Hollowtech bb mtb and road compatibility

I currently have ultegra hollowtechs on my bike and I think the bb is on it's way out.

I have a brand new hollowtech bb here but it's mtb not road.

As far as I'm aware the only difference in the systems is that the axle is longer on the mtb cranks and to accomodate this the tube spacer in the centre of the mtb bb is longer and it comes with spacers. So I'm thinking I can take the tube spacer from my road bb and use it with the bearings and cups etc from this mtb one, anyone see any problems with this? The bearing cup parts look pretty much the same as the road ones but I guess they might not be.

For what it's worth Christ King does make two different HTII BBs, one for mtb and one for road. But even their site says the two are identical except for the widths. And they say contact them directly or ask a mechanic if you plan on mixing the two. If I'm not mistaken HTII uses the spacers for proper bearing preload. And although RaceFace and FSA uses HTII too some of their cranks require different spacers than Shimano mtb (1 on the left, 2 on the right iirc). So in short I agree, if the only difference is different spacing for proper chainline and preload, and if the differences between Shimano, FSA and RF can be resolved with spacers, there's no reason why the differences between Shimano road and mtb cranks can't be resolved by messing around with the spacers.

Have you got both on you? Measure them and see if they're the same. The internal spacer is just a spacer so you should just be able to swap them. The crucial length is the distance between the outside of the bb shell (so where the thread meets the shoulder on the bb cup) and the crank arm (so where the sleeve on the inside the bearing ends). If these match then you should be ok. The mtb bb may be wider, so the axle on your road cranks may not be long enough.

You follow? I spent ages trying to work out if a hope bb could be used on my gxp cranks, but they're different again.

The mtb bb might be wider but on 68 mm mtb bb shells you still need to fit three 2.5 mm spacers. So I assume even if they are wider, you can still make it work on shorter road axles with thinner spacers like the ones from velosolo. I assume the plastic preload cap can take out a little play too. All this is just assumptions though since I only have HTII on mountain bikes.

I have both but the road one is on the bike and I was hoping someone might be able to tell me before I bother stripping my bike down.

I'm pretty sure that the spacing between the bearing cups is determined by the plastic tube spacer in the middle, the road bb has a shorter spacer so that the cups sit 68mm apart (no spacers) and the mtb ones presumably sit 73mm apart so that you use spacers (in some combination) on a 68mm shell and none on a 73mm shell or the cups might sit slightly further apart than 73mm to allow alignment on a 73mm shell too.

So my hope is that the only difference between road and mtb bottom brackets is the length of the tube spacer but it seems like it might be a case of stripping my road bb out to make sure.

On a 68mm mtb bb shell you use three 2.5 mm spacers, one on the left, two on the right. On 73 mm bb shells you use just one spacer on the right (drive side). Makes sense that there are no spacer on a 68 mm road bb - that last spacer on the right on mtb presumably both makes room for the triple mtb crank and gives the correct chainline for a wider rear spacing (135 mm for mtb vs 130 mm for road).

Hold a ruler against the bb cups on your bike. Measure the distance between the bb shell, ie. the bit thats a part of the bike frame, and the crank arm/bb interface. Include any spacers in this, but make a note of how wide they are. If the mtb cups have the same dimensions, or you can get the same dimensions using spacers, you're good to go.

Then use the spacing tube that fits. Theoretically you could use no spacing tube, but stuff might start getting rusty so this is probably not a good idea.

Aye, if the mtb bearings are narrower I can make it up in spacers, if they are wider then I'm screwed.