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Hollowtech 24mm tolerance question

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Hollowtech 24mm tolerance question

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Old 01-12-19, 11:35 AM
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masi61
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Hollowtech 24mm tolerance question

If you are running Shimano Hollowtech cranks I was curious if anyone has run into a situation where the bottom bracket width varies by under or over 2mm (+) plus or (-)minus? And if the bottom bracket is a little fat, or a little undersized is it possible that the “crankarm fixing bolt” (that black plastic pretensioning bolt that goes on the non-drive side crank arm) can be let out ever so slightly?

i just don’t know if Shimano builds some flexibility into this interface to account for very minor variability in bottom bracket tolerances. Thanks for any insight you can add to the conversation....

Also, can triple cranks such as the Ultegra 6603 be installed without the proved non-drive side washer if this ends up being a better fit? I wasn’t sure if removing it might make the bearings more vulnerable to damage.

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Old 01-12-19, 12:05 PM
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Hmmm. You're asking about 2mm difference in width and also about it being a little bit fat.

I'm wondering if you might have an Italian thread BB.
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Old 01-12-19, 12:22 PM
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Originally Posted by Retro Grouch View Post
Hmmm. You're asking about 2mm difference in width and also about it being a little bit fat.

I'm wondering if you might have an Italian thread BB.
Actually I have a Klein that is 75.5 pressfit. I purchased cups for Shimano 24mm through Phil Wood. The width of my Klein bb, and the protruding part of the Phil Wood cups added up to 92.9mm

when I measured my 2 English threaded 68 mm road bikes with Shimano outboard bottom brackets, the width I came up with is about 91.6 for the double crank bike (1.3mm less), and 94.0 for the Ultegra triple bike (counting the big washer that triples require) or 91.2mm width without the washer ( so within +1.1mm or -1.7mm without).

Iím trying to get a handle on if Iím going to need to have the Klein bottom bracket faced or not.

If I install the Phil Wood pressfit bearing cups and it isnít plug-n-play, how hard will it be to knock them back out again?
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Old 01-12-19, 12:29 PM
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Your 'wiggle room' is approximately the size of the hole that the plate pin fits through on the spindle (it doesn't actually function as a retention plate, but as a go/no-go gauge for proper installation). 4mm, If I recall correctly. Shimano is quite good at manufacturing tolerances over their side of the spectrum, so assuming you have a proper width shell, and you are using the correct bottom bracket/spacer configuration, if will fit with room to spare. Basically, your marker is the black plastic plate with the protrusion on it that fits between the fixing screws on the LH crank. If it closes down and goes through the spindle hole, you're fine. If the bottom bracket shell is too narrow, you can add a spacer on the spindle (one/both sides), whereas if it is too wide, you can exchange the spacers you have for ones that are slightly narrower. While I wouldn't personally have a problem with adding spacers, the correct solution to a too-wide shell is to have it faced down to spec. This is trivial for a shop that has the proper facing tool (not the standard facing/tapping set, but a tool that only faces).

They do, in fact make Italian threaded bottom brackets, so it would also stand to reason that if you install it properly, it should fit. The triple spacer, provided it doesn't negatively effect the chainline or fitment, can be traded for something smaller. My guess, knowing the installation procedure is identical for both bottom brackets, is that the Italian bottom bracket has a 2mm shorter width.

If it helps, you can study the exploded view document here.
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Old 01-12-19, 12:37 PM
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I imagine there is some variability in tolerance. The fixing bolt preloads the bearings. So bottoming out the fixing bolt, but still having side to side play means the bearings don't get the support they need from side loads. They will self destruct quickly. It also means the seals won't work as intended. You can place a shim between the bearing cup & the frame to take up the gap if necessary. My Schwinn has a .030 shim for this very reason. It was easy. I just took some flat shim stock, drilled a hole in it with a hole saw, drew a circle around the edge, then trimmed the remainder. The result was essentially an aluminum donut or washer. I blame the combination of frame refinishing, old external bearing cups, American/BSA bottom bracket adaptor, and alkaline bath treatment so that I could polish. My situation is rather unique, but the need for proper pre-load is not.

If you need to install a mtb crank on your road bike with 68mm wide bottom bracket shell, Shimano makes 2.5mm spacers for this purpose. You would need 2 to install a mountain bottom bracket, 3 for a road bottom bracket.

To your other question: The spacer is necessary. If you look close, there is a ridge or an o-ring embedded on the inboard side. It seals out water, grime & muck. One of the reasons things work out so well inthe longevity department is nothing gets inside to degrade the grease or score the bearings. The hollow tube between the cups seals out water ingress from the frame side, the seal on the spacers (3mm non-drive, & 6mm driveside) seals out water ingress from the environment. The path it is blocking would be between the inside bore of the inner race and the axle. Then, of course there is the seal between the inner & outter races on the bearing itself.

Hope I helped.
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Old 01-12-19, 12:37 PM
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The amount that the LH arm fits onto the axle is variable. I suspect Shimano places the min/max WRT if the little peg on that "plate" that fits into the small hole in the axle. Having not measured the amount of fit range I speculate it's a mm or two. Andy
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