Circlip on output shaft
If your gearbox is from the period when BMW thought a circlip (actually a snap ring) was not necessary on the front bearing on the output shaft then I strongly suggest that you have a groove turned in the shaft and fit one.
I rebuilt my gearbox without installing one and so had to rebuild it again in about 15,000km. BMW seem to have relied on the bearing being a tight enough fit on the shaft for it not to move. Mine moved. The bike has now done about 30,000 with the re-rebuilt gearbox and it's excellent.