jacampb

ok - having some issues here. let me just get to the point - what's the problem with using a 68mm bottom bracket in a 73mm bike. The left side cup doesn't have any sort of stop as you screw it in so it can screw in far enough (it seems) to go around the shell. also when it is installed there is clearance enough for the left side crank arm and the offset is minimal. what am I risking here? Same question for putting in a 73 into a 68. Is the only problem crank offset?


Thanks
Josh

Jeff Wills

In general, yes, what you say is true for modern sealed-bearing BB's. Back when 73mm bottom brackets first appeared, they were mostly loose-ball type. The extra width spaced the bearings further apart, making it a bit more durable.

As long as the cups hold tight, and the crankarms spin freely, I don't see a problem substituting one for the other.

`Orum

Is the only problem with a flat tire that you can't go as fast?

What I'm trying to say is, it might seem like only one problem, but that one problem causes other problems. External cup BBs should have no play in them when the cranks are correctly installed. As many BBs are now being made capable of fitting both 68mm and 73mm shells, there's no excuse not to do the correct assembly when possible. You can probably mechanically assemble a 73mm setup on a 68mm shell, but then chances are your axle will slide left/right 5mm, which you don't want, even if you're on a single speed. You also might be able to assemble a 68mm setup on a 73mm shell, but chances are your left crank arm will not engage enough of the axle and ad will be over-torquing the area it does grab, and could loosen more easily and even pop the cap that compresses the left crank arm onto the axle (prior to clamping it on with the pinch bolts) right off when it comes loose.

In short, I'd never chance it. Many EC BBs will fit both 68mm and 73mm shells anyway with the proper assembly.

Edit: FWIW, I've seen just a few tenths of a millimeter be enough to cause issues with a FSA EC crankset. The bike in question had never had its bottom bracket shell faced for external cups, even though it came with them! Shame on you, Bacchetta--though to be fair it seems almost every manufacturer does this. Anyway, the axle had a slight amount of play that could be felt, and would creak when under a lot of force. Looking up the tolerance on BB shell spacing in the FSA docs, I found it was very small, and just a few tenths of a mm too wide was, well, too wide. Got it faced, it's now within tolerance for width, it doesn't creak, and there's no play.

JonathanGennick 

Your chainline is slightly changed. But if everything fits and the cranks go around and the bike shifts, then why not?

On some of my external-bearing mountain bikes the chainline is biased outwards more than is ideal. The bikes came that way. I don't see why a slight bias inwards would be any worse of a problem.

Dan Burkhart 

From the OP's description, I'd say he's talking about a cartridge style, not external. If so, if the cups both tighten, the cranks don't strike the chainstays and the chainline is within workable tolerance, I'd say you're good to go.
MTB cranksets with external bearings come with 3 spacers to acommodate either size, with or without E-type derailleur.

jacampb

I am talking cartridge style. Sorry. And yes both cups tighten, crank arms spin freely. Thanks for replies

Josh

`Orum

Ah, yeah, with cartridge style you can get away with a LOT. Your distance from the center of the saddle to the pedal will be a little different, which might bother some.

As far as being mechanically sound though, It's just external cups that you need to worry about.