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.
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.
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.
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.
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 (http://www.2strokeworld.net/forum/index.php?topic=1725.0)
I went and checked my Prox again and they measure the same at bottom than at 25% up.
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