Are Shimano Hollow Tech 2 bottoms brackets reliable when fitted with two 2.5mm spacers on the drive side? This to me is 5mm less thread inside the shell, and coupled with external bearings seems to me to result in more stress on the cups and premature failure? Can anyone convince me otherwise? Do you have this set-up, had it long term and consider it reliable on an MTB? I'm no stress engineer but need reassurance before investing in this technology.

I've had that on my bikes. One lasted 6 months due to being overtightened, another lasted more than 2 years and was still running fine when I swapped frames. It's all in how tight you nip the cranks up to the BB or not as the case may be.

are the spacers to get the right chainline ? My old sq. taper BB is well too long, at least 7mm, but it still seems to work and shift fine, so you could remove one spacer

imo stress is not an issue, water is

Got a 3 year old hollow tech bb on my bullit, runs fine. I had a raceface version – bearings gash after about 3 months..

We do love old mother shimano.

If you don;t run the recommended number of spacers then isn't the plastic inner sleeve too long? I guess you could leave it out or modify it. Also presumably the axle may poke out too far the other side but the I guess that may not cause a functional problem.

Don't use them then. If you are fretting that much about proven technology, then use something else. I'd not want you lying awake all night all concerned.

Surely if you don't use the reccomended spacers, when you come to fix the non drive side arm there will be an extra 2.5 mm sticking out??

Thanks guys. Spoke to my LBS today and the mechanic said he only fits one spacer to the drive side on a 68mm shell. He seems to have the same concern I do, not enough thread inside the shell. Anyone out there running one spacer each side on a 68, and happy with that set-up? TooTall, I'm wide awake, slept like a log. 😀

One on each side here. Cranks and bottom bracket sleeve fit fine.

IIRC the RH cup is deeper than ther LH cup, presumably to allow for this.

Normally 2 on the drive side,1 non drive side on 68mm.
Mine lasted for 2 years like that.

Never had any trouble at all, surely this is the most common set up there is? Been well impressed with longevity on my XT BB as well – outlasted the hubs & freewheel on the same bike & still going strong…

2 drive side spacers is what Shimano want you to do, it's how they've designed it, and presumably the whole thing's been measured and tested like this…

here's a wacky idea: maybe best to follow the instructions?

My SLX one has been in 2 frames over the past 2 years so far, been ridden through all kinds of shite & water and it's still fine as the day I got it.

I have NEVER heard (not on STW where everything fails) of a HT2 BB shell failing or causing damage. I've heard of bearings failing prematurely due to incorrect fitting or crap sealing, but never the BB bearing shell.

5mm less thread inside the shell, and coupled with external bearings seems to me to result in more stress on the cups and premature failure?

Don't be daft.

OK. Convinced. 😀

I have to agree with Nickc….. Shimano designed it to work with 3 spacers across the BB for 68mm shells and 1 spacer for 73.
If you run to few spacers the insert wont fit, if you leave that out you'll get water ingress issues. You'll also have 2.5mm of axle sticking out the end which will mean you have 'floating' cranks. That really will give you a strange adjustable chainline!! Messy!!

imo stress is not an issue, water is

Arrh, I'd agree with this, I find them far more likely to seize up from water ingress than have the bearings wear out.

Crikey – I think we all agree… 😯 😕