nunavutpanther

I'm upgrading the bottom bracket of my 1980s Motobecane, which comes with a squre taper BB, 68mm. I want to upgrade into Sram GXP team English BB. the drive side does not completely threads in, but it just goes half way in (see the photo). The non drive side threads in completely. As the bike is very old, I cleaned it deeply with DW 40, dried it out and then put grease before threading the BB in.

Any idea what the problem could be?

philpeugeot 

Sounds like you might be trying to screw an English-threaded bottom bracket into a Swiss-threaded bottom bracket shell. Don't try to force it in, you'll have to source a different bottom bracket.

nunavutpanther

I measured the old and new ones and both have the diameter 34.8mm. Shouldn't the diameters be different if one is English the other one Italian/Swiss? Though the old bottom bracket is shorter (photo below).

merziac

@nunavutpanther

Nope, Italian is bigger, all others are effectively the same but different pitch, direction and orientation.

You will want to stop right where you're at and carefully evaluate the threads, doesn't take much to muck them up and keep you on the road to ruin.

Narhay

Does the old cup go in nicely now or not?

merziac

Good question, time to make it a chaser if not.

nunavutpanther

It does go in by just hand force as much as it did (pic attached); Since it's very old it took DW 40 and some effort to take the old one out. So I still haven't destroyed the thred shell?

squirtdad

what if any markings (stampings) are on the cup?

nunavutpanther

I didn't find anything!

merziac

You should be ok, I would encourage you to make that cup into a chaser.

nunavutpanther

I see; So you suggest that the problem is with the thread shell not being entirely smooth. And the thread shell should be English? How can I figure that out? Also the old cup is shorter and I think the new cup goes in smoothly as much as the old one used to go; So does making the old cup a chaser help with that?

3Roch

Based on my limited (3 bikes) experience with Motobecanes of that vintage you might want to look in the bb shell to determine if the threads are fully cut far enough into the shell to accept your new bb cup- which has a much longer threaded shank than the original..

nunavutpanther

I just measured and there is enough threading in the shell. Though you might confirm that Motobecane of 80s have English threading?

3Roch

actually the one with the closest vintage (80) has french threads however it's a Le Champion and threading seems to vary with models.
The reason I brought this all up is the fact that I had used a VO french threaded bb in a earlier LC and it worked out fine but in the 80 the threads did not go as far into the bb so I had to use a "normal" fixed cup etc. Remember that the threads are likely not cut full depth at the far end of the threaded section.

merziac

Maybe, was`the cup you took out fully seated before you started? If so then the shell is compromised, if not then you may have the wrong cup, bad threads or both.

My big point is, if you have the right cup and the threads are compromised from trying the new cup, then a chaser made from the original cup may fix the threads if you are careful. I use anti-seize and one step 1/4-1/2 turn forward, two steps back to work through the bad threads. Tedious to be sure and I actually use a right angle pick to scribe/clear the bad threads before I employ the`chaser. All of this is dependent on being certain you have the correct cup to make the chaser out of. It can be used after the fix if necessary but I always find another good cup and keep the chaser for future use.

Hondo6

If the frame is indeed Swiss-threaded (and reputedly at least some Motobecanes of that era were), you may be out of luck in finding a new bottom bracket at all - particularly if you're looking for a modern external-cup version. My understanding is that that BB standard is now completely obsolete and was also rare even 30-40 years ago. But perhaps someone with more knowledge than I on this forum can point you at a source.

Sunlite does make a "repair bottom bracket" that is threadless and intended to be used in stripped bottom bracket shells (Velo Orange used to carry one, but I think they've quit doing so). But I don't think Sunlite makes it anything but JIS square taper. And your selection of spindle lengths might be limited as well.

hokiefyd 

I once owned a 1982 Motobecane Jubilee Sport with a Swiss threaded bottom bracket.

Mr. 66

Have you tried putting the two cups to each other to compare the threads? Also the outside of the drive side cup will have no ring, or ring(s) around the spindle opening that designate the threading on your drive side cup.

3Roch

Here's a thought...
will the original non drive side (adjustable) cup screw into the drive side (the side you are having problems with) of the bb shell? If it will not likely you have swiss threads.

merziac

This far down the line, JIS/ISO is mostly of little consequence.

Most of the crank arms have been on and off enough times to stretch and conform the square taper.

It can even help in finding a spindle to fit an odd chainline, chainstay, chainring set up if it moves the arm in or out for being one or the other.

Hondo6

You may be correct if the OP re-uses an existing or used crankset - or not, depending on that crankset's precise condition.

However, his OP indicates he's trying to install an external-bearing BB in order to use a (presumably recent) SRAM crankset. That implies he's looking at replacing his existing crankset.

If he finds he has to use a repair/threadless BB and opts to install a new square-taper crankset - or a used one that hasn't been pulled/reinstalled many times - then the BB taper may matter. It likely won't be an issue if he tries to install a new square taper set; best I can tell, most of the ones being made today are JIS. But if he tries to install an older European crankset - either NOS or "lightly used" - it could possibly be an issue.

Vintage_Cyclist

Phil Woods used to make a Swiss threaded outboard bottom bracket cups, specifically for use with SRAM GXP. Don't know if they still make it. Pricey of course; probably more so than that frame.

merziac

This has been discussed at length here many, many times, even with new parts many have been mixed and matched over the years, with little or no drama.

The OP is planning on using a modern outboard bearing crankset so this is mostly moot.

Again, my main point is to use the original fixed cup to make a chaser if it was originally correct and will be able to repair the threads if they are compromised.

I will also tell you there would be no need for a hack/repair BB if I were working on this, I have never failed to restore any BB threads no matter how bad they are messed up.

Hondo6

No argument regarding using a chaser to restore the threads if compromised, and I wasn't implying that your advise wasn't sound. I'm also certain you could restore the threads if they're damaged. (Not sure if I could or not, never had to do it so I haven't tried.)

But if the OP (1) indeed has a Swiss-threaded BB shell, (2) can't find another Swiss BB (or can't afford a Phil Wood BB, which I understand can still be had with Swiss cups), and (3) can't restore his current BB to usefulness, he may be left with precious few other options if he wants to keep using the frame. One of those options would be to use a repair/threadless BB.

My intent was simply to highlight one potential "gotcha" if using a repair/threadless bottom bracket: it's very likely available in JIS square taper only. You're IMO likely correct that that limitation won't be a problem. I've always thought it best to know about a possible issue in advance and make an informed decision regarding accepting a potential risk, even if that risk is small.

Erzulis Boat 

This is a common problem.

Although it may appear that you have enough threads in the BB (on the frame) to install the "modern" component, you don't.

1- The BB threads (in the shell) of many vintage bicycles were deep enough for the fixed cups, but are not deep enough for the extended internal lengths of "modern" BB's.

2- What I do is to lathe turn the modern BB to thread root diameter for the portion that will not thread in. As stated above, it may look like you have enough thread, but the original BB tap had a pretty generous taper (to allow easier threading for steel) and it simply doesn't go in enough.

3- Modern bicycles have a lot more useable thread in the BB's due to the design of the aluminum system.

4- I have had to do this quite a few times.