Prox piston measurements vs Wiseco

[Alain2]

Does a new 64.75mm (2.549") Prox piston measures at exact size 2.549"? Or it is machined to account for piston-to-wall clearance?

I am measuring my used pistons at 2.5465". I'm am still working on my measuring technique so I may be of by +/-0.0005. Measured on a 90 degrees angle to the wrist pin and at 25% of the piston length from the bottom of the skirt.

My understanding is that Wiseco pistons are made to account for piston-to-wall clearance, i.e. piston measuring at 2.545"-2.546".

My cylinders bores are measuring at 2.5505-2.551",  0.0000" out of round, 0.0005" taper.

...for a total of 0.004-0.0045" piston-to-wall clearance.

Unfortunately I didn't measured new when I put it together few years ago, they were apparently machined for 0.0025" piston-to-wall clearance.

Engine was working ok, I'm just doing winter inspection and cleaning the carbon deposits on the pistons.

[m in sc]

all pistons are designed for x clearance depending on material and use.

that being said, wiseco and pro x are 2 completely different animals because of 1: material and 2: metric vs sae overbore scales.

ive always measuered overall at the bottom of the skirt, thats the widest part of the piston. 

[Alain2]

Ok thanks. The measurement location I used is the one recommended by Prox on their website. I realized that Wiseco recommended the same location you are using, bottom of the skirt.

[m in sc]

for rough home measurement, i slip the piston in the bore and check with a feeler gauge at various points in the bore. seems to have served me well.

[SoCal250]

Piston measurement is usually done near the bottom of the skirt as Mark said.
There's some great info here from our resident engine building expert, Dr. Supertune: Tip of the day series- Piston clearances

[Alain2]

I went and checked my Prox again and they measure the same at bottom than at 25% up.

[tony27]

Most piston manufacturers make there pistons undersize by the clearance amount so the cylinders are bored to the nominal size, a piston marked as say 66.00 will be for a bore of exactly that size & will measure the required amount of clearance under that.
A friend bought a bike that had been bored as though the size marked on the piston was it's actual size, in his own words "it sounded like there were 2 of them trying to get out". Even thought the bore was round & parallel we had to order the next size up wossner piston & bore the cylinder to fit, runs like a dream now