Shimano Hollowtech BB Road->MTB BB Compatibility

So, I got a frame with a nice Dura Ace Hollowtech II bb, but the cranks I have to put on it are Deore Hollowtech II.

The cranks fit fine with one 2mm spacer on on both side (as apposed to the 2 on the DS that you would normally need for a MTB BB on a 68mm bb shell), they pre-load nicely and spin well.

But I have read (Technical FAQ: Shimano road and mountain bottom bracket compatibility) and heard (no link) that they aren’t technically fully compatible but I can’t find any evidence of this being a problem long term.

So has any here got any thoughts? any one used this combo long term? fails? wins?

Just a different outside width. 92mm for mtb vs 89.5 or 90 for road. The MTB cups are wider. If you’ve spaced out the road cups so that the preload width is correct, the only problem will be decreased bearing longevity from using a smaller bearing.

Also, mtb cups are about $40. Save the D-A ones.

  • Joel

Outside width being the width of the spindle or the actual width of the bearing?

thanks for the quick reply,


and yes I am considering just buying some XT cups.

I’ve had an Ultegra BB with Deore cranks in my 29er for about a year and a half - I had the BB (used) in the spares cupboard so bunged it in with a couple of spacers. It’s holding up fine. To be fair it doesn’t get a pounding, mostly commuting, a few kerb drops, some JRA and a bit of light single track now and again.

Just replaced two shims no hollow tech bb with generic mtb ones, both bikes get out in all weathers, have compact road cranks and serious problems with the real shitmano ones.
$27 delivered from Taiwan, proper seals, would never bother with a shitmano road one again.

outside width meaning that, when installed on a 73mm BB shell with one spacer OR 68 shell with three spacers, the distance between the outside surfaces of the black plastic bearing seals which the spindle passes through.

  • Joel

So, in theory with exactly the right spacers (probably not just two 2mm’s) it would be spaced just as a MTB BB?

Yes, but with narrower cups, therefore possibly smaller bearings, therefore less bearing longevity. Also lighter seals for less drag, more ingress of beer.

  • Joel