Final Drive ratio

[r1200rst]

Has anyone ever come across a /2 final drive without a ratio stamp? It is not in my possession so I can't count it myself.

[20774]

In my limited experience, I've only seen stamped final drives. Are you should that it belongs to a BMW /2? There's not much to go on in the picture.

[r1200rst]

Yes they are for a /2. Not the greatest pics but they show it's real.

[kinterridge]

The 28 teeth on the splined flange is correct for a /2 (R50; R50/2; R60; R60/2; R69; R69S.). 4 options from what I can tell on the crown gear sets, if no stamping you'd have to count the teeth, 27:7, 27:8, 25:8, and the sidecar rig had 26:6.

[20774]

To determine the ratio, I think it takes more than counting the teeth. Pick one of the teeth and turn the final drive exactly one revolution while monitoring how many turns the u-joint at the front of the drive shaft turns. Those number of turns of the front joint will let you figure out what the ratio is. It will be something like three full turns so 3.0/1 or 33/11. If it's slightly more than three full turns, it could be 3.125 or 25/8.

[r1200rst]

I've asked the seller to try to count the ratio by rotating the input. But the difference between 3.125 (25/8) and 3.375 (27/8) might be hard to distinguish.

[20774]

Possibly. If it's not quite 1/4 past 3 turns, then 3.125. If slighly more than 1/4, 3.375. Do it several times to get a consistent result.

[20774]

Further info I've read indicates this is likely a pre-war final drive or possibly for one of the early R51 plunger bikes and may have been a replacement.