GS750/GS850 cylinder interchangeability?

[Michael Graziosi]

I have a 1979 GS750 (parts) engine and a 1980 (or '81 or '82- not sure but no kickstarter) GS850 (parts) engine. I want to build another 750 engine for my 1978 bike and have an RC Engineering 870cc piston kit (found a new old stock kit). The stock 750cc pistons are 65mm and the RC kit pistons are 70mm. Can I use the 850 cylinder and bore just one millimeter to 70mm or should I use the 750 cylinder and bore 5mm to get to 70mm? Will heat be a problem going to 70mm with the 750cc cylinder. The current 750cc engine (runs very well and 30K miles) in the bike has the Yoshimura 850cc kit and a aftermarket oil cooler. The bike is currently ridden on the street and drag raced and that's the plan with the new engine.

 

[Nessism]

5mm over is too much for the liner. You will have to use the 850 cylinder.

 

[zuluwiz]

Isn't RC Engineering Russ Collins old outfit? I haven't heard his name for a looooooong time.

 

[Michael Graziosi]

That is correct. Quite an innovator from what I read up about him. The piston kit I have is still in the original box from probably the early/mid 80's.

 

[Suzukian]

The only other option would be having new liners pressed in. 5mm is way too much, as was already posted.