Bottom end interchangeability

[SeanW]

The crank on my 80/7 has a damaged big end journal due to an oil pressure failure, so am thinking about just bunging all the bits on a used bottom end, like the 100rs one on eBay at the moment.

The 800 is a 1980 bike; is there any simple rule for knowing what bottom ends I should be shopping for, eg models / years etc?

 

[Rob Farmer]

The crank cases went through various modifications over the years and were steadily improved all the way through. If your bike is a 1980 you will ideally need the heavy flywheel crankcases to match your barrels. The 1981 onwards machines had a different sized barrel that was machined to take the O ring. you can fit your barrels to later crankcases but you would probably be best advised to fit a base gasket.

Probably easier to stay with 1980 cases.

 

[SeanW]

Cheers rob

Are the 80 and 100 cases fundamentally the same / interchangeable?

The rs cases on eBay are 1980, is the reason I ask.

Also I currently seem to have one original barrel and one later (o- ring) barrel, but this didnt s eem to be causing any issues when it was running....

 

[Rob Farmer]

If the RS bottom end is a 1980 I'd grab it. 80 and 100 crankcases are the same.

I gave you some misleading info in post 2. The 1980 bikes were a hybrid. I take it yours has a points bean can and a finned timing case ? The 80 bikes were heavy flywheel engines but had nikasil barrels and single row timing chains that were the norm on the later machines. It sounds like you have one nikasil barrel and one non nikasil barrel. You need to have piston rings that match the barrel type so you could be in for some interesting barrel wear in the future.

Just had a look on ebay, there are two available. When you get the bottom end have a look at the camshaft number stamped on the end of the cam.

 

[SeanW]

Just double checked. The 80 is a 1979, not 1980. Sorry.

Has single row timing chain, though

Think neither barrel is nikasil, at least they both weigh way more than my 100gs barrels.

Would 79 stuff go on an 80 bottom end?

 

[Rob Farmer]

If it's single row then it's what people refer to as a 79/80 machine. BMW years go from Start of September to end of July. So a September 79 bike will be a 1980 machine if that makes sense?

Nikasil barrels on all machines from September 78 (79 model year onwards)

 

[SeanW]

couple of pics of barrels, in case they aren't what they're supposed to be

they both have liners that stand a few mill proud of the barrel at the head gasket end - not sure if that indicates anything significant (only mention cos my GS's aren't like that)

manufacture month was feb 79 in case that helps

 

[SeanW]

I also noticed when comparing these to my GS, that the pushrod tubes seem to be positioned quite a few mm further inside the barrel.

is there a typical/ideal position for the tubes that makes them more likely to seal nicely at the cases?