Boring a 250 to 300?

[Thompo]

Mornin,

I know the done thing to get a 300 barrel and head etc but they are hard to come by...

My 2011 250 with S3 cylinder has a bit of piston slap , piston only has 20hrs on it (was sold bike with "new piston" fitted, still has some of the black break in coating so seems legit) im guessing the barrel needs plating (no damage or worn areas) or a next size up piston. No idea on total hrs.

Is it possible to get the 250 bored and plated to take a 300 piston, are the castings the same?

Don't want to spend the money getting a size up piston, finding its still slapping and need plated anyway, when for a similar cost (+ Head) I could convert to a 300.

Cheers

[gasgasman]

I would say, not worth the trouble.
You would have to machine grooves in the cylinder for the head o-rings.

[pscook]

You can get the cylinder measured and purchase the appropriate sized piston for the bore, assuming that the bore is round. I believe that there are four sizes of pistons available, so unless your bike was fitted with a C or D from new, then you should have room to drop in the right size piston. Again, assuming that your bore is round.

I think that the cylinder itself has an alpha marking on it indicating the original/OEM piston size, hopefully someone else here will point out where to look on the cylinder for the letter.

[Zman]

There are multiple posts for big bore kits. You can overbore a 250 but as I recall you can not go all the way to a 300.

[RudolfHucker]

Assuming that the barrel is not badly physically damaged, the normal route would be replating.

The UK leader in this is Langcourts in Weston super Mare. Give them a ring and see what they recommend. Normally you would buy a size A piston and send it with the barrel so that Langcourts can finish the bore to fit the piston and you are back to new. They can also repair damaged cylinders by welding up and machining before replating.

If you try to go from 250 to 300, you may also need a replacement CDI unit as well as the head. That assumes that the crank and rod are the same on both models. The whole thing is starting to spiral...

KTM used to do a 250 to 300 update kit and there were loads of parts in it and they were rarely successful.

[Zman]

You can run the 250 cdi on a 300 without issue.