69.25 piston

[baker250r]

Ok guys. measured my piston today with the caliper... and i used like 2 diff ones to make sure it was correct. i got 69.25mm so i need to order rings for this thing, and i cant find anything for that size. only 69 or 69.5 does anybody know where i can get rings for this piston. i got some of the carbon off the top of the cylinder... and here wut i think the part number is.

526m86950 everythign is correct... except the last 9. i cant quite make out wut that is. but i think its a 9.

any help would be helpfull alot. i dont knwo if its a wiseco, prox or niks piston.

[wilkin250r]

Your part number looks like a standard Wiseco part number. Are you sure it doesn't say 526M06950? That would be a standard piston for the 86 crank. The 6950 at the end indicates a 69.50mm piston.

[baker250r]

yea, its 86 piston. do they even make a 69.25 piston?

[Eddiesanders250]

not that i know of

[jon370r]

Did you measure it 90 degrees across from the piston pin? Up by the crown it will measure much smaller. Sounds like you have a .140 oversize piston. 1mm equals almost .040,so .040 x 3.5=.140
66mm stock +3.5=69.5 mm piston

[baker250r]

yea, i was measureing at the top... so ur sayin i shoudl measuer more at the middle/bottom?

[punker69q]

You need to measure the skirt, 90 degrees from the piston pin. Pistons are usually oval, but not of 0.25 mm (that's 0.01''!).

No offense, but are you sure you can read a caliper correctly? Or is the caliper worn out?

Here is a cool link for those who don't know how to read them.
http://www.physics.smu.edu/~scalise/apparatus/caliper/

[baker250r]

yea i can read one correctly... i jsut didnt knwo where i had to measure it at. im using a digital one... maybe i better use one with a vernier scale?

[punker69q]

Nah, digital ones, when in good working order, are accurate to less than a 0.001'' / 0.02mm so this is not the problem. Maybe your measurements are just rights but you have some kind of custom 69.25mm piston made from a 69.5mm casting/forged blank??? :confused:

[wilkin250r]

Is the piston used? It would be odd for it to be worn out that much, but not completely impossible.

Like Punk said, you should be measuring the piston skirt, NOT the top. Also, your measurement should be directly front-to-back, not side-to-side.

Oddly, pistons are not perfectly round as you would normally think. When they heat up, the dome expands more than the skirt, so the piston manufacturer intentionally makes them smaller at the top, to allow for expansion. Likewise, the piston expands more side-to-side, along the piston pin, so they are smaller there as well to allow for expansion. They are slightly oval-shaped, and tapered from top to bottom.

If I were you, I'd also measure the cylinder-bore clearance. Is there a reason you are dead-set on using this piston instead of buying a new one?

[baker250r]

Well... I conjered up enuf money to get it done right. so i took it to a guy, and hes sending it out, with a new piston to get replated, and imgettin port work done too... so when i get it back, ill let u guys know wut a real 250r feels like. lol