gooflophaze

Building an engine, 84mm bore, trying to Target .017 on the top and 0.18 on the second. Machine shop gapped the rings out at the .0065 multiplier putting them too large. Got some replacements from summit racing, and unground we were measuring .022 on the top ring. Got another set from 949, .015 on the top ring, 2nd ring is .024. Is this normal? Or should we send it without worrying about a couple thou over?

Should also mention I've checked the bore to the best of my ability, which is basically telescoping T's and vernier, so it appears to be on size.

LeoNA

You need to use a dial bore gage and micrometer on the piston. Can you use the .015" gap rings from 949 on the top and the tightest second ring on the bottom. .015-.017 on the top is fine and the second is not that critical, .016-.020. The top ring has a barrel face and the second is a scraper profile. You have to use the correct ring for each location.

gooflophaze

Yeah, went thru the box of new rings (from 949), all the 2nd rings are 0.025-0.027

LeoNA

What pistons are you using? This is a common issue when ring pack availability is low for a specific application. The ring gap will increase by .003” for every .001” increase in bore. So your issue is most likely an oversized bore.

gooflophaze

Supertech 9:1's - and oh yeah, that whole pi thing is coming back to me. Wish I could rent a dial bore gauge.

technicalninja

If your running forged 9:1 this should be a boosted engine and your intended rings gaps may be too tight.
I would run the tops at 20 -22 and the second a bit looser.
This is for 15 psi and lower.
I'd add a bit more gap for higher boost.
I'd also ask for 949's suggestion and probably give their suggestions more weight than anything I got from the internet...

gooflophaze

Yeah, I know to err on the side of looser spec - but https://www.miataturbo.net/engine-pe...-follow-81272/ appears to bless the targets.

Current motor is bone stock with probably 250k+ miles on it, probably 20k boosted. It's tired. Trying to have an engine that drinks less oil (I know no oil is unrealistic).

LeoNA

I would have the shop that bored the block double check the piston to wall clearance. There is nothing wrong with running .0005" over the recommended clearance but if its more then .001" then that is the reason your end gap is large.

I personally do not run large end gaps. The end gap is to prevent ring butting and has to do with the expansion rate of the rings material and the location which dictates its max operating temp . Traditional dished pistons have a fair amount of piston above the upper ring. The second just needs to net more clearance then the top to make sure there is a pressure delta. For these engines I would run .016"-.018" for both. The upper ring will always have a smaller gap because it runs much hotter. Both rings will wear in a little and the gap will get larger. There have been many tests to see how much it takes to butt the rings and its much more then is obvious. The ring manufactures don't take into account every engines design and use. If it is going to end up that you have .020 on the top and .024 on the second then run it. There most likely is nothing you can do other then move up in bore size and decrease piston wall which can cause other issues if you go to far.

gooflophaze

We've called a few local machine shops - Due to covid, many are backed up with 6 week lead times. And we're trying to get this built, swapped, broken in, and boost turned up and tuned by MATG - if that happens this year. Urge to send it is rising.