I've never used any SRP pistons before this. My experience is with either TRW/Speed-Pros or a high expansion type like a 2618 alloy both of which seem to like looser clearances in the .006-.008 range.
The recommended clearance(.0015-.0025)for these SRP's seems rather tight for a forged piston. What clearance are you guys using for a street/strip motor that occasionally will see moderate spray?
How is it for oil consumption?
I would rather be a little too loose than too tight.
These are P/N 142986 to be used in my .030 over 454.
Thanks in advance for your input.

Randy

My TRW's recommended the same tight range of clearances. The local machinist stated that I could run clearances in the upper end of that range if I was confident I would never overheat or detonate the engine. He defined overheating that would begin to cause problems at 210°F. I ran 0.004", same as every other forged piston in small blocks I've assembled, and it works just fine. For your big block, 0.0045"-0.005" should be just fine.

Yep, they do spec a tight clearance. But the expansion rate is much less. I've used SRP's for years, excellent pistons. They are much lighter than TRW's, make sure you have your engine balanced to make up for it. Lighter reciprocating assembly means higher RPM capability too...

I broke a KB in my motor last summer. The block was already +.060 so I did a light hone and installed +.060 SRP's. Now the engine uses NO oil and flat out runs - no piston slap either.

I actually think the SRPs seal better now with the 1/16" rings than they did with the KBs with 5/64" rings. I was very impressed with the SRPs and plan to put them in pretty much everything thing I build from here on out.

So, Eric, exactly how much clearance did you finally end up with? It's like I said earlier, it's better to be a little too loose than a little too tight! I'm just looking for the amount of p/w clearance folks are running with these pistons and whether or not it yielded good results.
Thanks for the replies and keep 'em coming!

Randy

I think SRP wanted a 4.060" bore and if I recall I had like 4.063-4.065". Was a little looser than recommended, but what can you do . . .

Shoot, there are some high mileage engines out there that run fine with standard pistons and bores that are worn so bad +.030" won't clean the bores up. Not exactly ideal in a performance engine, but they can go pretty loose before you have problems.

I would stick to SRP's recommendation. If they say 4.030 go with 4.030. Directions will come with your pistons or it might even say in the catalog.

I thoroughly read the directions that came with the pistons several times. Then I miked the pistons at the exact place they specify (1/2" above the end of the skirt and perpendicular to the pin). The finished dimensions of the pistons at room temp(85*)would only allow .0015-.002 at the 4.280 finished bore size that they specify.
I carefully torqued the heads onto the motor with the head gaskets I plan to use and inserted my dial bore gauges from the bottom of the cylinder in case it had something to do with the torque plate hone.
The clearance I came up with in ANY hole was never greater than .002. I guess I'm old fashioned, but I still feel that's too tight to run a forged 4032 piston.
So, i guess I need to drag this puppy back to the machine shop and have 'em hone some more out of the cyls. I hope some others post their clearances and results.

Tandy

The clearance is built in the piston. The cylinder should be honed to the exact oversize (.030" or whatever). SRP is a quality company and I wouldn't expect to find any variation. It is alway a good idea to mic the pistons to be sure. My .030 over 350 Wiseco's mic-ed (sp?) at 4.026" for .004" clearance.

Tandy,
Piston technology is changing very rapidly. Before you "ruin" your block by honing it too much, what not call JE and tell them your concerns ??? I bet that they will give you a good explaination as to why you should keep the tight clearances. The design of the piston is jsut as important as the material when determining expansion rates. A tighter piston running clearance will result in a lot less piston rocking, much better ring life etc.
As far as overheating goes, if you ever measured how much the bores grow just during the honing operation, you would wonder if that is really a true statement. Now excess combustion chamber temperature, such as from lean running, detonation etc can cause the piston to grow and possibly be a problem. On Marine engines you have to run more clearance because they run too cool !
Anyway, I put more trust than most in the manufacturers who spend thousands of dollars perfecting designs so we wont have problems with them. That policy has treated me very well.
Just my opinion,

Bill:
I called JE/SRP and basically was told that the clearance sheet is just a guideling and that each application will require an appropriate clearance. They started by telling me .0015-.0025 then said that if I was going to to race this motor, I "may" want to add "some" clearance. Then he said that if I'm going to spray it, I "may" want to "add some more" clearance. I never could get them to commit to an EXACT number.But till I got done adding up all the mays and somemores, we were up to .004-.006.

Pat:
If these had miked at .004 clearance, I'd probably be not having this thread but they ended up at LESS that .0022 when the block was finished to 4.280.

Randy