Short skirt pistons and clearance questions

[Lyndon302]

911 2.0L High comp.
Just a general question.
My Wiseco have a really short skirt when compared to JE pistons.
I would have thought with piston rock it would run a tighter tolerance with a short skirt. Am I wrong or it just does not matter.
Lyndon

[Speedy Squirrel]

I think that, if you measure them, and you should, you will find that the diameter at the skirt is bigger than the diameter at the top. This is to account for the crown getting hotter than the skirt.

When warmed up the will even out a little, with both at the desired piston to cylinder clearance.

The desired clearance at the skirt may have been designed to be a little tighter than the crown to help piston stability, but usually not. The whole point of the short skirt is reduced mass and less friction, so they are designed to run loose. Piston stability is one of the trade offs for the short skirt. You can make more power by reving higher than sealing tighter, to put it simply.

[stanglife]

Originally Posted by Speedy Squirrel

You can make more power by reving higher than sealing tighter, to put it simply.

That’s been my story for years!

[Carbster09]

Speedy, this makes sense. I recently had a high performance Norton Commando engine built with a short piston / long rod combination. The builder told me that once started I need to keep revs at ~2,000 rpm for 5 mins, or until warm to minimise piston scoring. It is a PITA, but I follow this religiously. The bike makes alot of power, but I question if it was all worth it; moving so far from stock.