[C-Leigh Racing]

If you have a 330 cylinder on the engine now & running it, your cases have the right size bore & will be the right size bore even if you go to a 350 Pro-x cylinder.
On all of the Pro-x cylinders, & ESR cylinders, the skirt part of the sleeve that fits down into the cases, all of them are machined to fit down into that 3.350 machined out opening of the 250R cases.

Only other thing, like if the 330 cylinder is one of the bolt on type, then one of those cylinders the skirt is only like 5mm long, so the spacer plate provides the clearance needed to bolt the cylinder down onto the cases & no machining of the cases is needed, because the skirt will no go down into the cases.

It is fairly easy, to see if the cases have been machined or if the cylinder is the bolt on type, simply pull the carb & reed cage & with a light, look down into the cylinder & you can see over to the other side of the cases & cylinder skirt & can tell right off what you got. Not that hard to do even with the engine still in the frame. I've done it many times, even with my big head.
Neil

 

[C-Leigh Racing]

If your asking about the full length of the sleeve, from the head down to the cylinder base, if it is the same OD as the 330 sleeve, then no, the 350 sleeve will have a larger OD all together than the OD of the 330 & the 330 casting must be bored to the proper size for the 350 sleeve to fit in.
Now in the skirt area of the sleeve, the OD in that area on the 330 or 350 will measure out the same.

If you get a sleeve from LA Sleeve, be sure once you have the old sleeve out, to measure the OD of that old sleeve, because over the years LA Sleeve has changed their machining on the new sleeves & the full length OD of the sleeve is smaller than years ago.
Normally though, you will only run into this problem, if the cylinder you have is an old early model Pro-x casting, specially the 310 & 330 cylinders.
Neil