Which piston?

[veloduke]

1975 250 Desmo

I was looking to fix an oil leak from the cylinder head, thinking that the o-ring between the head and barrel had perished. There was also oil at the end of the exhaust pipe.
Right, so off with her head, so to speak.
The o-ring had indeed hardened, but I took off the barrel while I could, and found that the top piston ring had corroded with the barrel, creating a groove large enough to pass oil.
No problem, a rebore will fix it - except that this turns out to be a 239cm3 engine.
Am I right in believing the barrel can be bored to take a standard 74mm piston?
There are some pistons available for various 250 models, were they all the same, or are they model specific?
Appreciate any advice, thanks.

[Jon Pegler]

It is possible to bore out a 239 cylinder to 250, it just takes a few more cuts.
As regards pistons, there is quite a choice, although finding standard size 74mm pistons is a bit harder than finding oversize ones.
Most of the pistons for sale are high quality forged items, being sold at a high price.
Years ago, Meteor pistons were available. They do still occasionally come up for sale. While they are usable for a road bike, they are of poorer quality and are a cast piston.
I think the Vossner pistons made in Germany are machined from solid billet. They do however come with an odd size of piston ring ( 1mm, 0.8mm and 2mm scraper ) which may make ring replacement difficult in the future.
The choice is yours. It's still going to be expensive.

If anyone wants a set of replacement 239 rings, for a 72.5 bore, I still have a couple of sets available. They are quite hard to get hold of if you need a set.

Jon

[veloduke]

Thanks Jon.
I think I'll go for one of Nigel L.'s Omega forged pistons.
As you say, standard bore size (74mm) isn't easy to find.