bottom bracket spindle length question

blriggs

Hi Guys,

I’m a bit of a bike building newbie / novice.

So really hoping someone can help with this, as struggling to get a definitive answer on google.

I need a new Bottom Bracket for my 1990 Alpinestars Al-Mega DX (pre e-stay) mountain bike.

The bike will be running a triple crank set Shimano FC-MT60 (170) with (C-46, C-36 & C-24 rings).

With 135mm Shimano DX FH-M650 7 Speed rear hub.

And Shimano HG70 7 Speed Cassette 11-28 T

Frame size is 18” (it has over sized tubes).

Rear drop out width is 133.2mm (measure inside edge)

And shell width is 73mm

I went on the Sheldon Brown site and it said i need a:

122.5mm square taper BCD 110/74 to work with this crank

But the Bottom Bracket that bike came with, is a SKF with a spindle length of 126.5mm with a drive side of 24mm & none drive side of 19mm (but i’m not sure if this is the right size bottom bracket because i think it is original & i'm not sure how it functioned as i never got to ride the bike, beofre i stripped it all apart).

I’ve got a bit scared of ordering the wrong size BB Spindle length (like a 122.5) and getting issues with chain line etc, because of the fatter seat tube (which is 34.9mm diameter) and it throwing the front derailleur out further.

If you could please help me with what size & what model BB i need i would massively appreciate your help with this so i can order one asap to get this build moving forward.

Thanks,

Ben

mully79

Nice bike. I'm sure square taper cranks sat in different places depending on the crank itself. Changing cranks might mean a different BB due to changing offsets.

The easiest solution is probably to stick the crank back on the bottom bracket and hold it up to the BB shell in about the right place.

It should measure 50mm from the middle chain ring to the centre of the downtube.

blriggs

Thanks so much @mully79, that makes a lot of sense, i would never have thought to try this, so massive appreciation for the help. Will try this out over the w-end hopefully.