smr21

Bought a 1978 Trek TX500 frameset and starting to build it out. Found a Shimano Ultegra FC-6750 crankset at a LBS that sells secondhand parts, cleaned it up and it's in decent shape. Asked the guy there what bottom bracket I needed with that crankset and he sold me a SM-BB52. Got home and found the install info from Shimano for the crankset, and on that pdf it states a different BB to use (BB-6700).

Hmm...did some more digging and found a compatibility chart from Shimano and the BB52 isn't listed as one of the bottom brackets compatible with that crankset, but there's a bunch of other ones that are listed as compatible that I'm sure I could easily find.

My question is - do I believe the guy from the LBS and go ahead and install this BB with this crankset? Or do I return this BB and get one of the ones recommended in the Shimano compatibility chart? Or a third option that I haven't thought of yet?

csport

I think BB52 is a "mountain" BB, and it has additional spacers to use with the 68 mm BB shell to make the "mountain" cranksets (compatible with the 73mm BB shell) fit.
It will probably work with a "road" crankset if you use just one of the three 2.5mm spacers. Or maybe no spacers? Try it and see.

Edit: forgot that the dust sleeve which goes between the BB cups may be too long if you use less than all three spacers.

maddog34

use two spacers instead of all three.... and place them as needed to make the crank arms as even as possible when they pass the chainstays... i usually test fit with both crank arms facing the same direction until the cranks look to be the same distance from each stay, then mount/clamp them in the proper orientation.

Road Hollowtech II cranks are about one 2.5mm spacer narrower than MTB HT II cranks, as i just measured both to verify this fact.
all recent Hollowtech II BBs that i've used are made to work with either crank type.

and the spacer instructions should be mostly ignored, as per my experience with HT II BBs... sometimes they're spot on, other times they make people break into tears and/or seek help online if "followed".

and then, there's the BB kits that came with FOUR spacers, but the instructions were for THREE.... Hoo boy.

sometimes ya just gotta use your eyes and wing it.

smr21

Thanks, that makes sense - it's listed as compatible with all the MTB cranksets in the compatibility doc I found. Interesting that the BB didn't come with any spacers in the box, but I imagine that'll be easy enough to sort out.

smr21

Thanks maddog34 - those are good tips, especially test fitting with the crank arms facing the same direction.

And will ignore the spacer instructions, I'd prefer to avoid breaking out in tears as much as possible, although sure there will be plenty of opportunities as this is my first full build.

maddog34

since you're going for a rebuild,, set the hubs to have a tiny amount of play in the bearings.. if you don't, once you tighten the QR, the tension becomes too tight and the hub bearings will overheat.
Yes, the hub's AXLE COMPRESSES slightly from the QR tension.

note the 1982 trek 930 Custom listed in my bikes list.. it was a bare frame/fork when i brought it home new.. i will never sell that bike. it still wears the Shimano 600 Arabesque crank installed way back then, and the D.A. BB and headset from the same build.... i ran across the original DA/Rigida/DTSwissDB Front wheel i built up at that time.. my first Bike wheel build, now sporting two curb lip dings.... those dings led to my present Co-op shop... ten years ago, i went looking for a new wheel and brought home a trek 820 MTB that needed EVERYTHING done... i spent more on tools than parts.

smr21

Oh man sounds like a lotta adventures in bike building! The vintage Treks seem to have quite a following, I'm pretty excited to see how this one turns out. Will keep that tip in mind once I figure out wheels (considering building them - there's a local bike co-op that runs wheel building classes twice a year...could be fun).