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I bought a Longitude 2016 last year but ever since I've noticed that the bottom bracket comes loose after around 200 miles and needs tightening. By this I mean not the cranks but the little circular hub which is used to tension things up - sorry can't explain this better but I think it's an 8mm allen key and I notice if you over-tighten it this stops the cranks from rotating freely. (I'm tightening this first and then the crank arm correct?). It is a hollotech type BB. Would replacing the entire thing help with this? Is it a common issue?
Thanks
FM
If you're talking about the 'pre-load thingy' you could either remove it once you've tightened up the crank or nip it up once the crank is tight. Once the crank is tight I don't think it serves any purpose.
If you over tighten any screw the threads will stretch and it will then work loose. If its just there to add the preload, then once you have tightened (to the correct torque setting) the crank, you could remove it.
I thought the final tightening of the crank arms actually gripped the torque cap. Hence loads of people strip the tiny hex nut.
My bikes: Hollowtech II
Very important: to re-tighten the crank a couple times.New BB: I torque the crank to the recommended torque (torque wrench) and bike a couple times. Re-torque ... bike again a couple times and re-torque. 3 times or so until the torque will stay.
This plastic "cap thing": looks a bit different on my bikes - but assume the function is the same.
When everything is new assembled, clean, greased (chain not on yet) I use this plastic cap thing to push the crank (axially) a bit onto the BB bearings. Very little (close to none) torque to be used. When you rotate the crank you will feel the right amount. The crank should spin smooth and freely - but still there needs to be a bit force axially onto the crank before torquing the above mentioned screws. Reason: there is a very small seal vulcanized to the crank and this needs to push against the BB to prevent dirt from getting into the bearings...
Once the crank is torqued the plastic cap thing is clamped and won't move any more.
Many forget to re-tighten the Hollowtech BB. In the Shimano manual it's mentioned that it's necessary to do so. If this procedure is not followed the crank might move a bit "away from the BB" and then it's easy for dirt to damage the BB bearings...
The 8mm Allen key thingy there only needs a very gentle nip up. The pinch bolts need to be tight. Use a torque wrench, 12-14Nm iirc.
Check the pinch bolts aren't coming loose (again with torque wrench) for a few hundred miles, expect to see.some movement on them but if the crank keeps loosening after a couple of times nipping it up you've wrecked the splines by letting the pinch bolts get loose and riding on it, and need a new crank or cranks.