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Odd BB issue on Merckx carbon frame

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Odd BB issue on Merckx carbon frame

Old 10-20-17, 06:54 PM
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Odd BB issue on Merckx carbon frame

I'm startng a build of a Mercks LXM carbon frame, and ran into an unexpected issue with the BB. I'm using Campy Chorus carbon crankset (50/34) which is an ultra-torque crankset. So the BB cups screw into the BB (the DS cup has a left-hand thread) and all seems OK. However, the crank teeth (Hirth joint) don't meet in the middle. I used the binder bolt and was able to draw them together, but there is play between the left and right cranks indicating that they aren't fully meshed.

Hmmm. That's odd. Definitely English thread, the frame is from 2008. measured the BB width, and it is a bit over 70mm. Didn't expect that. I thought that 68mm was pretty much the standard for road frames now, and this frame isn't THAT old.

Seems like the best approach is to face the BB on both sides down a mm so that the overall width is 68mm and the chainline is unaltered.

Have I missed something? A quirk with the Merckx carbon frames? Or just this one?
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Old 10-20-17, 07:21 PM
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I suspect that the thread in the BB shell are misaligned.

I'm surprised to hear of that in an old Merckx, but anything is possible. You might try this alternate method.

Install the two bearing cups, but don't tighten them fully. Then install the cranks and tighten them. The spindle will pull the cups into line using the float that usually there. Then finish by tightening the cups.

Keep in mind that this is a kludge, and the misalignment will increase bearing wear. The right answer is to use a proper BBC tap set, where the taps areally kept aligned by riding on a common shaft (search for an image of a bottom bracket tap set).

Hopefully, the misalignment isn't too bad, because the taps Willa shave the threads in order to improve pose proper alignment.

Also have the shell faced because sometimes that's the issue. When the cup is fully tightened it bears on the face which cams it out of alignment.
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Old 10-20-17, 09:19 PM
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Originally Posted by FBinNY View Post
Also have the shell faced because sometimes that's the issue. When the cup is fully tightened it bears on the face which cams it out of alignment.
I'm going to the LBS tomorrow to have the BB faced and narrowed to 68mm.

In addition to the BB being wider than 68mm, I also suspect that the faces are not parallel. Everything I could see and the resistance I felt when tightening the cups makes me think the threads are good. When I removed the cups the threads were clean, so I'm fairly sure the issue will be the BB faces aren't parallel.

I'll probably ask them to chase the threads, just to be thorough. Hopefully it will fit properly and the crankset will fully mesh and turn freely.

Then it will be on to the next build issue - isn't there always another one waiting in the wings?
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Old 10-21-17, 10:43 AM
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Sure it's not an Italian threaded BB? 70 mm sounds like it. It wouldn't be threaded lefthand but since Italian is larger you could probably force a left hand cup into it. Googling around, sounds like it should be Italian.
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Old 10-21-17, 12:22 PM
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Back from the LBS - the threads are definitely English and are nice and clean. They don't have a facing tool, and they measured the BB at 70.5mm wide. I found another shop that can do the work, but they aren't close and are closed on Sunday and Monday. So it's on the road on Tuesday to get this work done.

I'm tempted to pick up a facing tool, but considering that I will probably only use it (maybe) 3-4 times, the $150+ price tag seems more than it would be worth to me.

I'll report back once the BB is faced and the crankset is installed to let you know how things go.
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