.0025 clearance piston to bore... and a subaru dont even run oil squirters.

http://forums.nasioc.com/forums/show...1061799&page=7

 

thats for 4032 alloy Mahle pistons. they dont expand much if any. likely very similar to alloy in your stock piston. stock factory piston to wall shoots for .001.

 

hmmnnn

A 4G63 has a Cast Iron Block and is more like a RB26, than it is to a Scooby which has an Alluminium Block...

None the less:

4032 Alloy piston requires less clearance because it has low silicon content. This means that it expands less than a piston with no silicon, but since the silicon is fully alloyed on a molecular level, the alloy is less brittle and more flexible than a stock piston... (So yes its expansion rate is closer to stock)

2618 performance piston alloy has less than 2% silicon, Both because of the higher than normal temperatures that these pistons experience in their usual application, and the low-silicon content causing the extra heat-expansion, these pistons have their cylinders bored to a very loose cold-fit. This leads to a condition known as "piston slap" which is when the piston rocks in the cylinder and it causes an audible tapping noise that continues until the engine has warmed to operational temperatures. These engines should not be revved when cold, or excessive scuffing can occur.

 

Hey Sal... I like happy slappy pistons... did that subaru engine that won this weekend on the street car challenge and I use .0048 for clearnace and it runs LA Iron sleeves...

My opinion, is that high rpm and higher boost engines need to run more clearance... KW(HP)= KJ/s and Energy translate in heat... no one can tell me what the piston temp go to...and that where it count...on the side scirt... we only work with water temp and egt...

The time duration you run the engine also plays a role... 500whp street car and 500whp circuit car is way different.

 

For Sure on a race spec motor 2618 performance piston with extra clearance for boost and high rpm is the way to go - no question about it...

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Piston to wall clearance is different for each application. The design of the piston (how it will expand) and the temperature of the environment will all change the required clearance. In addition, you have to consider how much the bore (block) itself will expand. A cast iron block expands much less than an aluminum block. In fact, some applications with aluminum Nikasil plated bores run extremely tight clearance (under .002" with forged pistons). Basically, if the cylinder is made of a similar material, it will expand at a similar rate as the pistons...except that it is exposed to less heat compared than the piston, therefore you still need clearance! Make sense?