Shimano Bottom Bracket

JesseD

My bottom bracket finally gave up the ghost on Sunday and sounds like someone has swapped it for a bag of spanners, so I popped into my LBS at lunch to buy a new one.

The BB on the bike is 68mm english thread and had what I believe to be a Shimano Tiagra level Hollowtech 2 BB originally installed, however my LBS didn't have any in stock and as I needed it for tonight sold me a MTB BB part number SM-BB70 which has spacers included to move the width of the BB between 68mm and 73mm and said to try it as it should fit without spacers.

Anyone able to confirm if it will or won't work as if it won't I will return it and try another LBS to see if they have any in stock (the one I went to was closest to work).

Chainset is 105 (5800)

dgunthor

yes, will be fine. the cups have more threads (wider) to accomodate the mtb spacers is all - just leave the spacers off as they said

k-dog

The cups will thread in the same distance - because that's dictated by your frame - but the outer dimensions are slightly different - the road cups are about 1mm narrower.

So, the total overal width of what you have is about 2mm more - so you'd have to make sure that there's enough axle sticking through to be gripped by the NDS crank.

JesseD

Will that mean I will need to adjust my shifting on the front mech as the over position of the chainset will be 1mm over (I know its a tiny increment but it could make a difference?)

k-dog

Yeah, if it fits you'll need to look at the FD - 1mm is more than significant for that. You'll need to start from scratch - limit screws then cable tension.

arlowood

JesseD wrote:

Will that mean I will need to adjust my shifting on the front mech as the over position of the chainset will be 1mm over (I know its a tiny increment but it could make a difference?)

Just take it back to your LBS and order a road specific version. You don't need the hassle of faffing with front mech adjustment and/or crank shaft length

keef66

I googled the part no. and Jenson USA says it fits 68 and 73mm BB MTB shells but says it's not compatible with HT2 road cranksets. That's probably why.

JesseD

Fitted it and had to use a spacer (?) but it works and shifting is fine.

The centre plastic tube of the BB was too long so I just swapped in the one from the BB I took out and all is well.

Guess I was lucky!

JesseD

OK i am having issues with the BB creaking so decided to reinstall the bb using all the original parts and following the instructions by shimano and used the spacers as recommended however, when i went to reinstall the ranks the axle doesn't seem to come through the BB far enough and when I put the crank arm on the axle only comes about halfway into the arm which I fear is not enough and will cause damage.

If i take the spacers out the supplied plastic tube with the BB is too long for me 68mm BB and the one i was using is knackered as i managed to stand on it and cracked it.

How far into the crank arm is the axle meant to extend, my understanding was that it should be almost fully through?

mamba80

Shimano road BB's do not have any spacers, only their mtb equivalents on a 68mm BB, your using a mtb BB, then you dont need the spacers.

there is maybe 3mm max from end of BB axle to outside face of crank, so for you its not enough.

JesseD

This is what I thought however when the spacers are removed the plastic guide tube is too long for the BB shell, so looks like I'm back to the shop tomorrow to get a shorter plastic tube.

Thanks Mambo80

JesseD

Ok went back into the LBS this morning to see if they had a shorter plastic tube which they didn’t, the mechanic there said that the tube was there just to keep the water out of the bearings if it gets into the frame and that it is by no means integral to the working of the BB, so technically I could take it out and just use the cups in the frame and all would be fine.

Seems plausible to me given its only a plastic tube but can anyone confirm that’s all it is for, is there another use that I do not know about?

Would using the BB cups without the tube work, I am off to France tomorrow morning early and they do not have another BB in stock to fit.

Thanks

keef66

Yes, the plastic tube is there just to keep any water away from the spindle / bearings. And it keeps internal cables / wires away from the rotating spindle.

OTOH sometimes with internal cabling it's necessary to omit it because otherwise there's insufficient room in the BB shell.

So yes, fit the cups without it

JesseD

Great, thanks keef66