schiavonec

I spend some time searching and did not get the specific answer sought after, but I cannot believe this topic hasn't been covered before. Perhaps I'm not using the right search option(s).

I have a 1980s Centurian with a traditional loose ball bb with lots of crud and an annoying grit sound when rotated. (The bike is a fixie conversion - all weather beater, fun to ride.... bike) I have a shimano Un72 sitting in the parts bin. The shimano cartridge did not thread in easily and stopped about 6mm or so into the travel. I never force things through for fear of stripping the threads so I stopped.

My question is whether there is a TPI issue? (Visual inspection showed that they look similar.)

I'll take a stiff brush to really clean the bb shell threads this evening, but is the desire to switch this out ill advised for compatibility reasons? Anyone encounter similar difficulty?

Thanks in advance for any assistance,

Chris

USAZorro

Obviously, your Centurion objects to being used as a fixie

Actually, these are both Japanese, but the problem could be due to one of several things. It could be TPI issue, but I suspect there is an imperfection somewhere in the threads. First, clean very thoroughly. Double check the threads on both. One sign you have a threading incompatibility issue is the appearance of fresh wear on one side of the threads. If you see this, forget forcing that Shimano in there without re-tapping.

Next, look for burrs or signs of cross threading. Try putting the old one back in and see if it is tough at the same place. If so, the problem is likely on the frame. If not, look at the Shimano. If you find it, you might be able to use a fine file to remove the objection. Once done, try again. If it catches in the same place, apply force for about 1/8 turn and see if turning gets smooth again. If not, take it out and find something else.

Remember to always apply grease to all threaded surfaces, or the Shimano will likely be the last bottom bracket your Centurion will know for the rest of its life. That would be almost as much of a shame as running it as a fixie.

sydney

Actually, I would check for interference from the screw that might be holding the cable guide onto the bottom of the BB. FWIW, if both the BB and frame have English threading(there is no Japaneese) there will not be a TPI issue.

USAZorro

Correct. All Japanese makers that I'm aware of manufacture to the English standard.

schiavonec

All - Thanks for the feedback.

I don't think it is the screw as athe cable guide is on the top of the bb shell, but will check for other obstructions.

I'll just have to clean it up nicely and give it another go. The loose ball setup isn't a bad option, but had an extra in the bin and I don't mind playing around.

Thanks again

sydney

Actually, Shiamno makes Italian thread BB. And their square tpaer cartridges Are JIS not ISO.

USAZorro

Sydney. Thanks. That's the third thing I've learned today - makes this a better than average day for me.

sydney

Ya sure that un-72 is engish thread? Drive side should be marked 1.37x 24. If the old one came out ok the new one ought to go in ok. Put the left side cup in part way to act as a guide for the cartridge, and be sure the cartridge isn't cross threading.

schiavonec

Thanks again to all for the feedback.

Not sure why, but it just doesn't want to play nice. The old cup fits fine. Screws in and out with easy and needs very little pressure. After cleaning, the sealed unit went in a bit further with ease and then stopped flat. I checkd the threads and there were a few 'burrs', but it did not appear to be cross threaded. (nothing a file would or could fix.) Oddly, the un-72 has a slight flange like profile (very small, but discernable angle) and seems to seize up when the widest part starts to thread in (which seems to be more of a TPI type issue and the old unit is pretty flat, i.e. no flange.) Additionally, the threads on the original seem to be obviously 'deeper'.

Drive side is marked 1.37 x 68, but that is std shell size metric. The original doesn't have any marking at all.

thanks again.

USAZorro

If you have something else handy, I'd abandon effort with that Shimano bracket. If you really want to use it, it sounds like you'll have to get the threads on the frame tapped.

sydney

English threads on his bike should match english threads on a current shimano BB. I've yet to have an issue that way.Something else is going on and I have seen so mamy ways to screw up,I'm not gonna start guessing.

schiavonec

Found the problem:

The un72 is from mid 1990s (95 or 96). There is nothing wrong with the thread count and it is English thread. I stopped at a lbs and asked to see a 'newer' square taper un72. The threads match the cup threads exactly and are MUCH deeper. After chatting with a mechanic, he mentioned that they weren't tapping the threads all that deep back in 95; thus, the difference I noticed between my cup and the un72 in terms of thread depth is the reason (too much metal and not enough thread on the un72).

While grateful to know the reason behind the issue, it really just took a glance at a new un72 to figure out the problem. Not sure I want to drop 40 on a bike I paid $85 for though. It is more of a fun, beater fixie and the 3rd in my stable (although used more frequently than the others).

sydney

Maybe you have one that is off spec,but otherwise I am having a hard time getting my head around the idea that shimano was misthreading english BB for a period of years. I been using un52/72 english thread BB from that same vintage as yours with no issues. Shimano had also been turning out english thread BB for years before that,so they obviously knew the drill.

schiavonec

A pic is worth a thousand words and maybe the one I have is out of spec per your suggestion. When placed side by side with a newer one an obvious difference is presented regarding the thread depth.

Since you have one that works, care to swap?