Minimum Clearance Ross Forged Pistons

[Luc]

What is the minimum wall clearance you would dare to apply for forged pistons?

Please share your experiences.

[Preyupy]

Each manufacturer specifies clearances for their pistons. It depends on the material, thickness and shape of the piston skirt. Forged pistons will usually be set with a much larger clearance than a cast piston ( forged grows more than cast with heat). I have not used Ross pistons in more than 35 years so I can’t recomend a clearance specifically for them.  What bore are you running? For a 89-90mm bore with CP, JE, Venolia pistons I usually run between 0.006-0.0065” measured 1” up from the bottom of the piston skirt. 

[Luc]

The pistons are Alloy 2618 T61. Bore 89,25mm. Ross client page ( http://www.rosspistons.com/wp-content/uploads/2016/08/Piston-Install-Auto-Ross.pdf) states 0,003 for street use, 0,006 for turbocharging.

This raised my question:

My machineshop thinks that 0,003 is even too much. They do forged pistons in turbocharged Audi, Volkswagen with a clearance of 0,05 mm that is 0,002 inch. Never caused problems. At my time of visit they had an Audi S3 five valve engine that had to be build with Wiseco pistons. They choose a clearance of 0,002. 

 

Please share your experiences. What clearance did you choose and how did the engine behave regarding clatter and oil consumption. Did it fail with less clearance?

[TobyB]

You are NEVER going to go wrong with what the manufacturer says.

Because even if it goes wrong, they will then make it right.

 

I ran 0.003" with them for a racing application, and have had no problems.

They made very little noise, even dead- cold.  I was surprised how quiet they were.

 

I would also listen to Byron, above.  

 

A piston that gets too tight is... ugly.

 

t

 

Edited June 13, 2018 by TobyB

[Preyupy]

OOPS I screwed up last night typing on my phone,  Typical CP, JE, etc forged piston clearance is .0035" (not.006)  on a 89-90 bore.  Cast is roughly half of that.  You do need to be careful with forged pistons on a street car to warm it up a bit before you take off, they will rattle in the bores a bit and under the worst conditions you can crack a skirt. 

[kbmb02]

Another (similar) data point: the manufacturer recommends a minimum based on forging alloy, usage, design, etc. At 0.003” you’ll hear them a little at startup - but that’s better than the alternative. On most race engines with 2618 alloy I’m at/around 0.004” (my custom 4032 forged street pistons can go tighter = quieter). -KB