Understanding Gearbox ratios

All gears can have different ratios in a SACHS engine. By following these instructions you will be able to find out what ratios are in the gearbox without taking it apart:

Select first gear, and then turn the crankshaft 31 revolutions. Take out the plug will make it a bit easier. If the mainshaft turns 4 revolutions you have the late, helical first gear with 8/29 teeth. This is normally fitted in all Messerschmitts after mid 1955, and also in all other later dynastarted engines. If the mainshaft makes 4.5 revolutions you have the straight cut first gear with 9/29 teeth.This is normally fitted in all kickstarted engines and also in early 1955 KR200 engines. As far as I can remember I have only found straight cut first gears in 200cc engines with the 20mm crankshaft.

Now select second gear, and turn the crankshaft 20 revolutions. If the mainshaft turns 5 revolutions you have the normal gear with 13/24 teeth. It is fitted in all SACHS engines except 150cc motorcycle engines with engine number under 1343492. If the mainshaft makes 5.5 revolutions you have the very rare high gear with 14/24 teeth. Through the years I have seen several 150cc engines, but that gear I have only seen in handbooks.

Now select third gear, and turn the crankshaft 21 revolutions. If the mainshaft turns 8 revolutions you have the early, high third gear with 17/21 teeth. It is fitted in all SACHS engines up to 1957. Unfortunately, I don?t know if it changed in the beginning, middle or end of 1957. The reason for change was that the teeth on the twin slider gear were too fragile. But there is no reason to worry if you make normal gear shifts, and it gives more sensible steps between the gears than the later version. If the mainshaft turns 7.5 revolutions you have the late, low third gear with 16/21 teeth. It is fitted in all SACHS engines from 1957 and later. As far as I know SACHS only made dynastarted 200cc engines from that time.

Finally select top gear, and turn the crankshaft 11 revolutions. If the mainshaft turns 5.5 revolutions you have the low top gear with 20/19 teeth. It can often be found in scooters and motorcycles. They are fitted in all Nobels and also in late Fram-King-Fulda (Swedish Fuldamobil S7) and also in engines for lift cages in television antennas. If the mainshaft turns 6 revolutions you have the high top gear with 21/18 teeth. It is found in all KR200 engines, and also in dynastarted KR175 engines. I know that the handbook?s figure of the total ratio suggests that it should have the low top gear. But I have four dynastarted KR175 engines at home that can be turn over, and they all have the high top gear. They also have the helical 8/29 teeth first gear, and also that doesn?t agree with the handbooks. Unfortunately I have no kickstarted KR175 engine so I can?t tell the top gear ratio for them. But the mechanism for the kickstart always has straight cut gears, so first gear is definitely 9/29 teeth. The high top gear can also be found in early Fram-King-Fulda and also in 150cc and 175cc motorcycle engines, but only if they have the additional letter G. Swedish graveyard tractors always have high top gears, but I am not sure what the snowmobile Sputnik has.

If you want to make your own calculations of the total ratio you also has to remember that the primary chains sprocket on the crankshaft has 16 teeth and the clutch case has 34 teeth. The KR200 has 13/30 teeth in the final drive chaincase and the figure for KR175 is 16/38.

If you have any comments on this please contact me on three.three@swipnet.se

Anders Johnsson, Sweden