Brose Mag S Rebuild Question

[kamloopsmtbguy]

Hi Greg!

I am currently rebuilding a Brose Mag S motor due to an exploded sprag clutch. My question is, how far do I press the right-hand crankshaft bearing (PLB20122) onto the torque sensor shaft?

I measured before removing the bearing, and it was about 18mm, but I’ve also seen 17mm quoted as the correct distance.

I’m making my own tools from scratch, so I don’t have the press cup that sets this depth by bottoming out against the end of the shaft.

Thanks!

 

[Greg Watts]

Hi Greg! I am currently rebuilding a Brose Mag S motor due to an exploded sprag clutch. My question is, how far do I press the right-hand crankshaft bearing (PLB20122) onto the torque sensor shaft? I measured before removing the bearing, and it was about 18mm, but I’ve also seen 17mm quoted as the c...

@kamloopsmtbguy - welcome to one of my favourite Friday evening pastimes: Brose motor surgery. The exploded sprag clutch is practically a rite of passage at this point.

The PLB20122 is confirmed as a 34x47x7mm right-hand crankshaft bearing. The reason the eBike Motor Centre sells their 4-in-1 press tool (PLA00936) is precisely because this depth is critical -

it's machined to set the right hand crankshaft bearing at the exact depth on the torque sensor. That tool acts as a physical depth stop, so without it you're relying on your measurement, which is exactly the situation you're in.

Your 18mm measurement is your best reference here, honestly. The fact that you measured before disassembly puts you ahead of most DIY rebuilders. The 17mm figure you've seen floating around may reflect a slightly different motor revision or simply someone measuring to a slightly different reference point on the shaft. Since this bearing seats onto the torque sensor shaft and the depth affects how the torque sensor, needle roller bearing, and sprag all interact, erring on the side of your own measurement is sensible. If you're off by a full millimetre in the wrong direction, you'll either get binding or excessive play in the torque sensor assembly, and you'll feel it when you trial-fit the crankshaft assembly back together.

One note on orientation: the seal with integrated collar must face OUT, sealing into the housing. Easy to get backwards when you're focused on depth.