Convert R45 to 650
 

Has anyone put r65 pistons/ cylinders on R45? Exactly same bike so is it a straight swop?

Check out UKGSer forum, go to the airhead section and post up your question there - you may have to pay a £12 subscription to post but if you’re working on an airhead it will be a good investment for further questions /help you might need.

I have a pre 81 model R65 and I think you will need to do more than just change the cylinders & pistons to get the full benefit of the larger capacity. The heads and carburettors are also different part numbers on the R65 to the R45 and there was even a lower power version for the German market.
If you are looking for outright performance from the R45/R65 series there is “proper” 800cc conversion kit available.
But just as flipflop suggests join one of the many airhead forums - there is even a dedicated BMW R65 one which you may find the answer in.

As has been said it is not as straight forward as it seems but for some good advice head straight here.
https://advrider.com/f/forums/airheads.85/

Replacement Kit in Germany available:
https://www.siebenrock.com/motor/power-kit-860cc/
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Watch: Manual under "Downloads"