Sachs A & B Motors
What's The Difference?
by Alan Buehner
Originally printed in the 2004 issue #23 of Still….Keeping Track
The Penton motorcycles from 1968 thru 1975 were equipped with 100cc and 125cc Sachs motors. Since the 100 and 125's used the same bottom end they could easily be switched over by changing the piston, cylinder and head.
The early Sachs motors (from 1967 thru 1971) were classified as the "A" motors which came with a large diameter crank and thin selector rod with a 8mm selector key. Because of problems with over shifting and not being able to adjust the shifting mechanism, the motor was updated around 1970.
The "B" version was equipped with a smaller diameter crank and a thicker selector rod with a 10mm selector key. All Sachs motors came 'With a serial number I.D. Tag which was riveted to the top part of the ignition cover. However, because all ignition covers are interchangeable and the ignition covers were subject to damage if a chain was thrown, you cannot go by the the LD. tag to identify which version motor that you have.
The following photos show the differences.
The top motor case is an "A"motor with a 8" diameter crank opening.
The lower motor case is a "B" with a 7" diameter crank opening. Note the double w,ill around the crank area. Also, the opening in each is deeper to hold the thicker crank.
NOTE: Check the front mounting hole. Early "A" motors have an 8mm hole. Some "A"s and all "B"s have a 10mm hole.
The difference between the "A" and "B" motor cases is obvious when you see the center case gaskets. The "B" center case gasket is thicker around the crank area to match the double wall construction.
The "A" crank (left) is larger in diameter than the "B" crank (shown on right). Some "A" cranks came equipped with aluminum rods, however both can use the same steel rod with the only difference being the length of the lower pin. Both cranks use the same size M-20 crank bearings.
SELECTOR RODS AND KEYS
The "A" selector rods are thin in diameter and have a threaded end for attaching the shift linkage. When compared to the "B" selector rod, you can see the problem the early"A" motors had with over-shifting because of the smaller grooves for the pins to catch.
The selector keys as shown are not interchangable because of the difference in diameter.
There is only one way to identify a "B" motor case without having to split the cases, and that is by the cast slit in the case to route the Motoplat ignition wiring (see top motor case in photo). The "A" motors came equipped with Bosch points and condenser ignitions and it's loose individual wires were routed through a round rubber grommet through the round hole in the motor case as shown. Because Sachs built their motors to accommodate the bike builders, there are several versions of cases. There are "B" cases with round ignition wiring holes! Some motors were still being equipped with the Bosch ignitions even though the Motoplats were available.