What's to stop an English threaded 68mmm road BB being used on an mtb? Is there a fundamental difference? Lesser seals, or just branding?

Ta.

Just keep the tube from the centre – it's a different length. Other than that, I've done it and it works. Lesser seals? maybe, but no evidence yet.

Thanks, thought that may be the case. Tiagra ones are 12 quid at CRC at the moment.

LBS sold me a road HTII BB last time my hardtail needed one. Couldn't be arsed to go back so continued to ride with it. Operationally speaking it didn't feel any different to the MTB one, but only lasted one winter compared to the 2 years I usually get out of a MTB BB, so I'd guess the seals aren't as good.

Ridden an Ultegra one on the crossbike for 3 1/2 yrs it's only just failed. Don't think there is a difference.

For shimano they're a different width – each cup is 1mm wider than an mtb 1 – ie a set of ultegra cups will be 2mm wider than xt for example.

If you buy a hope road BB it's just an MTB one with 2 x 1.0mm spacers….

Col.