The number stamped in the crankcase (from -10 to +10) refers to the size of the crankshaft and camshaft gears installed by the BMW factory. After 60 – 70 years of service, the crankcase may have grown, which means that the original gear set may not be the optimal set anymore (0.01 to 0.02 mm play between the 2 gears).
One way to determine the correct gear set number is to measure the c-t-c distance between the crankshaft and camshaft. This value must then be compared to the nominal c-t-c distance. This is impossible without very accurate measuring tools. The difference in tooth thickness between 2 consecutive gear numbers is only in the order of 0.003 mm
A simpler method is to use a “dummy” crankshaft and camshaft, with a close sliding fit between shafts and gears. This “trial & error” method is accurate, quick and let’s you test different gear sets without damaging the gear bore (because of the press fit with the original crankshaft and camshaft).
Note: the BMW R51/3 – R68 repair manual mentions that newly fitted gears can cause a slight humming noise that will (almost) disappear after a certain running-in period. (see below)
Photo below: the crankcase is stamped +4.
A new +4 gear set resulted in a backlash of 0.025 mm.
A new +5 gear set gave the desired backlash of 0.015 mm.
