I don't know the Splitdorf magneto very good, but here is an extract that may be of help.

Service information Splitdorf magnetos, model S, NS, SS & B. RH rotation Motorcycle Magnetos 42 Degrees. Setting the rotor break.

Seldom is it necessary to reset the rotor break, except when a new rotor assembly is installed in a magneto. However if the rotor break is found to be improper, caused by wear of the cam or bumper on breaker bar, a new cam or breaker bar should be installed. The rotor wings on all S, NS,SS and B magnetos are set symmetricly, except those models of the S and NS magnetos used on motorcycle engines. On the motorcycle models, the distance between the ends of the rotor wings are much larger on one side than the other. This is done to obtain the irregular firing order of 201 deg. and 159 deg. as required in the 42 deg. type of motorcycle engine. Before attempting to check the rotor break, have the contact points set at .020" and the breaker in full advance position.

Turn the rotor shaft until keyway in the drive shaft of the rotor is in downward position, and the wide opening or brass insert shows in the timing slot of the left hand side of the frame, looking from the drive end of the magneto. Insert gauge, Part No. 3296, which is .204", in timing slot; turn rotor in direction of rotation (right hand) until edge of rotor wing presses gauge against the top of timing slot.If a gauge is not obtainable, use a No.6 standard drill. In this position the cam should be set so that the wide nose or lobe of cam is just striking the bumper on breaker bar, and the contact points are just starting to open. Drill and pin the cam in this position, taking care that cam does not shift while drilling and pinning it. Use drills and pins mentioned above. Check setting of the rotor break after pinning the cam, to take certain that the cam did not shift.

Left Hand Rotation Motorcycle Magnetos 42 Degrees

The rotor break on left hand rotation magnetos is set the same as on right hand magnetos, except that the gauge is inserted in the timing slot on right hand side of frame.