UN300 - less than 300 miles and it's toast

[Tootyred]

Just wondered ........thought the pressure levels on the crank might be exceeding the rating of the bearings?

 

[Ugo51]

I have strong legs...sadly not that strong to destroy bearings in a few hundred miles

 

[Tootyred]

But in all seriousness that does seem very odd. Unless it is loose, one other thing could be the faces of the bb are not parallel? Although they would need to be some way out as the threads would still be in line...but it can make them come loose.

 

[Ugo51]

You know...it might.
It would be hard for me to check that, but I'll have a go.
The original BB was so tight that it took a 1m long extension bar on the wrench to loosen up the NDS. It was insane. Maybe that's the only way to keep it tight, although with the English thread, it shouldn't really come loose.
Anyway, I'll check tonight if possible.

 

[Woz]

That is odd - there's a mechanical issue or you are pedalling wrong. Maybe take up canoeing or darts instead?

 

[Woz]

Tootyred is spot on, but re-read what he says about the faces being someway off to cause this issue.

The older UN5x are essentially a hunk of hardened steel, and made with good precision. While it's not out of
the question you could have purchased a bad one, I'm starting to think the cause may be more related to
your assembly technic rather than the bike frame causing the damage.

Use plenty of grease during the assembly, start by inserting the cartridge on the DS, wind in several turns, insert the NDS
cup, check it all centres reasonably well, wind in several times. Then go back to the DS and tighten up. Don't be shy here,
make sure it's properly nipped up and the edge is firmly against the BB shell. Then go back to the NDS cup which demands
more care to find the sweet spot - too loose you ruin the BB - too tight you ruin the BB.

When you just start to feel the NDS cup butts against the rest of the cartridge STOP. Check that the spindle turns freely.

Add another half turn to the NDS and STOP. Check that the spindle turns freely.

If it feels notchy, you have gone too far and already ruined the BB; basically created dimples in the bearing grooves.

Continue like this reducing to quarter of a turn - there will be a point where you will feel the spindle turn ever so slightly less freely; at this point
you must STOP and undo by about a quarter of a turn. What this means is there is sufficient compression on the cartridge
to keep it all in place (will not undo when riding) and a tiny amount of bearing pre-load (will not develop immediate slop, but will
run in with time, and eventually wear out).

It's difficult to describe it any better, someone else may have other technics, but this technic has worked well for me over many years.