jbrow1

I've put three cartridge BB's in three different older bikes. One went like butter, two had my hands up in the air in exasperation b/c they kept wanting to cross thread. Had the shop chase the threads and everything, but they just went so difficult it was very frustrating. Funny thing was though, the old bearing cups would thread right in no problem. Ended up getting the two difficult ones in and all is well. Just curious why the stock cups go in so easy and the new BB so difficult.

joejack951

Either the tolerances of the threads (new cups might be larger) or the lead-in for the threads isn't as substantial. I've witnessed the same thing FWIW.

FBinNY 

Often these problems occur because of imperfect axial alignment alignment of the threads on either side of the BB shell.

Traditional cone/cone BBs also need near perfect alignment, but poor alignment manifests differently because the cups don't engage each other. What would only cause marginally shorter bearing life with a traditional BB, can make it neat impossible to install certain cartridges.

If the cartridge BB is one of those where the two cups slide over the cartridge they can bind on installation if the threads are even a hair off. If either thread is off line, it'll cause the cartridge to be at an angle within the shell, passing too close to one side on the other side.

Chasing the threads often can't help because the tap tends to follow the lead of the old thread. Whenever I encounter one of the binding BBs I check by assembling each side separately and checking how the cartridge fits, and whether it's leaning close to the opposite thread.

When I find the problem I try one of two workarounds. One is to install the good side most of the way first, then try to work in the bad side. The other is to try to bring both sides in together, sort of splitting the difference. Either way, you need to stop bringing the left short of the estimated final position, then tighten the right before continuing.

If the thread mis-alignemnt is especially bad, I don't even try any cartridge system with cup/cartridge overlap.

jbrow1

Thanks for the replies guys. FBinNY, that's about the exact same process I had to use on the two that were so difficult. Thought the one was maybe ruined, but all is well. Glad it's not just me

zebede

I have always thought those plastic "cups" on cartridge bottom brackets were there because the mfg. saved money. Do you think that they also might be there to compensate for mis-aligned threads?

FBinNY 

I can't speak for the makers, but I don't think so. First of all, they don't help that much, if at all, but mainly because non-axial BB threads shouldn't happen.

Eons ago BB shell manufacturers started tapping both sides of the shell simultaneously on a purpose-built machine where the taps both rode on a single shaft ensuring axial alignment. It's only in the last few decades, with the advent of Tig welded frames, and CNC manufacturing that folks started tapping BBs in two separate operations. The problem is that a very slight error in fixturing can cause misalignment.

This is a problem that was licked almost a century ago, and only occurs now because folks forgot why it was done the hard way.

BCRider

This sort of thing is why I've always said that book learned engineers should have to come out of their offices and work the machines that make the stuff they designed and then work at putting the stuff together. And if the stuff is too big to do that then they should at least take a humility pill and talk with the labour guys that do the machining and assembly of such things. As you say it's all too easy to forget why some things were done that way.

Mondoman

^^ Goes quadruple for software engineers.

Kimmo 

...Don't forget this overlapping cartridge BB you can use in pretty much anything: https://store.velo-orange.com/index.p...-brackets.html

rccardr 

I always thread both sides of a cartridge BB in partway before attempting to snug 'em down.