .017" (2nd), but still the 'wrong' way around gapwise from what I've read. Neither set came with any paperwork beyond identification of which ring is which and which way up they go.

    Having re-read the relevent sections in respective rebuild manuals, both sets are within tolerance. Only the Chevy rebuild implies that the second ring could have a bigger gap. And I've now spoken to a professional engine builder and he reckons it's just fine the way it is.

    I guess the moral of the story is don't believe everything you read on the internet.
    The Butty Run - Warwickshire breakfast meet, 1st Sunday of every month
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