Replacing UN72 with UN55

[mgronow]

Hi,

I've just been attempting to replace the UN72 on my partner's thorn with a UN55, to find that the left hand cup doesnt go all the way in by a lot...perhaps 5-7mm.

I've definitiely got the dimensions correct, which I took from the UN72 I took out (110mm x 73mm).

Any ideas would be very much appreciated.

Best Regards
Martin

[cycleruk]

Have you tried the old cup with the new cartridge?
Put the old and new side by side and compare them.

[beardy]

I would have expected a Thorn to have a 68mm BB.

Which Thorn is it?

It could be that it isnt the original BB and previous owners made a mistake. 5-7mm overhang fitting the difference. I can not see any reason why it will not be OK but the chain rings would be 2.5mm to the side of where they were meant to be, no big deal.

[gaz]

First of all, take a look at the respective LH cups. If you slide the LH cup from the UN55 onto the UN72 does it sit 5-7mm further out? If you slide the LH cup from the UN72 onto the UN55 does it sit in the right place.

Inspect the LH thread and if you have an old style steel BB cup run it in to test.

After that I'd be inclined to compare the UN72 and UN55 to see if there is any difference in shell length (mislabeled 68mm UN72?).

IMO the least likely possibility is that there was a washer fitted between the RH cup and the BB shell on the old UN72.

Edit - others have posted whilst I was gathering my thoughts.

[Brucey]

my guess is that a 73mm UN72 was respaced to 68mm by fitting other cups to it, and this is how the confusion has arisen.

cheers

[mgronow]

Hi All,


Many thanks for all the informative replies, armed with this info' Ive just attacked the beast again.

Here's a few interesting points...
The thorn (which we've owned from new) has undoubtedly got a 68mm shell (as pointed out by beardy)
The old BB seems is marked 73mm but has one of those LH cups that'll disappear into the shell if there's room (rather than having a lip to stop it going all the way in).
Ergo...the bike has had a 73mm bodged into a 68mm shell from new (as pointed out by Brucey)...nice one thorn....and presumably has been aymmetrical since then...

I found a 115mm x 68mm (UN55) lurking in the garage and thats gone in just fine (to my eternal relief) but I must say it doesnt seem to be a patch in quality compared to the removed UN72.

Many thanks for the help and inspiration...
m.

PS The 73/68mm UN55s have the same length body but different LH cups..I'm not sure what this means but online wisdom suggests some spindle lengths are aymmetrical which is very naughty IMO....

[thecycleclinic]

A good replacemant for the UN72 is the tange seiki LN7922 it also has a hollow axle and is the same weight.