Road Fan

I've been looking for a 113 mm BB cartridge to try to match my SR Apex chainset to a 1984 Trek 610 (C&V bike content!), and the local co-op gifted me with a very smooth UN73 with the right threading and spindle length. The BB length however is designed for a 73 mm shell - it's even marked on the label and I didn't see it. I haven't tried to install it yet to verify but I've had the problem in the past.

I think I only need a non-drive side cup salvaged from a worn 68 mm Shimano BB cartridge.

Any better ideas about a "minimum fix?"

Anybody have one of those to spare?

Soody

You just need two 2.5mm spacers, preferably metal.
Only 1 if the NDS cup isn't flanged.

It's the same size as cassette spacers.

Road Fan

Duhhh, you're right! And I even have a few of those in a parts sorter box!

Thanks a bunch!

Road Fan

So I tried it, and the 2.5 mm spacers have play relative to the drive side threads. I have not found a 3 mm spacer on the parts market, for whatever reason. I recall I tried this once before and the 2.5 mm spacer cracked due to the off-center compression stress due to tightening. How Shimano expected 68/73 to be a satisfactory replacement part is beyond me! So I don't think those spacers are a good idea, without some sort of packing to fill the gap or a slightly wider spacer. I don't think I get to customize my spacer without a lathe and good chops!

The "bike shop solution" is to get the correct BB cartridge, which in this case would be a 130 mm spindle 68 mm BB shell bb cart with English threading and JIS tapers. Shimano seems only to offer the UN300 now.

Does anyone have enough experience with those to comment on quality and durability?

There seem to be dozens of companies making square taper cartridge BBs today, and several are cheaper than Shimano. I really like the way those older UN71 and UN73 parts move, however. They also seem to have been hard to kill. I'm not against BB surgery to make a repair, but I'd like to get this 1984 Trek 610 back on the road.

squirtdad

I recently installed a UN300, don't have a lot of miles on it, but it seemed to be of good build quality, including metal cups, Does feel like a upgrade form UN26 and not far from the UN55

I would not personally hesitate to use an UN300 again

Road Fan

Thanks! I liked the 55. The UN 26 was all oversized steel, it seemed, but it turned decently well.

delbiker1 

In the line about the bike shop solution, did you mean a 113 mm spindle? I have a UN-55 with the spindle measuring 115 on my vernier calipers. It is barely used. PM if interested.

repechage

Sometimes gifts are not gifts. They are white elephants.
purchase the correct parts or assembly

Road Fan

In my town if I go to LBS and talk about any sort of a BB problem, such as in this case, what I think is a chainline adjustment, their proposal is to sell me a UN300 at full Shimano list price. In this case that part is an appropriate solution, but I've heard the same thing for bikes with vintage Campagnolo. When I say it's not suitable, I hear "Oh, no, I've done it before."

I may get back to you, but at the moment I think the 113 is the best solution. I'm trying to narrow down from a 118 mm spindle and I think it needs 2.5 mm per side to match the rear wheel chainline and to minimize the Q-factor. I took a free 113 from the local co-op, but it isn't a good fit since it is designed for a 73 mm bb shell rather than a 68 mm shell. It would have needed a 2.5 mm spacer on the drive side to fit. That might still work, but the ones I can find do not install centered on the BB axis.