71 R50/5 upgrade to 600 cc?

[jdvorchak]

The owner of the 71 BMW R50/5, I'm working on, was asking about the conversion to a larger displacement engine for future reference. I told him I thought it was an easy upgrade to R60 but can't seem to find anything on the web about the conversion. R60 pistons and Cylinders? Any machining needed on crank case? Heads and valve covers? Cam? I advised him to ride the bike as is for a while before making any attempt to upgrade.

[Major Softie]

Cams are different. Some may have used the (milder) R50 cams with a larger motor. That cam has both less overlap and a little less lift.

R60, R75, and R90 cylinders/pistons bolt right up. '76 and later have larger spigot external diameters, and thus require machining either cases or cylinders to fit.

R50 had much smaller valves, so one would probably have to switch heads along with the cylinders and pistons.

[jdvorchak]

Thanks you sir. I'll let him know.

[Major Softie]

Oh, and I would expect the R50 heads to have far too small a combustion chamber for the larger pistons - especially the R75/R90, so it would probably yield an unacceptably high compression ratio.

[sprints@pldi.net]

It may be much less problems to find a larger used engine

[robert]

It may be much less problems to find a larger used engine

The voice of reason.

[jdvorchak]

Thanks for the replies and suggestions. I'll let him know he has options.