BB UN-51 won't fit?

[Martini-night]

Does anyone here file down BB threads with success??
Hello, just replaced my bottom bracket from a BB-CS10/20 (I can't remember exact model and as i threw it out it had plastic threads on one side) to a better UN-51 ,but the UN-51 doesn't screw all way inn?? About 2mm space gap on Right side??
Tights perfectly all the way even by hand but on drivers side don't go all way through. About 2mm
I toke it out ,clean the shell etc etc but still won't thread all way through on drive R side..
Both BB are same measurements,lenght etc..
I'm no retro bike expert but I thought all shimano early-mid 90's bottom bracket were all the same?? Thread-wize?...
Or should I just file a few end threads on the BB to make it fit??

 

[d8mok]

Are the new cups and old cups the same threaded length?

Does your frame have shallow threads in the shell?

 

[Martini-night]

I don't have the old bottom bracket anymore.. Aye new one fits and tights perfectly all the way even by hand but on drivers side don't go all way through. About 2mm left. I toke it out ,clean the shell etc etc but still won't thread all way through..
I might file down the end threads on the new bb to see if works

 

[Martini-night]

Are the new cups and old cups the same threaded length?

Does your frame have shallow threads in the shell?

 

[FluffyChicken]

It would normally work as that looks short still, they do have and some other makes do have longer threads and I've seen plenty with the ends mashed off.

Do you have an all metal (old) cup, they're great for forcing through the end of the threads, often get rusted and dirtied up and can be rough.

If.you need to file them down go for it.
Do check it is not the flat part not jamming against anything as well.

 

[d8mok]

Yes if the cup thread is longer than the shell.

I tend to swap it for one that’s the correct size as I’ve drawers full.

 

[d8mok]

I’d start by measuring the thread lengths first though

 

[FluffyChicken]

Quick check the old BB and shell is 73mm, as that's a 73 not an 68 shell BB

You are fitting the drive side first? then the NDS cup is fitted.

 

[Ippikin]

^^Yes that is indeed the most likely explanation. OP measure your BB shell!

 

[Martini-night]

Quick check the old BB and shell is 73mm, as that's a 73 not an 68 shell BB

You are fitting the drive side first? then the NDS cup is fitted.

Aye she'll is 68..but still the BB should fit?? Picture: the BB I've bought and doesn't fit