Track car piston to wall clearance

[Def]

What are you guys running for piston to wall clearance (and ring gap) for track motors? Mfg recommended clearances for a street/strip engine?

[CodyAce]

I ran what CP suggested, can't recall what it was off hand.

[Def]

Yea, I've got the spec sheet for my pistons, but I don't recall the piston to wall clearance changing for usage. I'd think a track engine would want a few ten-thou more than a street/strip engine?

What about ring gap?


You get lots of piston slap when cold?

[CodyAce]

There is a number on the CP tech sheet that I had, as I know on the engines I've used CP's with I just gave my machine shop the sheet and asked him to ensure it was in spec

Ring gap I ran on the tighter side of the 'turbo' measurement on that same tech sheet, as it's already looser than the street/NA measurement. I had always read you don't want to have it overly tight with a turbo engine to prevent the rings from butting up under boost and potentially chipping a piston (granted that was more prevelent with cast pistons, I figured it couldn't hurt, and the CP sheet seemed to also indicate that more gap wasn't a bad issue.

On mornings where it's sub 50ish* you can hear a smidge of piston sound, which is obviously amplified as it's in a garage and the sound bounces. However it goes away very quickly (a few seconds at best). On my roomates old car I actually had the sides coated with the Swain piston coating, and selected the option to have it thicker than normal which swain also suggested was common on forged piston engines (I figure they had much more experience than I, so I ran with it) His engine makes no noise at all, regardless of temperature. While you'd think this may be because the coating is 'too thick', we had to tear the engine apart when he lifted his head, and there were ZERO signs of scoring or slap after roughly 15k/20k of miles on it (and he beats the living **** out of his car, whereas i'm a grandpa)

[cutter]

If you are running cp pistons just do what the sheet says. I thinks its like .0035. I only had 2 track days on the engine but it saw 22lbs of boost and 400+ hp everyday. After I sold the enegine the next owner blew it up and I tore it down and the skirts and everything looked great. It had about 10,000 miles on it. I will see if he still has the pistons and take some pictures

[cutter]

My friend built his wrx with a few extra thousandths piston clearance and every time he started it, it sounded like the Macarena under his hood.

[Def]

Speaking of coatings, I was thinking of a ceramic coating on piston tops and wear coating on the skirts. Local shop I trust does them for $150, but I'm a bit wary of the potential for flaking and/or wondering if it's worth the money. I'm getting a bit tired of being $100'd to death with this engine build, so I think they're just going in as-is.

The other decision is the main bearing oil groove machining and running GTIR bearings for better rod oiling. But I'm having trouble finding specs for the groove width (I've seen 1/8" wide, but no specs on how far out it should extend and the depth). The shop has never done it before, and I trust them, but still... you know how first-goes sometimes turn out. Thoughts?

I've also decided to go against Calico coated bearings, they're a hair over $100 more for an SR set of bearings. I think I'm just going to run ACL Race bearings straight up (just having the machine shop verify clearances to make sure nothing is amiss, although lots of people skip that step... seems like a good place to spend ~$100-150ish).

[CodyAce]

for $150, but I'm a bit wary of the potential for flaking and/or wondering if it's worth the money. I'm getting a bit tired of being $100'd to death with this engine build, so I think they're just going in as-is.

I really don't see the need for the top thermal coating on a street car really. Then again, I also can't vouch for how 'well' it works against detonation and or pitting from it.

As far as the side coating, after seeing Matt's, and others, I'm gonna say I'm a fan of it for sure on a forged piston setup.


The other decision is the main bearing oil groove machining and running GTIR bearings for better rod oiling. But I'm having trouble finding specs for the groove width (I've seen 1/8" wide, but no specs on how far out it should extend and the depth). The shop has never done it before, and I trust them, but still... you know how first-goes sometimes turn out. Thoughts?

I think this is more of a 'can't hurt' idea, but then again there are countless others that do not have this modification, that have a ton of miles, that get abused, that don't have issues. Not saying that is reason to 'not' do it, but just something to consider. With that all said, I wouldn't trust any shop to do that, and would send it to someone who has done it before, and won't worry. I know JWT offers the service through whatever machine shop they use.


I've also decided to go against Calico coated bearings, they're a hair over $100 more for an SR set of bearings. I think I'm just going to run ACL Race bearings straight up (just having the machine shop verify clearances to make sure nothing is amiss, although lots of people skip that step... seems like a good place to spend ~$100-150ish).

I recently had a set of ACL's plastigauge right, and the crank was good, yet still knock. Unsure if I misread it or something else went amiss yet. With that said, every engine with OEM Nissan bearings has not had an issue. Gotta get the block back to see what I may have ****ed up/what did **** up.

[Def]

If I have to ship the block then I'm not doing the oil groove. Im positive the local shop can do the groove, but its annoying that no one lists specs for it. Another instance where the Nissan community is a little too hush hush for its own good.

JWT cuts them by hand with a cutoff wheel. That's not hard to replicate ..

[Epstein]

I bet you could rough out some specs just by looking at the bearing. Say, as wide as the feed hole and just as deep. IIRC, it's more about pressure than flow. Surely someone on the FWD side has measured the GTi-R grooves.

[gawdzilla]

i ran what the piston manufacturer (wiseco) recommended for the ring gap, and filed them accordingly. i did a 1mm overbore and bought pistons that were sized to that, so i guess i didnt really "measure" the piston to wall clearance besides verifying the bore was correct and true. no piston slap at all, unlike several built wrx/stis i've heard running CPs. maybe that's just the norm for those motors?

The wisecos I ran also came with a skirt coating which gave me a warm fuzzy feeling. left the tops shiny and untouched.

speaking of warm fuzzy feeling, i liked how oem/nismo bearings came with the oem tolerances (i think 6 sizes in total?) specifications, instead of other aftermarket bearing manufacturers that simply have over/oem/under (3) sizing. sure you can still get the tolerance in spec, but it's not AS precise as oem size offerings.

[Tower240sx]

Run the pistons with the standard .001 per inch of bore guideline, loosening it up beyond that just accelerates piston skirt and cylinder wall scuffing.

For any track motor you are better off on the loose side of ring gap tolerance, butting rings is very bad, very very very bad, and a track day car will reach thermal uniformity and expand the rings more than a drag or street motor.

as for the oil groove, even a 1/8 slot in the stock (for gtir) location will suffice, you are just trying to increase the time oil pressure is exposed, the flow rate is restricted by the bearing clearance alot more than a 1/8 slot could restrict it.

piston skirt coatings are nice but a good forged piston won't scuff much anyway, and remember how much heat leaves the piston through the skirt, do yo want to insulate that? only if you have enough ring gap...

Crown Coatings seem to have alot of potential but it's hard to get any real data on how much they drop the pistons temperature...

[cutter]

The wisecos I ran also came with a skirt coating which gave me a warm fuzzy feeling. left the tops shiny and untouched.



From what I have read on Endynes forum you actually want to scuff piston tops and knock down any sharp edge to reduce potential detonation. I have built 3 engines and used OEM bearings in 2 and the coated acl bearings in 1. The only thing the coated bearings are good for is if you loose oil pressure for a second (roll) they protect your engine. If you are running an accusump or a dry sump they are not really necessary. All of my pistons came with a coating on the skirt.

Where did you get your pistons?

[Def]

I have CP 8.5:1 pistons, no sharp edges on them to knock down, but I am having Endyn do the machining and measurements.

[Epstein]

i ran what the piston manufacturer (wiseco) recommended for the ring gap, and filed them accordingly. i did a 1mm overbore and bought pistons that were sized to that, so i guess i didnt really "measure" the piston to wall clearance besides verifying the bore was correct and true. no piston slap at all, unlike several built wrx/stis i've heard running CPs. maybe that's just the norm for those motors?

That's the norm for Wisecos. I ran them in my motor for this reason. They use different materials (something about high vs low silicon content and thermal expansion), tighter PTW clearances in the 0.0025" range, and have offset wrist pins (like stock). I would have chimed in earlier, but I never pushed that motor for extended periods. Just a lot of 40-140's.

[Def]

Wiescos were my second choice, but I got a much better deal on CPs and people seem to be pretty pleased with them, so that was my first choice. They are machined very nicely, that's for sure.

[cutter]

I have had bad luck with weisco. My first 2 engines were built with weisco and I actually had 1 piston skirt crack and the second set the ringlands got damaged with no signs of detonation. I once heard them called dirt bike pistons or 1 race pistons. CP for the win