Having slight issues with crank/frame clearance, the non-drive side.
I have a 113mm axle.
If I get a BB with a 120mm axle where does the extra 7mm go? Will each side be 3.5mm longer or will one side be 7mm longer and the other the same?
I recon 3.5mm on the non-drive side should do it but obviously need that extra length on the left
Cheers,

Will each side be 3.5mm longer or will one side be 7mm longer and the other the same?

This. Could mess with your chainline though…I had issues with drive side clearance on a frame so tried 122mm axle which didn’t work well, compromised between clearance and chainline with 118mm in the end

the BB is designed to centre the axle (of any length) in a given BB shell.

So assuming you stick with the same shell size, then yes, you will end up with +3.5mm on either side of the axle.

There are some BBs though that allow you to shimmy the BB around in the shell. I have one, and cant remember the name right now. Posh one too.

One crank fouling and the other not?

Weird.

What cranks are you trying to fit?

That’s what I wanted to hear Stoner, thanks.
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Al, it used to be fine, but I’ve fitted a BB mounted ISCG adapter and guide which has moved the BB a few mm to the right, pulling the non-drive crank in towards the frame. I’ve got an HT11 BB, tried this with an X-type chainset, axle length is fine but it’s a triple spider, I have a single ring on the middle position and a bash on the outer, the holes where the bolts for the granny ring would go foul on the bolts of the guide.

Squares are Middleburn RS8s on UN54 Rob, the X-types I tried are RS8s with a Chris Bling BB (not in the spares box unfortunately, bit fancy for that, nicked off the XC bike)

Shimano E-type BBs will have an offset built in to deal with BB mounted mechs and chainguides, probably worth looking for one of those to keep it all centred.
Edit- or maybe a 73mm shell version if yours is a 68mm.