More piston clearance questions

[CyFi6]

Hello all, i searched the section for all the questions on cylinder wall to piston clearance with forged pistons and got a basic idea of the situation. More power means more heat which in turn means more expansion of the piston. My situation is i am doing a rebuild with .040" Probe forged pistons and i am trying to figure out a couple things. First of all, how do i know what cylinder to piston clearance i need for my power goals? I want to make a max of around 350 RWHP, but will probably be running around 12-13PSI of boost on the stock setup for a long time. Ideally, i would like to build the engine for a little less horsepower and keep it reliable and avoid as much slap as possible.

The guidelines for the Probes are about .015"-.045" clearance. Can anyone guide me in the right direction for what clearance i would need? Obviously i dont want to do damage by having the clearance too small, but i would also like to avoid slap, as the engine is not going to be making a lot of horsepower and reliability is key.

Second, i received my Probe pistons with rings from Driftmotion. They came with NPR rings. My question is what are the materials in the NPR rings, and what surface finish should i ask my machinist to put on the cylinder walls? I know that chrome rings are harder that cast etc and need a different surface finish for perfect and quick seating. Thanks for the help.

 

[Adjuster]

All rings as far as I know are cast iron. They can be coated, and I'm sure there are various metals added to the material, but all rings except your oil control rings, are cast iron. (Again, that I know of.)

I'm a big promoter of a hard break in for a new motor. Ring seal is the key, and if you don't break in the rings in a few miles, they will take thousands and thousands of miles to break in.

Use whatever pattern, and cut the ring supplier reccomends, and then break in the motor like you stole it, and you will be fine.

 

[suprarich]

If you have a chrome top ring then you will want a finish of 35 to 40 degree cross hatch with a 220 grit conventional stone ( or 325 if a diamond stone). Start with normal load on the hone machine to .001 of final size. Then reduce the load to 50% untill .0002 of final size, then reduce to 20% load to final size. After honing, have the bores swept with a flexible plateau honing brush. this will get rid of the jagged peaks on the cross hatch.

For probe pistons I have been using a .0038 to .0040 piston to wall clearance. A general rule of thumb on the rings is bore inch size x .0065/.007 = min top ring gap. Bore inch size x .007/.008 = minumum 2nd ring gap. Oil ring I like to keep .016 to .017 with .015 being the min.

Hone cylinders in a stagered pattern to keep the heat down on bores next to the one your honing. Make sure the block is fully cooled during final bore checks because our block grows a good bit from the heat of honing.

 

[supraguru05]

along this same line of questions. i just had a block bored for some weisco pistons. the wiescos are coated and the box says a bore of 3.2874. with a suggested clearance of .0025. the block was bored to 3.2875 for some extra clearance and after hone he bore was measured at 3.2885. the pistons on a tight mic on the coating was at 3.286 which means the final clearance was right on 0.0025. I am worried this is to tight for 400 hp, however i think the coating will wear in slightly. any opinions

 

[suprarich]

Do not do any hard pulls with a cold motor. I would make sure the motor was fully warm prior to any kind of run. With a cool or cold motor, even a smaller or medium pull on the motor will grow those pistons much faster than the iron block will grow, thus closing your tight clearance even more for a short time.