250-500cc Another Piston Thread

I used a Wossner, they are forged and have A through D sizes I beleave. Only a Wossner wrist pin bearing will work with the piston and rod. I think if memory serves me right they are the same as the KTM,s. Very good high quality piston.

Vertex would be the first choice, measure the bore and the old piston make sure you dont need a new plating.
No reason not to use cast.

Don't get hung up on cylinder size, wiseco 66.4 will work in abcd cylinder. I replace small end bearing because they are cheap and its easy. Yes go for the 2 ring piston.
Cant think of any reason for going cast apart from quick warm up.

It's more than theoretically true, it's practically true.
I'm pretty sure the wr250's only came with a,b or c cylinders. Most people can't measure that accurately!

All of the pistons use the same same ring regardless of letter size, so the only issue with ring gap would be when your cylinder is worn out and ready for a re plate.

It is both practically and really true. People get stuck on the piston sizes that have a maximum size difference of .01mm thinking it is a big deal and don't look at the the reality that is way less than a 1000th of an inch, .00039 ten thousands of an inch maximum to be more exact. Good luck measuring the difference even with a micrometer.

I measure by eye after half a dozen cans.

With expansion/contraction & machining tolerances I wouldn't have thought 0.01mm would matter SFA! I am no mechanic however. Maybe 0.1mm might be an issue!

I love vertex pistons which are cast. But if you are really prefer a forged piston the Wossner pistons are nice too.

I only got a wossner for my 300 because the 165 slugs from Walt are wossner n look beautiful(you know what I mean-almost to good to put in ya bike). No probs with the oem vertex for 100 hours each

Shawry 0.01 may not be an issue, 0.1 may cause a cold seize.
I currently have a 93 cylinder on my bedside draw with a brand new wossner piston the reason its not in my bike is with 0.06mm clearance as stated by wossner the thing seized in four places.

Now 0.06mm may usually not be a problem but it is a steel liner so your metals now expand at different rates. 0.01 can be an issue in some curium-stances.

Measure by eye dude you will be suprised at how acurate your eyes are.

XXXX Cans. It's summer here now so several!

Did you get a chance to replace your piston yet? My 2010 WR250 just started doing the exact same thing. When lugging at very small throttle openings Im hearing a piston slap type noise. Almost sounds like pre ignition or pinging. Went to the next richer metering rod in my Lectron and started running 50/50 av gas and pump gas. Seems to have reduced it some but not completely. Ran pump gas no problem all summer without hearing it. Compression is 185 and piston looks good looking in from exhaust and intake. Im second owner bike has 1200 miles on it. Just dont want to have a catastrophic piston failure

I'd like to do my top end this winter, also. It will be my first.

I want to go the same route you did with the Wiseco.

Would you mind writing where you bought each component?

I.e., piston, rings, clips, pin - Wiseco direct?
Small end bearing?
Gasket kit, o-rings - Athena direct?

Your help is appreciated.
Mike