Let analyze the principle behind a starter mechanism.
On a car, where the space is less restrictive, the starter
is composed of 2 major distinct components: the motor and the solenoid.
On most motorcycles, space restriction doesn’t
permit to fit a bulky starter with a solenoid. Manufacturers have adopted 2
other solutions: an electric motor in conjunction with a free wheel and/or a sprag clutch. This is not new as 40 years ago the English and Italian bike had
already fitted their electric starter with this system .
On the early K100 (1984), BMW used a free
wheel to drive the crankshaft from the starter. The free wheel had only 3
rollers and was quickly replaced in 1985 with a sprag clutch.
From 1985 on, BMW adopted another solution for the K100: the sprag
Here is a very comprehensive animation of a sprag clutch (Springs are not represented on this animation).
And here is the actual sprag clutch of the 1985 K100.
In order to keep a permanent contact between the inner and outer race, the BMW sprag clutch is fitted with small springs (not visible on the previous picture) applying a small constant pressure as illustrated on the following diagram. (This is just an illustration to show the principle.)
A dirty system will prevent the sprags or
springs to move freely and consequently render the system inefficient. The
clutch will start to work erratically and the starter will spin without driving
One solution found by many riders is to flush the engine to try to get rid of the sludge. Some people found that running the engine with good quality diesel engine oil will dislodge the sludge and solve the problem. 88 from the k100-forum found another good solution: With the right side (crank cover) off you can access the oilway holes in the sprag clutch. Turn the engine over gently you line it up where you can get a shot of cleaning oil in there.
Hope this will help you to understand the link between the sprag clutch and the crankshaft.