Reversing a chamfered BB shell (Mavic BB)

[MartinB123]

Halfway through pulling the bits together to build a frameset up.

The existing BB is a tired Mavic threadless jobbie and I've learnt tonight that the bottom bracket shell would have been chamfered to suit (no wonder I can't get a BB to thead in!)

My question is, can this be rectified so the shell can be refaced/re-cut so a conventional bottom bracket can be installed?

Thanks

 

[foz]

Probably yes, but it would need the metal building back up again, then rethreading and facing. I imagine getting the threads lined up would be pretty difficult, unless one side can be done first and then tapped from the inside out... A modern threadless BB is probably an easier option

 

[mdvineng]

I've done it with a UN73 BB unit OK. Just carefully clean the start of the threads in the shell as best you can until the bb can be screwed in. I wouldn't go down welding metal on etc as cutting new threads to line up with the existing is a task you don't want.

 

[Nob]

Get Hambini on the case

It could be done if you have a machining background

 

[Jonny69]

Get Hambini on the case

Don't even mention that idiot.

 

[Nob]

Don't even mention that idiot.

 

[MatBH5]

I had a titanium frame with Mavic champhered finish. In that case, external bottom bracket cups (Campagnolo) threaded in (and out) just fine.

 

[onegoodbike]

Hi,

It might be worth checking to make sure there isn't a pre-existing problem with the threads in the B/B shell.

The Mavic bottom brackets were often used as a 'frame saver' option when the threads for standard B/B's were compromised in some way.

Hopefully this isn't an issue with your build, but thought it worth mentioning just in case.

Matt

 

[danson67]

The chamfering process only affects the first 3mm or so of the thread, and at the time, many BB shells came with the chamfer already cut, so the thread could still be all OK in there.

If the MAVIC BB was used because the thread was damaged, as @onegoodbike said, then there are a few ways of revitalising the thread.
In order of cost and invasiveness.

-Run a standard BB tap through any existing BB thread, if you're lucky it will be OK.
-If there never have been threads (Rare, but has been seen). Leave chamfer and tap to BSA both sides.
-Ream out damaged threads both sides and tap to Italian thread, find an Italian BB axle to suit.
-Ream out and tap both sides to Italian thread then fit ELDI BB thread sleeves to create the BSA thread. (Pictured).
-Add some material to any damaged areas with braze, then re-rap to BSA. This will damage paintwork, of course.

I can do any these, if required.

BB thread damaged on a customer's much-loved Nicolai.
Align the BB shell in 3 axes on the mill:



Ream out old threads:



Tap to Italian:



ELDI Insert:



Insert fitted (Loctite 638 Retainer):



All the best,

 

[MartinB123]

Wow!

The chamfering process only affects the first 3mm or so of the thread, so as @onegoodbike says, the thread should still be all OK in there.

If the MAVIC BB was used because the thread was damaged, then there are a few ways of revitalising the thread.
In order of cost and invasiveness.

-Run a standard BB tap through any existing BB thread, if you're lucky it will be OK.
-If there never have been threads (Rare, but has been seen). Leave chamfer and tap to BSA both sides.
-Ream out damaged threads both sides and tap to Italian thread, find an Italian BB axle to suit.
-Ream out and tap both sides to Italian thread then fit ELDI BB thread sleeves to create the BSA thread. (Pictured).
-Add some material to any damaged areas with braze, then re-rap to BSA. This will damage paintwork, of course.

I can do any these, if required.

BB thread damaged on a customer's much-loved Nicolai:

View attachment 678609

Ream out old threads:

View attachment 678610

Tap to Italian:

View attachment 678611

ELDI Insert:

View attachment 678612

Insert fitted (Loctite 638 Retainer):

View attachment 678613

All the best,

Wow!! Looking good and sounding positive.

It's already Italian (Basso frame) - I'll take down to my LBS at some point and see if they can re-chase to start with and go from there. I guess I could always get another Mavic BB fitted worse case.