Not being funny but you are doing it the wrong way round, Shimano shaved the flange off the left cup at one stage to stop people getting it confused with the right hand side cup which is the fixed position cup that you fit first, in fact the right hand side is fixed in place on the cartridg in most cases these days.
Put the right in first, wind it all the way in, then insert the cartridge if it's not already an all in one unit with the right hand side cup, then wind the left hand side cup down on to it until you have a happy medium between it being tight and not binding on the cartridge bearing. The right hand side cup is the one taking all the torque and locking that BB in place, the left hand just stops it contacting the BB shell under drive by supporting it. You can partly fit the left first as a receiver and guide for the cartridge as you thread it in from the right if you like but you must not wind it all the way in.
It is perfectly normal for part of the left hand side cup to stand proud by a few threads, brute forcing it in to be flush is acting like a worm drive and pushing the right hand fixed position cup out, if you keep doing that you will kill the threads on the cup. You should not be needing anything but a smear of grease with metal cups and effectively none at all with plastic ones as they are already Delrin/PTFE impregnated plastic, PTFE tape should not be needed.
Also check you are not mixing and matching cups from other manufacturers, some brands look the same and take the same splined tool but are just that bit smaller on the internal diameter.
I suspect you are having problems getting your BB's to stay put because you have been doing it the wrong way round, you have effectively been damaging the threads on the cups so I am not surprised you have experienced problems with them unwinding, keep fitting them the wrong way round and forcing the right hand cup home last and a metal cup could make the actual BB shell thread go baggy as well.
Not being funny but you either misunderstood my post or I did not make it clear enough.
I normally fit BB like this: Non drive first (finger tight) this normally goes a bit more than half way but never all the way.... the only way to do that is by using a BB tool.
Having said that, in this case I began to try all kind of things to get it to work and as it happens, I fitted the non drive side all the way to test the BB shell and threads on the cup ( both working correctly) - on this occasion I left the cup all the way in before I fitted the drive side with the same result as before.... (this is not the way I fitted the BB first time round)
I did not use lubricant for the thread part of the plastic cup but for the inside, where contact is made with the BB.
As I said before, the fault was with the BB..... fitted a new cup ( finger tight) went in nearly all the way, maybe 6 threads short..... then the drive side, all the way in, followed by tightening the non drive side. Both sides look fine now. After 50 miles ride everything is ok.
The bottom line, you are right, some people could easily fit the left side all the way but I was showed to do it the way I described before and I never had a problem.
Thanks for your post anyway, I'm sure it is useful information.
Andrew_s oooppps, looks like I was overdoing it, thanks god it's not a carbon frame
