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I have an issue with my BB / chainset which has happened twice now and I'm scratching my head. My set up is: Shimano GRX chainset, Ultegra 68mm BB, BB shell has been faced following delivery (frame is new, I have built the bike, I've fitted many similar set ups and never had this problem).
A few weeks ago I noticed play in the BB/Chainset interface (could rock cranks slightly back and forth (well side to side) in the BB under load). I stripped down, replaced the BB (I had fitted an old one so thought the bearings may be shot), preloaded using the Shimano tool to finger tight (and double checked) and tightened NDS crank pinch bolts to torque. Everything seemed fine for a few rides.
On my commute this morning I noticed a creakig bearing noise from the BB area and have checked the cranks and the play is back again. Only been able to do a quick check, but the pinch bolts seem tight still. I'll have to cycle home on it and can then strip and rebuild there, but given it has happened twice fully expect to have the problem return. Any ideas on what could be causuing this and how to remedy long term?
Have you measured the BB shell? If it's been faced it could be ever so slightly undersized. Should be easy to cure with a thin spacer.
Bottoming out the splines not preloading the bearings?
BB shell is not 68mm (a bit less?)
Try a thin spacer on one side just to check?
What’s the shell width on the frame?
BB shell ‘should’ be 68mm, will get the calipers out later and check. I did wonder about a spacer on NDS, sure I have a few in the bits box. If it’s that simple then great.
There’s not a huge amount of play, so what is the risk of having already damaged the splines (probably the biggest cost risk)?
You won't have damaged the splines as they were torqued up tight, or if you had damaged them you would have seen that when you swapped the first bb out.
The only other thing to check is if the crank axle is fretted in any way at the bearing positions that might allow for movement. This doesn't normally happen with HT2 though if you use shimano bbs as the plastic top hat prevents it. It's more common on dub or 30mm set ups with direct bearing to axle contact.
I did wonder about a spacer on NDS, sure I have a few in the bits box. If it’s that simple then great.
Check, but I think HT2 on a 68mm shell should have 1 spacer on the NDS and 2 on the DS (all 2.5mm).
Watching with interest as my monstercross developed an odd knock in the NDS BB/crank area during yesterday's ride when going for it uphill. Ten year old Deore crank on an allegedly upmarket Nukeproof BB bought during the CR fire sale. I noted some negative reviews at the time regarding the design lacking a top hat such that the crank axle sits directly on the bearings...
So an update. Made it home and dismantled. Callipers show 67.3mm so lost some material in the facing, found a 1mm spacer and fitted, however have noticed the drive side top hat has disintegrated, I assume under the movement of the axle. So new BB ordered which is a pain as the current one is only a few weeks old. Will fit that and see how we get in from there.
If the rest of the BB is OK you can replace the top hats
https://www.kineticbikebearings.com/top-hat-bearing-cap-for-bb-for-6805-bearings-in-bb90-95-hollowtech-ii-pack-of-2.html
Will order some of those for the current bb, thanks