[S/st 54]

What is a ideal piston to wall clearance on a N/A motor?

Even with tape on the piston during mock up these babies seemed a bit loose.......


Not gona say what I measure but I'm curious what you guys think is ideal.

[mike leech]

.006-.008

[Tom Vaught]

It depends somewhat on what the piston material is.

A Piston will always grow until it reaches its Thermal Stabilization Size (under load).

The old California Pistons grew so much that sometimes you would have .009" piston to wall clearance when the piston was at room temperature. If you made it tighter the piston would scuff or seize.

Some of the newer piston design people say you can run the things at .005" clearance.

I have always been nervous of that deal and always had at least .006"-.0065" minimum. A engine like Mike's might need .008" to not scuff the pistons.

JME

Tom V.

[S/st 54]

Originally Posted by mike leech

.006-.008

LOL that's what I thought! I'll give someone a screamin screamin deal on a set of gas ported cv-1 Ross pistons! That have a. .043 .043 3mm ring pack. Set up for a 4.5 stroke and a 6.700 rod.these babies are brand spankin new




Actually anyone know how Ross is to deal with? Who's a good contact over there?

[cgeise]

It also depends on skirt design -- but a lot of the newer stuff is getting tighter -- the long stroke really causes piston rock with loose pistons upsetting ring seal -- the worst problem I ever had was with a set of Ross pistons, just saying.

[mike leech]

Running Ross Pistons right now. You have to measure them at the skirt across the pin. check the piston to find out exactly where they're supposed to be measured. If you measure at the very top of the piston they will measure way more than what you actually have.

[Tom Vaught]

No issues with the local guys "Diamond Pistons" or the California Guys "CP Pistons" on either work or home projects.

Tom V.

[estep455]

Originally Posted by cgeise

It also depends on skirt design -- but a lot of the newer stuff is getting tighter -- the long stroke really causes piston rock with loose pistons upsetting ring seal -- the worst problem I ever had was with a set of Ross pistons, just saying.

x2.

[PONTIAC DUDE]

Originally Posted by cgeise

It also depends on skirt design -- but a lot of the newer stuff is getting tighter -- the long stroke really causes piston rock with loose pistons upsetting ring seal -- the worst problem I ever had was with a set of Ross pistons, just saying.

X 3

[David Holmberg]

Originally Posted by cgeise

It also depends on skirt design -- but a lot of the newer stuff is getting tighter -- the long stroke really causes piston rock with loose pistons upsetting ring seal -- the worst problem I ever had was with a set of Ross pistons, just saying.

Yup, you should hear the original LS1's on a cold start.

[mike leech]

That's why I prefer a shorter rod than the pontiac community thinks is ideal. I'd rather have a stable piston and lower the pin a touch. Helps with rock and is like coatings when it comes to power. No differance

[dda72]

Originally Posted by mike leech

That's why I prefer a shorter rod than the pontiac community thinks is ideal. I'd rather have a stable piston and lower the pin a touch. Helps with rock and is like coatings when it comes to power. No differance

X2. Again Mike is correct.

[Scott Roberts]

Did the machinist not have pistons in hand before boring the block? Any competent m achinest wants the pistons before taking the block to final hone...

[mike leech]

Jason how are you measuring the piston please describe.

[leeklm]

Sorry to hijack the thread, but this topic is timely... My new forged SRP piston instructions call for a minimum of .0025/.0030 wall clearance for a 4.2" bore (see attached). My local engine guy made the bore 4.185, which gives us about .0035 clearance.

Based on the instructions, i assume this will be fine for an aluminum head 455 street engine?

[screamingchief]

Originally Posted by leeklm

Sorry to hijack the thread, but this topic is timely... My new forged SRP piston instructions call for a minimum of .0025/.0030 wall clearance for a 4.2" bore (see attached). My local engine guy made the bore 4.185, which gives us about .0035 clearance.

Based on the instructions, i assume this will be fine for an aluminum head 455 street engine?

Originally Posted by Tom Vaught

It depends somewhat on what the piston material is.

SRP pistons are 4032 vs. most dedicated "race" forgings being 2618.

The two materials have differing expansion rates thus different P2W specs.

The pistons these guys are talking about are all 2618 I'm sure.

And usage also comes into play,high CR,boost,nitrous all can require more P2W as SOP due to the additional heat the pistons "sees".

My first thought when I saw the "What is a ideal piston to wall clearance on a N/A motor?" was .005" to .007" as a good "middle of the road" number for the bulk of applications,but then again I prefer the 2618 pistons as SOP as well,but yeah the 4032 pistons (and whatever material TRW/FM forgings are made of) can indeed run closer than that in many applications as well,they just are'nt as strong as a 2618 piston would be.

HTH

Bret P.

[Half-Inch Stud]

My understanding:

Cold: clearance-spec tight Skirts, loose&oval slug heads.

Warmed-up: slip-tight skirts, expanded heads that are closer to round & not scuffing.

[S/st 54]

Ok measured them across the skirts..... Much better but still loose. That's with a cold block and a warmer piston. Thanks guys. It will be just like everything else on this motor. It will "probably"work but not what I wanted. Sure think I've spent way to much on this motor to not have exactly what I wanted.just my 2 cents. Thanks again guys for the help!

[Jeff Kinsler]

Originally Posted by S/st 54

Ok measured them across the skirts..... Much better but still loose. That's with a cold block and a warmer piston. Thanks guys. It will be just like everything else on this motor. It will "probably"work but not what I wanted. Sure think I've spent way to much on this motor to not have exactly what I wanted.just my 2 cents. Thanks again guys for the help!

it will be fine. run her hard!!!

[S/st 54]

Originally Posted by Jeff Kinsler

it will be fine. run her hard!!!

Yeah but I really am looking for a specific HP. # and I know things have to be right to achieve that number..........the thing is I have already spent a healthy amount to try to achieve that # which shows I'm serious about obtaining it. Not the typical "I'm gona" that so many have said and NEVER FOLLOWED through with and do not even have a starting point let alone any kind of track results to get there feet wet...........just sayin.........I just don't want to wait 6wks to get a. "Correct " set of pistons.