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Piston ring end gaps

Engine Building Fundamentals

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Hello there I’m curious to know what piston ring end gaps u use on a built Evo 4g63, top and bottom ring

Fuel, boost level, power adders?

I've been using JE and it's a weekend car and occasionally doing street pulls and drag events. I have been following the JE spec sheet where the bore x 0.0050 for the top ring and Bore x 0.0055" for the 2nd ring which yields to 0.017 for the first ring and 0.0019 for the 2nd ring. This is for a 85.5mm 2618 JE piston. 4032 would go tighter than that.

What i noticed is that since i'm running on ron 97 almost 100% of the time, as you go beyond 1.8 bar - 2bar boost, noticed the ring butts are just a little bit shiny meaning they have butted. Probably it's my tuning or ignition but it was tuned at around a lambda target of ~0.8. This was running on a GT3076 turbo.

I'm currently rebuilding this motor and maybe i would try adding around 0.005" to the rings and see how it goes.

Might be down toi interpretation, but 2 bar of boost is beyond what I would call moderate - using the 0.0055/inch they recommend for drag use may have been wiser. I would suggest going to 0.006/inch (increasing 2nd as required) might be smart as it will give a little more headroom for more boost/power adders and the slightly greater ring gaps will have little affect on power.

You were very lucky to dodge a total engine failure as if they'd butted up slightly harder there would have been no further expansion possible, forcing the rings into the cylinder wall(s), leading to immediately seizure as they overheated and were forced ever harder into the bores.

Piston material shouldn't have any significant affect on ring gap, just piston-bore clearance.

thanks for the reply Gord. Yeah, on the next build i'll be using the drag spec recommended by the piston manufacturer. The 0.006 you mentioned i believe it's mm just to clear it out for other readers. the recommended for drag spec is 0.0185" (0.0057mm) 0.006mm probably leave some safety margin i would say with the higher boost i'll be targetting. I'll might start to play around with e85 and see how it goes.

Oops, missed that in editing - I meant 0.006"/inch :-) As you imply, it's a ratio of 0.006:1, so can be used for any measurement used.

For an 85.5mm bore, that would be 0.513mm, or 20.2 thou' which would be a nominal 0.020".

If you have a spec' ring gap of 18.5 thou', I would suggest using a go-not go with a 19 and a 20 thou' feeler guage - if a 19 goes through, but a 20 doesn't, it's 'right'.

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