Bottom Bracket Dilemma

[mgronow]

Hi

Ive just fought the cranks off and the BB out of an old Raleigh (its taken 3 days and involved drilling out one of the cotter pins and applying the blow torch but I got there in the end...)

Now the word on the street says that old Raleighs have 26tpi BB threads and who am i to contradict except the plastic cups were marks "FAG BSA 24tpi". Is it possible that in the past someone has just screwed in a plastic BB 24tpi into a 26 tpi BB shell would this be do-able, would it stay put?

Both the left and right threads have a bit of a bald patch. What should i do...tap 26tpi and by a phil wood BB...tap 24tpi or go threadless (theres a velo orange threadless that looks nice...)

Options as ever much appreciated.


regards
martin

[Brucey]

if someone has tapped the shell 24 tpi already then you will get about 8-10mm of 'thread' between bald patches with centres that are spaced 12.7mm apart (in theory).

People do screw 24tpi plastic cups in so that might be the answer too.

If (once clean) a 24 tpi cup screws into the threads then you have your answer.

Installing a threadless BB is an easy way out and saves any real hard work I guess.

cheers

[mgronow]

Hi Brucey,

The bald patches are roughly circular about the size of an index finger nail....the threads there are very worn and flat (almost non existent). One patch each side at the lowest point of the bracket...almost like theyve been worn off there by rubbing...


cheers
martin

EDIT: I lie...one patch at bottom and other side at top...much like an ill fitting BB moving??

[Brucey]

it is just possible that they are an artefact of original manufacture or perhaps from a previous BB extraction. Or like you say some dreadful wearing event of some kind.

I'd suggest trying a 26tpi cup and seeing how it goes in, if you have one. If threadless is a good option, do that; you can always revert back later on (although you may have to remove a few burrs from the threadless unit).

cheers

[531colin]

Sheldon Brown on Raleigh threading...http://sheldonbrown.com/raleigh26.html

The "bald patches" could be just bits where the bottom bracket shell is far enough out of round / oversize that the tap has just skimmed over the top rather than cutting a decent thread, its pretty common on cheap BB shells, much less so on decent cast shells, which are more accurately made as well as being thicker.

I would try a standard 24tpi cup, if you have one loose somewhere....if it fits, you are home free....I would rather do that than threadless if I could.

Edit......I think 24tpi plastic cups that had been screwed into a 26tpi steel BB shell would show some signs of distress to the plastic threads....as Brucey says, after about half an inch of thread, the steel shell is going to expect a valley where the plastic cup has a peak, so to speak.

[FixedGuru]

You really need to measure the inside diameters of the shell at different positions and check that the dimensions suit the bracket that you are trying to fit. If it has appropriate dimensions, you can try running a set of bottom bracket taps in to reform the threads. However, you are unlikely to find any recent cotterless chainset or bottom bracket will fit the frame, unless you choose to either tap it to the standard English thread (and ream down faces to 68mm), tap it oversize to the Italian thread (and ream down to 70mm) or fit a Phil Wood BB for 73mm width. You may still find that the chainset won't clear the rear chainstays with the reduced bracket width.

Threadless BBs won't usually help as they are usually for 68mm wide brackets, not 73mm.

Check all the dimensions before you start!

[ferrit worrier]

It's times like this when a thread gauge is invaluable. However a 1/4 BSF bolt has a thread pitch of 26 TPI and a 3/16 BSW bolt is 24 TPI, if you can find one of each and in reasonably good condition see how it rests in the thread of the BB shell. If the bolt sits flat that's the one, if it rocks it means the pitch is out and is incorrect. HTH

Malc

[mgronow]

Hi

Ive just tried and i really can screw this in by hand even though the threads are a bit dirty...does the thread look sound?


cheers
m.

[cycleruk]

The thread looks fine to me but what is the other side like?

[mgronow]

Hi

Thats the left cup your seeing, alas i mashed the right cup getting it off
I couldnt get the left crank off so i tried unscrewing from the right in an unparallelled act of optimism...which resulted in serious cross threading as the left crank pressed on the left cup and i carried on turning...

regards
m.

[531colin]

Sheldon says Raleigh 26tpi shells are a non-standard width.
Just see if your shell is 68mm, but I think that BSC plastic cup has been in a BSC threaded shell.

[Brucey]

most raleighs were about 71mm width so some (most?) threadless units won't fit without some adjustment. However some raleighs were wider than that, up to ~76mm.

It isn't the end of the world if you file ~1mm from the ends of the BB shell BTW.

cheers

[mgronow]

Hi

70-71mm is about what I measured. I've bought the same model BB from sjs so ill give that a go. I'm sure my LBS will face it for a modest sum but a nice flat block and a bit of emery cloth might do the trick...


Cheers
M.

[mgronow]

It's times like this when a thread gauge is invaluable. Malc

I just had a quick look online and theres a bewildering array of thread gauges available...would anyone mind recommending one?

thanks m.

[Brucey]

if you have anything else threaded 24tpi, that will work as an effective thread gauge.

Also an M6 bolt is 25.4tpi, so will either be too small or too great for 24tpi or 26tpi respectively; any lateral shift when 'rocking' in partial mesh will indicate which is the case.

cheers