So I am using shimano un73 (68x107) with 75's. The past week the non-drive side of my BB has been loose. So I went to the shop today, to find out that the shell is 70mm.

Will prolong use of this kill the shell, or continue to loosen?

I am thinking of just getting phil wood BB with the cups that fit 68-73mm.

Thanks for any advice.

Edit: two shops and no one has a 70mm in stock.

Rty to find just an axle intended for 70mm BB and same cups as yours. Campy ones of the right period will be marked SC-70-xx, but they come in 109 or 102, AFIK

have 1mm machined off either side of the BB shell?

what the hell sort of bike has ISO threading and 70mm shell? probably something got measured wrong, and your problem resides elsewhere. occams (sp?) razor.

****ed up, I know. Well it was a custom frame I got 2nd hand.

The one with stiffer BB. Like a custom TT bike for a pro.

I have a taiwan Univega with a 70mm BB shell with english threading. I'm using a UN-53 with one of those metal non-drive side cups (not the right term, I know) without the lip, it threads past the edge of the shell to hold that side of the BB snugly in place.

IHOP helps this.

I have an old fuji "special road racer" frame that had a 70mm english thread BB shell. I ran a 68mm sugino 75 BB on it for a couple of months, with the lockring barely hanging on by a thread (no pun intended). After repacking the bearings recently, the lockring gave up at the furthest point of a 25 mile ride. I had a LBS reface the shell so it's now closer to 69mm. Not perfect, but better.
Note that this will change your chainline slightly. Only 1 millimeter, really.

Oh, be sure to check the LBS' work if you go this route. The place I took mine to charged me $35 for doing little more than sanding the paint off of the edges of the BB shell. In retrospect, I have spent much more on this frame than it is worth...

Thanks for the responses, I am going to go w/ a Phil BB.