Is 0.002" too tight for Supertech Pistons
 

Is .002" / 0.05mm too tight for Supertech forged pistons? The Supertech instruction sheet says 0.00276" / 0.07mm MINIMUM clearance (no max clearance listed), but I am measuring a consistent 0.002" on all my piston to cylinder bore clearance. I gave my machine shop my 83.5mm Supertech pistons with instructions to bore and hone. I am wondering if they bored it to the specs for a cast factory piston ( 0.0013 - 0.0023") . This is for a ~300whp build with occasional track use.

Should I just take the block and pistons back and have them bore/hone to increase clearance to a minimum of 0.00276"? What is the max clearance limit for a Supertech forged piston? The Supertech paperwork did not provide an upper limit. If I have them open the bores up, what should I have the gap increased to?

I’m doing a new engine with the ST 8.7 pistons and I’m going to use .003-.0035”. I personally would not run less then .003”.

+1 and +1
I'd worry about less than .0035
.0035 to .005 would be my range.
A little too loose and you might have minor piston slap during warm up. It might be a little noisy...
A little too tight and you won't have to worry about piston slap as it will destroy the pistons the first time you run it (especially under boost).
Forged pistons expand with temperature a significant amount. When you are on boost the crown temps go up significantly over NA use.
Turbos COOK pistons. Ceramic coating help combat this. Getting rid of the squirters make this worse.
I researched how much is too much sometime back and it appears that the drag race crowd will use up to .010 without ill effects. That's 10 thousands clearance!
Too much in my book but 4 or 5 should work just fine.
I'd be very leery of a machine shop that bored for forged at .002. They wouldn't get my work anymore.
I'd have also requested a specific clearance BEFORE they started the job...

I am taking the block back tomorrow to have them confirm my measurements. I will have them open it up to 0.0035" clearance. While the Wiseco alloy seems to like even more clearance, it looks like the general consensus is that 0.003-0.004 is all that is needed for the Supertech 4032 alloy. That should meet my 300whp 98% street and 25 track needs.

Are you using a dial bore gauge? If you break the entire gauge down, calibrate it and measure, can you repeat the measurement 3-5x times and get the exact same answer?

Measuring with a dial bore gauge takes practice. Do not use a snap bore gauge.

I have a telescoping bore gauge which I guess is the same as a snap bore gauge. I was getting very repeatable measurements.

I took the block in and he measured it with a dial bore gauge. He measured 0.00280" to 0.00281" which met the Supertech minimum clearance measurement that I provided with the pistons. When I dropped it all off I did not realize that additional clearance would be needed. Why don't Supertech list it on their spec sheet like they do for the different ring gaps based on severity of use?

In any case, he will be opening the clearance up to 0.0035". I don't think that Supertech's 4032 alloy needs much more clearance for my intended use.

Any track use runs the risk of ceasing a piston when you are running boost. But .0035" is very good. I'm running .0035-.0038" and I have pushed 27psi of boost.

snap bore gauges just don't work because you are doubling up on the amount of measurements. gauge to bore and then you have to measure the gauge itself. 2 sources of error.

Next you need to decide on ring gaps!

Should I not use the gapping guidance provided with the rings???
Wisco 1st ring gap 0.0165" second ring gap 0.018". Or 3.285" x 0.005/0.0055 respectively.

are you going to file the rings yourself? I'm going to do a similar 300whp goal build and was curious about the rings. I have read that they never come out of the box at the right gap.

That .005/.0055 sounds pretty good to me. I went slightly larger at .0055/.006 and it is a little too much blowby (gas smelling).

I'm at .019" for the first ring, .023 (second) and .017 (oil rings).

.0165"&.018" is perfect.

I was planning on hand filing the rings for exact fit. My brother has a ring filer that I might use.
My ring gaps shown above are from the Wiseco guidance that came with my rings.
https://cimg6.ibsrv.net/gimg/www.mia...d9d6efc810.jpg

I thought you had checked them and they were .0165 & .018". Most people struggle filing rings. The ends end up not parallel and the gaps are not as even as they would like. They end up big because they were chasing them. Check the rings and see where they are. They might have too much clearance out of the box.

Only time I have seen gaps too big is if the bore is too big. I have started gapping these yet. I was planning on doing it last weekend, but found the piston to bore clearance to be too tight so I didn't even open the rings.

Just did a test fit and the top rings are all right at 0.0165" to 0.017" without touching them. The second rings are all around 0.023", should be around 0.018"!!! That's the spec Wiseco lists for Blown Race motors (0.070 x3.2850"). Even for the Supertech specs included with the pistons 0.023" is what is recommended for a Drag/Oval Track motor (0.055 x 1.25 x 3.2850").

Do I need to order oversized rings or just live with the second ring gap of 0.023"?

I don’t believe oversized rings are available. The second ring gap is not as important as the 1st.

Maybe I am wrong but I think you should use the piston manufacturer's specs, not the ring manufacturer's specs.

ring gaps
 

I ended up running the top rings at 0.018" and left the second rings at 0.022 to 23". I contacted several vendors with no luck and no willingness to provide any input other than what Wiseco lists, but finally got to Keenan Engineering who gave me some feeback that I should be OK with the larger 2nd ring gap. While Keenan did not give me specific ring gaps due to liability concerns, I do appreciate that they at least gave me some valuable input!

Hopefully this will work well on track and not result in too much blowby. Now I will have to make sure I have a good catchcan and valve cover baffeling setup in case I see a little more blowby.

Blow by would be caused by the upper ring and .018" is not bad. The second ring which uses a scraper profile is more for oil control. Everyone is running the about the same clearances because there is only a few ring suppliers. Most of the rings I believe are mfg'ed by NPR.