Flipped it when going up a boggy bank....afterwards kickstart was locked solid but engine seemed to turn a bit in gear pushing back and forth....strange!

Assumed a seizure so poured some oil down into the bore. After about ten minutes it freed up, started and i managed to ride the 12 miles back to the van.
Stripped the top end off and found the piston chewed up like it had swallowed something. i'm not convinced that it overheated having owned this bike for fifteen years and i know that the idle usually goes a bit tits even before the light comes on and i have a fan fitted. Can't find anything missing that it might have swallowed. What do you guys reckon on the condition of the Nikasil and where's the best place to source a new piston?

Bollox...can't figure out how to upload pics of the damage so best place to buy a new piston?

If you have an AR then it is a Nikasil bore if not then you have a steel liner. Is the bore bad? You may need a rebore or re-Nikasil.

Very difficult to tell without photos!

Upload your photos on to a hosting site and then copy a link to the photo on to a post on here.

Reasons for seizing other than overheating - worn out, oil pump knackered, hole in the intake system making the bike run lean ................

https://imgur.com/a/2eqEzWx

Another try at the images.

If that is your piston with the hell beaten out of the top then something has gotten in that shouldn't be there!

The bore looks fine but you need to find out what happened.

How does the cylinder head look like inside? Did the plug loose some insulator?

Cylinder head is peppered but not too bad. Plug is fine and nothing missing from the reed block.

This is the first time the top end has been off so i'm wondering if this is 'old' damage as the bike ran fine afterwards.

Can't say i can see any pick up damage that would suggest a seizure or find any cause.....it seemed to be smoking normally from the exhaust afterwards.

If that damage was pre seizure then it seems very likely that the issue was caused by overheating or a 'cold seizure' - sudden cooling of the cylinder when the piston is still hot. I had this once on a KTM125 driving through a ford. Unlikely though????

I would be very wary of using that piston again. There are some points that could get very hot and cause preignition and I would be surprised if the damage was caused pre-seizure.

Sorry I don't have an answer.

Ok, thanks for the advice.

There is no way i'll be using that piston again so just one final question..... where would you recommend i buy a replacement along with gaskets etc?
I usually use leisure trail for cables and the like.

When I rebuilt mine, I used a Wossner piston and Athena gaskets. If I did it again I would probably use a genuine heads gasket. Saying that the Athena one has been no trouble and that was 7 years ago.

I was wondering if the wossner pistons were any good...40 quid cheaper than a Honda one i believe.

Wossner are better than original.

Ok, so the Wossner turns up and is smaller in diameter than my original piston at 65.93mm. I assumed this was the standard size so now i'm guessing i need the next size they list, which is 66.40mm? [which i saw listed as oversize]. My piston measures at 66.30.

The piston also looks different around the gudgeon pin area but i'm assuming this is normal.

I know all of this sounds obvious but i just want to make sure i get this right second time.

I bought an oversized piston, my paperwork says 66.4mm and then had the barrel bored and powervalve blended to suit.

Duhh! Honda increased the piston size when they changed to Nikasil.

Bloke i bought the piston from does not agree.

The AR has a standard piston size of 66.40mm that is sometimes quite difficult to get hold of because it doesn't match any of the steel lined piston sizes.

The sizes for the steel liner are much easier to get hold of and perhaps cheaper for that reason. So a few owners with damaged liners and have it rebored and re lined to fit a more commonly available size.

Read this piece in 'Honda CRM250FAQS- Check here first' - viewtopic.php?f=20&t=5267

Ok thanks for that.

The guy i bought the wrong piston from has a 66.4mm piston available, though he says it's 'oversize' but if i bought it i wouldn't know if the rings were right for the Nikasil. [the seller is no help whatsoever].