I have a perfectly useable square taper BB over 10 years old but am looking to change the crank arms – so will new arms fit or is there likely to be a size diffence? (ie, size of hole/size of axel) Thanks.

Japanese taper and Italian Taper – slightly different angles

More info here: http://www.sheldonbrown.com/bbsize.html#square

So generally speaking if JIS (Jap) BB then stick with a JIS/Eastern made chainset

Aye campag cranks will sit further out on the axle than ja ones.

Almost certainly OK provided the axle length suits your new cranks.

However, there are two different square taper standards – JIS which is the 'usual'one – Shimano etc etc, and ISO which is Campagnolo. Depends who you talk to, but most people suggest you can put JIS cranks on an ISO taper and vice versa – but they will go on to different depths and there is a risk of cracking the crank taper.

It's an old shimano unit and i'm looking at fitting Truvativ isoflow arms – £15 from CRC, so probably all Jap.

Must type faster…

It's an old shimano unit and i'm looking at fitting Truvativ isoflow arms – £15 from CRC, so probably all Jap.

That will be spot on… And for the record JIS and ISO are not truly interchangeable. The JIS taper is longer, so you can fit (bodge) ISO cranks on a JIS taper (but they wil never fit a ISO bb again), but if you fit a JIS crank on a ISO taper they will work lose very quickly. Best sticking with the correct type every time.

the angle of the the taper is the same, it's just that JIS axle has a bigger cross-section, so an ISO crank on a JIS BB will sit further out than a JIS crank would, and conversely, a JIS crank on a ISO BB will tend to slide on too far up the taper.

Both scenarios should be avoided, given that BBs aren't that expensive…