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Hollowtech (II?) bottom bracket

Old 10-23-22, 03:44 AM
  #1  
alexk_il
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Hollowtech (II?) bottom bracket

I want to replace cheap Chinese single cranks on my mtb with double XT M785 crankset. The new cranks are a very tight fit for the existing no-name bottom bracket bearings. The BB is a bit tight for the old cranks too, but seem to work just fine.

Shall I just hammer the new cranks in or do I need to replace the BB?

Any recommendations for a decent quality BB in case of the latter? No plans to race, just building a commuting monster-cross.

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Old 10-23-22, 07:07 AM
  #2  
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Is the frame use a threaded (English) bottom bracket or one of the various press-in types? If threaded, any of the Shimano HTII models should work. That tight fit may be partially due to poor bottom bracket shell alignment. If the end of the spindle when inserted from the drive side doesn't align nearly perfectly with the nds cup before being pushed through, the parallelism of the faces of the shell should be corrected.
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Old 10-23-22, 07:59 AM
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When you say tight ?
Is the new crankset tight when offered up to a single bearing?
Test it in both left and right bearings.

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Old 10-23-22, 08:46 AM
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Seeing how Hollowtech cranks can use either a threaded BB or a pressfit BB, it might be nice to know what your BB shell is and even the model of bike so maybe the actaul size of the BB shell can be determined.

Shimano 2 piece cranks, some of which are hollowtech, should just slip right into the BB with no effort. Certainly no hammering required.
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Old 10-23-22, 09:37 AM
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I think alex_il means the spindle is tight as it slides into the bearing. It usually requires hitting the crank with the heel of your hand to get it seated properly. It shouldn’t require any more force than that, however.
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Old 10-23-22, 09:45 AM
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Originally Posted by cyccommute View Post
I think alex_il means the spindle is tight as it slides into the bearing. It usually requires hitting the crank with the heel of your hand to get it seated properly. It shouldn’t require any more force than that, however.
I've seen cranks enter either bearing without issue. But going all the way through is tight.
Bearing misalignment is to blame.
The fix for misalignment is very different than for bearings having an interference fit I.D.

So, alexk_il which is it ?

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Old 10-24-22, 03:32 AM
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Originally Posted by Barry2 View Post
I've seen cranks enter either bearing without issue. But going all the way through is tight.
Bearing misalignment is to blame.
The fix for misalignment is very different than for bearings having an interference fit I.D.

So, alexk_il which is it ?

Barry
No idea. How do I tell?

My jerk knee reaction is just to replace the BB. Is it not going to help? The BB is a threaded one with what seems to require a Hollowtech spanner.
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Old 10-24-22, 05:28 AM
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Originally Posted by alexk_il View Post
No idea. How do I tell?

My jerk knee reaction is just to replace the BB. Is it not going to help? The BB is a threaded one with what seems to require a Hollowtech spanner.
Slip the crank spindle individually into each existing BB bearing, but not through both. That will tell you if you have a single or double overly "tight" bearing in the old BB.

A new BB will help if it's a question of having an overly tight bearing in the old BB (e.g., one with a slightly undersized ID for the new spindle's OD). However, if the bottom bracket shell ends (faces) on your bike are not exactly parallel, the bearing bores on both cranksets (new and old) may not line up exactly - and the fit may also be tight when you try to pass the crank through the second bearing. In that case a new BB may not help much if at all.

Assuming your frame is metal, having the bottom bracket faced by a bike shop with the equipment and skills to do that would ensure the shell faces are parallel. But not all LBS do that any more.

If your frame is carbon, dunno what to tell you. Don't think facing one of those is a good idea, even if it's possible.
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Old 12-02-22, 03:02 AM
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Originally Posted by alexk_il View Post
I want to replace cheap Chinese single cranks on my mtb with double XT M785 crankset. The new cranks are a very tight fit for the existing no-name bottom bracket bearings. The BB is a bit tight for the old cranks too, but seem to work just fine.

Shall I just hammer the new cranks in or do I need to replace the BB?

Any recommendations for a decent quality BB in case of the latter? No plans to race, just building a commuting monster-cross.
OP here, status update.

Didn't have time to work on it, was too busy slowly working on other bike parts.

Anyway, ​​​​​​​the old BB finally failed yesterday, I replaced it with the new BB80 Saint, new cranks fit like a glove, all is sorted now.
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