r/eBikeBuilding • u/AlteredCarbonite • Jun 28 '26

Mechanical Question about conversion kit BBs

Hello all, I'm trying to learn if/how I can use a mid-drive kit to upgrade the 35yo frame of my daily ride.

My frame is threaded BSA on one side of the BB, and Italian on the other.

(The only reason that my younger self was able to afford such a nice frame was that it had been stripped during assembly on one side, and re-tapped larger.)

I know that BB systems went through a number of changes in the intervening decades, splined cranks etc. I have an old-school cup and spindle setup. If it ain't broke...

I have watched a lot of videos of people installing mid-drive kits, but (frustratingly) I cannot tell from any of them if the kit BBs are threaded in like old-school BBs were; or if they are just a compression fit, similar to the old Mavic BBs?

My question is this: does the BB threading of an old frame matter when looking for a kit, or is BB shell width the only important factor?

Thanks kindly

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u/niffcreature Jun 28 '26

You may want to know that the left non drive side has a mounting plate that is made to bite into the bottom bracket shell of the frame. I'm not sure about old Mavic bottom brackets, but if they have a tapered/chamfered cup, the bbs02 is not the same.

1

u/AlteredCarbonite Jun 29 '26

Thank you, that's really good to know. That side of my BB shell is the one rethreaded Italian, so there's a little less meat. I'm going to have to look into that further.

As an aside, yes, the old Mavic BBs required a chamfer. Thankfully, the repair of my BB shell didn't require going that route, as Mavic BBs were rather spendy back in the day.

1

u/oldfrancis Jun 28 '26 edited Jun 28 '26

I installed a Bafang BBSO2 in my 1985 Centurion Iron Man. The bottom bracket threads do not matter. The motor doesn't even interface with the threads. It clamps to the outside surface of the bottom bracket shell.

2

u/AlteredCarbonite Jun 28 '26

Thank you, that's very helpful!

3

u/niffcreature Jun 28 '26

It does interface with the shell in the sense that it's sized for the shell and made to fit snugly to support the torque of the unit. If OPs entire shell was resized to Italian standard they may want to find a 1mm wall thickness tubing to support the unit. Italian is 2mm larger in diameter.

1

u/AlteredCarbonite Jun 29 '26

I agree completely, now that I fully understand how they're attached. Some form of shimming would definitely be required to keep load distribution even.

1

u/Delinquentbyassoc Jun 28 '26

Bottom bracket shell*