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Stick to the clearances in the piston installation guide that comes with them.

I went on the loose side because I plan to run nitrous and a whole bunch of boost and on startup I have a bit of piston slap because of the loose clearance. If you go too big, it's something you have to deal with. Nothing major, but it just makes the engine a big louder for a little while at startup.
 

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Ninja Performance
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For the most part, a forged piston will call for a larger skirt to wall clearance than a stock cast piston due to growth. Most forged pistons call for 4-5 thou. MUCH looser than stock cast pistons. BUT, some forged pistons are made with that in mind, meaning a stock bore replacement forged will drop in AND give you the 4-5 thou recommended.

-Chris
 

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Al-key-haul
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Discussion Starter · #4 ·
For the most part, a forged piston will call for a larger skirt to wall clearance than a stock cast piston due to growth. Most forged pistons call for 4-5 thou. MUCH looser than stock cast pistons. BUT, some forged pistons are made with that in mind, meaning a stock bore replacement forged will drop in AND give you the 4-5 thou recommended.

-Chris
Thanks, that was my next question. So for example I assume the Ross pistons for the stock 91.1 mm bore account for the needed extra clearance right?
 

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never assume anything, if you can ck it yourself do so, or have the machine shop ck your bores before buying parts.

you must also consider the bore taper, if its exessive you may need to bore over anyway.

the newer piston alloys are allowing much tighter clearences now, Mahles can be 2.5 thou, CPs around 3 to 3.5.
 

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Ninja Performance
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Ross calls for 5 and the stock bore pistons account for that.
Everyone I have installed and checked anyway.
ALWAYS CHECK!

-Chris
 

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Ninja Performance
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Engines are quiet from start up. Ross used to recommend 6thou but changed to 5 thou a little bit ago. I called them and asked if it was a material change that made them go from 6 to 5 and they said no. With that, I guess you could try 4 thou and see if it is better, I always build for 5 thou and haven't had any issues. Maybe slight skirt scuff on high (600+AWHP) cars but not anything to worry about.

-Chris
 
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