R90/6 engine Guts into a R75/5?

[jjwithers]

I have a R90/6 block that has been sitting around in a shed since 1994. It has the timing cover on it and that's it.
So inside is the timing chain, crank and cam shaft... with the flywheel still attached at the back. No rods. No cylinders. everything has been exposed to lots of dust.

I have to do a complete rebuild of a R75/5 engine.

I looked on MAX BMW and it is showing the same part numbers for the crank and cam... but are they the same?

My question is... is it worth putting the R90/6 crank or cam into the R75?

[Garnet]

The cranks and cams on an airhead very rarely wear out, so your R75 engine may not need them unless they are missing. Most of the bottom end is the same from 1970 to the end of 75, then flywheel bolts, cam seal size, and cylinder spigots changed. So check when your R90 was built.

[Major Softie]

Cams are different on the R50 and R60 than the cam for the bigger bikes. I believe that's part of why the R50 and R60 are more prone to pinging.

Snobum shows them all here:
http://bmwmotorcycletech.info/cams.htm

Some of the differences he shows are just about the cam timing change caused by the change in the key/cam-sprocket, but not the difference between the R50, R60 and the bigger 247's.

[jjwithers]

Thanks guys. According to Snobum, the cams are the same.
I figured that if there is any reason to upgrade the R75 parts with my spare R90 stuff, now is the time. But the cam and crank appear to be the same. So back to my regularly scheduled programming....

-Josh

[Garnet]

Thanks guys. According to Snobum, the cams are the same.
I figured that if there is any reason to upgrade the R75 parts with my spare R90 stuff, now is the time. But the cam and crank appear to be the same. So back to my regularly scheduled programming....

-Josh

On sober second thought, there is a case for swapping in a 76 or later crank and flywheel. The larger flywheel bolts are stronger and more able to handle HP upgrades. Also the 76 and later flywheel will accept the more common, lighter and cheaper 9 tooth gear reduction starter. And if one wished to upgrade to the 81 and later ultra light flywheel the larger (11mm?) flywheel bolts are a must.

[jjwithers]

The VIN does point to a Feb 1976 manufacture date.
Will the R75 Rods and pistons work on the /76 crank?
I do plan on lightening the flywheel anyway.

[Garnet]

The VIN does point to a Feb 1976 manufacture date.
Will the R75 Rods and pistons work on the /76 crank?
I do plan on lightening the flywheel anyway.

Yes the rods are the same. Check the bolt size and count the number of teeth on the flywheels as BMW is famous for not always following the dates that I have posted.

[chasbmw]

Thanks guys. According to Snobum, the cams are the same.
I figured that if there is any reason to upgrade the R75 parts with my spare R90 stuff, now is the time. But the cam and crank appear to be the same. So back to my regularly scheduled programming....

-Josh

On sober second thought, there is a case for swapping in a 76 or later crank and flywheel. The larger flywheel bolts are stronger and more able to handle HP upgrades. Also the 76 and later flywheel will accept the more common, lighter and cheaper 9 tooth gear reduction starter. And if one wished to upgrade to the 81 and later ultra light flywheel the larger (11mm?) flywheel bolts are a must.

The larger flywheel bolts are from 75 on, I think that they went to the 9 tooth starter from either sept 75 or 76. 111 teeth on the flywheel = 9 tooth starter motor. Don't mix them up. 8 tooth Valeo and Denso starters are now available and are not much more expensive than their 9 tooth equivalents

[jjwithers]

I'd probably ditch the heavy old starter and get a newer lightweight starter. But yes... gotta match those teeth..