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BB386 to Shimano Hollowtech 24mm

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BB386 to Shimano Hollowtech 24mm

Old 03-04-21, 03:21 PM
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doherty555
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BB386 to Shimano Hollowtech 24mm

I have a road frame with a BB386 shell (width 86.5mm). I want to fit a Shimano Ultegra Hollowtech 24mm crankset so I bought a Prime PF30 BB and a pair of Prime BB386 to Shimano 24 mm adapters. This choice was based on a prime BB/adapter guide on Wiggle.

The problem is that the adapters add a few mm to the shell width and now the crank spindle seems slightly too short. There is not enough of the spindle showing on the non-drive side to allow the crank arm to fully engage with the splines. Only the inner pinch bolt on the crank bears directly on the spindle.

Was it a mistake to go for this BB/adapter combo? Perhaps I should have gone for something like the ROTOR BB386EVO TO 24MM BOTTOM BRACKET.

Any other suggestions?
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Old 03-05-21, 01:15 AM
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cpach
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Is it far out enough that the little retention tab fits in the spindle? If so it's probably reasonably within spec. The preload bolt shouldn't bottom out on the spindle because it needs room to do its job. Hard to tell what's going on without pictures.

If you asked me though, I definitely wouldn't have advised you go with an adapter--you loose the opportunity to run larger/more durable bearings and you introduce another element to be out of tolerance and creak. That Hope BB looks reasonable enough.
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Old 03-06-21, 06:01 AM
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doherty555
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No the tab doesn't reach the hole in the spindle.
I'm not sure I understand your comment about bearing size. The adapters simply reduce the inner diameter of the BB. The original bearings in the BB386 are still in use. The adapters don't contain any bearings of their own.

I just don't understand why these adapters are even advertised and sold. If the flanges on them increase the effective width of the BB shell to the extent that cranks can't be fitted safely (ie the retaining tab doesn't engage in the hole) then why would anyone choose to use them? Maybe I'm missing something.
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Old 03-06-21, 10:29 AM
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trailflow1
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Originally Posted by doherty555 View Post
No the tab doesn't reach the hole in the spindle.
I'm not sure I understand your comment about bearing size. The adapters simply reduce the inner diameter of the BB. The original bearings in the BB386 are still in use. The adapters don't contain any bearings of their own.

I just don't understand why these adapters are even advertised and sold. If the flanges on them increase the effective width of the BB shell to the extent that cranks can't be fitted safely (ie the retaining tab doesn't engage in the hole) then why would anyone choose to use them? Maybe I'm missing something.
Have you tried removing the outer plastic covers on the PF30 bearings, then try insert the adapters ?
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Old 03-06-21, 11:33 AM
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doherty555
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Yeah, that did the trick. It didn't occur to me that the presence of the adapters did away with the need for those little outer bearing covers. Thanks for the suggestion.
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Old 03-07-21, 01:10 AM
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cpach
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You miss my point. The bearings in a conversion BB are larger than the bearings in a BB386 BB because they accommodate a smaller spindle in the same size bore and will have better longevity and lower friction all other factors being the same, and the adapter is another failure point for imprecise tolerance. This makes the adapter solution (slightly, probably) worse.
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Old 03-07-21, 12:38 PM
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Originally Posted by cpach View Post
You miss my point. The bearings in a conversion BB are larger than the bearings in a BB386 BB because they accommodate a smaller spindle in the same size bore and will have better longevity and lower friction all other factors being the same, and the adapter is another failure point for imprecise tolerance. This makes the adapter solution (slightly, probably) worse.
I don't think is necessarily true. I bet many Shimano to PF30 BBs use 6805 bearings, which are 25mm ID, 37mm OD and 7mm width (ie the same as standard shimano BBs). 6806 bearings, which are used in PF30, are 30mm ID, 42mm OD and 7mm width. Same balls, but different race size and more balls. The load capacity is greater on 6806, but that's hardly relevant.

There are 2 reasons adapters suck. One is that shimano cranks have a wider bearing stance than PF30, so using a PF30 BB with adapters means that you're not directly loading the bearings. You're applying a torque to them. Plastic adapters are... plastic. There goes your precious stiffness.

The other reason, as you mentioned, is that you're introducing another source of movement/tolerance issues. The adapters have to be perfectly toleranced to not cause any play etc. but do you really believe that they are?

It's much better to get a dedicated solution like wheels mfg or hambini.
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