piston clearance

[bmoss85]

i called arias today to find what my piston to cylinder wall clearance should be. they told me .004 up to 20lbs of boost, .0045 to .005 if more than that. im not really sure how they figure that. they also said that the wrist pins were only rated to 20lbs of boost. how do you rate a wrist pin to the amount of boost that is being run?

if someone could enlighten me a little that would be great. im not sure what clearance i should run now or if i need to upgrade the wrist pin.

thanks for any help.

[ilikebigbutts]

uhh sounds like complete shit...clearance is based on cylinder bore and expected horsepower.... .004" sounds about right though for forged pistons.

[Adjuster]

4 thousands is a good number. Are your pistons coated?

ROSS reccomends 6 thousands, but they slap like crazy when cold when set to that number. (And worse when coated, as they do not heat up as much, or as consistantly as bare pistons do.)

My coated ROSS ones are set to 3.5k right now. :) Still slap when cold, and not quite as much when warmed up completely... Should have gone for 3 thousands.... :)

[bmoss85]

no they are not coated, but im going to have them coated.

[wh0wants2know]

I've heard that for ROSS pistons, what about JE pistons? Would those also be good at .004 or should I go more/less? I just bought a set and I'm sending the engine out to the machine shop tomorrow.

[bigboost7m]

Not only does it depend on horsepower rating, it also depends on piston material. Are those made out of 2618 or 4032? 4032 has a lower silicone content (2%, avg.) and also has less heat expansion, 2618 has a higher silicone (12%, avg) content and requires a little more piston to wall clearence. The 2618's are used in higher hp engines (big boost, nitrous etc.) While 4032 pistons are more for street, road racing, etc, where piston to wall clearences are to be kept kinda tight.

4032- .0035 (coated skirt), .0040 (uncoated)
2618- .0040 (coated skirt), .0045 (uncoated)
These are my recomendations, soley based on personal experience.

Pin Strength, Does Arias provide a wall thickness for the pin, and what material they are made from? Tool steel is usually the standard for racing apps.

[PPorting]

we run .004 in all our motors, a lot of people run that clearence

[suprahero]

My CP pistons called for a .003 clearance. I don't know what material they are made out of though.

[IJ.]

Not only does it depend on horsepower rating, it also depends on piston material. Are those made out of 2618 or 4032? 4032 has a lower silicone content (2%, avg.) and also has less heat expansion, 2618 has a higher silicone (12%, avg) content and requires a little more piston to wall clearence. The 2618's are used in higher hp engines (big boost, nitrous etc.) While 4032 pistons are more for street, road racing, etc, where piston to wall clearences are to be kept kinda tight.

4032- .0035 (coated skirt), .0040 (uncoated)
2618- .0040 (coated skirt), .0045 (uncoated)
These are my recomendations, soley based on personal experience.

Pin Strength, Does Arias provide a wall thickness for the pin, and what material they are made from? Tool steel is usually the standard for racing apps.
Good post and I concur ;)

I'm at 3 here but I run triple coatings on my JE's
Ceramic/Crown
Moly/Skirts
Thermal dispersant/Underside of crowns.

[bigboost7m]

Good post and I concur ;)

I'm at 3 here but I run triple coatings on my JE's
Ceramic/Crown
Moly/Skirts
Thermal dispersant/Underside of crowns.

Whats the thickness of the coating on your piston skirts, IJ?


The reason im asking is because I worked for Livernois Motorsports not too long ago, and we were doing testing on the 4.6 Modular engines, mainly experimenting with coatings. We had the piston skirts coated thick, or "double coated" (also experimenting with piston to wall clearences at the time too).
Anyways, after the break in and some low-med load dyno pulls, we tore apart the motor. We noticed that the coatings were almost completely gone. This was due in part to coating company actually applying a coating over the first layer. They would apply the first layer, let it dry, then put on a second layer. All in all, my point is never have your piston skirts coated twice, because the two layers can not adhere to each other. After the first layer is gone, your piston to wall is then increased as well. Thats just a heads up to anyone thinking of doing that.