Question - tune AC38 cups - depth

Discuss light weight issues concerning road bikes & parts.
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Bill F
Posts: 56
Joined: Thu Jan 08, 2004 5:08 pm
Location: Whitecourt AB CN

by Bill F

I am just getting ready to install my AC38 bottom bracket and have noted that the thread depth of the cups exceeds the thread depth in my bottom bracket shell. Do I need to tap the threads in deeper? Frame material is 7005.

popawheelie
Posts: 1021
Joined: Thu Sep 26, 2002 3:42 pm
Location: in Colorado

by popawheelie

That would be the strongest way to go. I was thinking of shortening the cups but they should support the whole bearing. If you were willing to risk it you could see how much of the bearing wouldn't be supported. Maybe 75% or more would be O.K.

by Weenie


Bill F
Posts: 56
Joined: Thu Jan 08, 2004 5:08 pm
Location: Whitecourt AB CN

by Bill F

On looking at the bottom bracket I agree with your advice and thank-you for it. With the design of the bottom bracket and cups reducing the depth of the cups will increase the potential for exposure of water/dirt into the bearings as well. The cups completely cover all the bearings/washers right over the shell of the bottom bracket itself. I would suspect the mechanical integrity of the bottom bracket would be compromised. I was fortunate enough to find a frame builder - Arvon Cycles (builds custom tandems/touring bikes and retro bikes for museums from steel) 250 km's away from home (not too many shops around here have proper taps for bottom brackets I found out) that was willing to help me out and tap the threads deeper. Charged me 20.00 dollars Cdn. It was well worth the trip. He is 70 years old and works out of a small metal shed beside his home on a gravel back road. He is a retired (sort of) Oilfield Machinist. He rode competitively when younger and competed in cycling in the 1954 Commonwealth Games. He still rides over 300 km a week. It was incredible - he forms and shapes tubes as well builds many of the mechanical components (hubs, bottom brackets, etc) on a metal lathe and milling machine. He builds bikes that can withstand very rough conditions on epic bicycle trips and adventure racing that allow easy repair on the road with a minimum of tools. My initial frustration with the problem was offset by meeting another fascinating individual in our world bicycling community. The guy was a regular renaissance man!

In future I’ll make sure I am more careful in specifying the depth of threads when having a frame built for me. Thanks for your help.

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