All 2st Vertex or Wossner Piston??

Hey guys, I'm running a 2007 WR 250...I've owned the bike since 08 and I'm pretty sure it's original everything. The bike has treated me very well, with good maintenance and no problems.
Anyhow, I'm rebuilding top end, and researching which piston manufacturer to go with, this thread has been very helpful!
The stock specs say that the bore and stroke are: 66.4 x 72mm. I'm leaning towards the wiseco piston kit, 66.40mm.
I'm just worried, I want to order the right piston kit...any advice?

I'd like to know how you work out which piston size to order as well. A. B. or C.
There are manufacturers clearances but how to measure without piston?.

Measure the cylinder bore diameter with a dial bore gage. This will give you the most accurate measurement and then you can order the appropriate piston. Any decent machine shop will have a dial bore gage and if you bring your clean cylinder to them measuring it will take less than 5 min.

The cylinder size (A/B/C) will be stamped on the outside of the cylinder.

Kyle,
Where is your cylinder stamped? I have looked mine over in the usual places and it appears I have an unstamped cyl. on my 09 WR250.

Sometimes the stamping is on the bottom of the cylinder on the base gasket surface.

The piston is also stamped if you clean it.

The piston sellers say measure and then make your own assessment as to which size you need Eg ebay
So not sure what tolerance is recommended
Mine has a c stamped on underside And measures 66.15 - with callipers - hard to get exact
Piston seems to be 66.05
Is C largest?.

Should a well used cylinder be honed ?
cheers

Yes, but but you'll get all kinds of answers, from no, to just use scotchbrite, to honing it will hurt it, to don't lube your chain because its bad for it.

s%#t stirrer!^

Lol I was stirring a bit, too! All good, and a polarizing topic. Bottom line is you need a crosshatch to properly hold and retain a barrier wall of oil. If it is shined away, it should be honed. If you feel unsure, let a professional shop do it. Here are some references on the topic:

http://www.nzcylinders.com/Cylinder honing.html

http://www.flexhoneblog.com/2012/11/the-flex-hone-for-niaskil-engine.html

Here's Millenium Technologies, employer of Eric Gorr, and platers for most of the top race teams. Notice the "light hone only" and "deglazing brush" in their price list. Call them, I promise they know more on the topic than any of us.

http://www.mt-llc.com/cylservice.php

Thanks Marc good info

I think I will start a new thread on my issues

I had my cousin come to my shop, he works in a machine shop, we measured the cylinder diameter, it read 66.34 on a very accurate vernier (spelling?) and on another tool to be sure. So now I know the diameter of the cylinder wall...?
The stock piston size is 66.40, is that ok?

I'm thinking it should be fine, what do you guys think? What was you experiences with cylinder diameters vs. piston sizes to order?

Also, I ran my fingers all around the cylinder walls, they are smooth throughout, but what about these discolouration? If it's smooth it should be fine or no?

according to your measurements you have .06 clearance, is that what your spec is? i would clean it well in a crosshatch pattern with a scotchbrite and run it if it were mine..i dont have any long term reports about wossner but the 2 i have bought im very happy with. substantial weights savings, especially on the bigger cc models. im a fan of having a machine shop measure it with the proper tools as well.

Are you sure you don't have those two measurements backwards? You are saying that the piston is bigger than the bore and if so it wouldn't fit. If it was the other way around you would have .06mm or about .0024 inches which would be about right for a cast piston.

Major oxymoron!! In precise measuring, we call vernier calipers " very near" calipers! For precise measurements of pistons, you need to use an outside micrometer. For the cylinder a dial bore gauge with a micrometer is preferred as Doug said. Verniers are good for setting oil heights, checking bolt lengths, verifying shim thicknesses, etc.