crankholio

Due to the plastic NDS cup cracking on my touring bike, I needed to replace it. I decided to pull the bottom bracket while I was at it just to see what was in there. I had the crankset changed by a bike shop several years ago, and they would have had to change the BB to do it. So for whatever reason, they used a Shimano UN54 labeled as 73 on my 68mm shell. They put 3 1mm spacers on the drive side.

I had never actually looked closely at 68 vs 73 BB's, but comparing it to a 68mm UN55 I have, I noticed that other than the spindle lengths, everything was identical. The BB cartridge itself wasn't any wider on the 73. Kind of surprised me, but it seems like the only difference is they make the NDS spindle longer and then make the NDS cup deeper. The plastic cup that cracked on mine was sticking out a few MM as it wouldn't go all the way into the shell. I used the metal NDS from my 68mm UN55 to replace it, which went all the way in.

So am I correct in assuming the only differences between a 68 and a 73 are the NDS spindle length and the NDS cup depth? And they're basically interchangeable with a spacer on the DS? Looks like that's what the bike shop assumed.

Nwvlvtnr 

Your asumptions are correct, I recal in the days of ISIS bottom brackets many manufactuers only offered them in the 73mm variety with a spacer and instructions to use it on the drive side with a 68 mm shell. This setup can also be handy for adjusting chainline. The left side cup sticking out a little is not an issue.

As for replacement cub Be sure that the inner diameter of it matches the outer diameter of the left hand side of the cartridge, I recal the plastic and metal Shimano cups having different internal diameters.