That's not going to happen. I'm sure there are some crazy big bore kits for some other model bikes and engines that are pushing this limit, but even the 351 BB kit isn't stressing the bottom end or balancer system to any great degree. On making the 250S into a 300, I didn't do any serious research on this myself, but I thought there was some info that indicated the crank and balancer components between late model 300's and 250's were the same.

I've been running an '06 250S with a 300 cylinder for a year now. I never noticed a vibration change or any other issues from upping the displacement...at least no negative issues. In fact I can short shift the bike a lot more now without having to wring the neck out of the 250 engine like I used to...and that with just a 300 cylinder. Starting from scratch and at full retail prices, I wouldn't consider anything less than BB's 351 kit. I got my 300 cylinder and piston brand new at a great price, or I wouldn't have gone there, though I'm very happy with my 300.

 

The 1999 cylinder arrived today and looks and measures good. The casting recess's in the base are 1/4" deep which leaves 1/8" of material before going into the water jacket, doesn't look like they will play much of a difference.

Once the gaskets get here and I get some time I will take more detailed pictures of the two cylinders.

 

are the tube dowels and head bolts in the right place?

 

Bolts seem to be in the correct location, won't be able to verify till gaskets get here.

Outside dimensions seem to be identical, just checked a few locations using dividers.

There is a difference with the dowels. The 300 only had one dowel though the 2001 model year. With a little more research it appears that 2002 is when they made the change in castings and added the second dowel. Not sure if I will drill for the second dowel or just leave it with one.

 

I'd just leave it with one.