My son has a Norco B-Line and the ISIS BB is past its best.

What I'm not 100% sure about is how to measure the BB shell to determine which BB I need.

Its currently got the original Truvativ Howitzer (or so I believe according to specs) on, although I read on a thread elsewhere that the Howitzer is a non-standard ISIS BB - any ideas?

The bike currently has a Truvativ Blaze double 'n bash crankset, but I've bought this, mainly for the 165mm cranks, and its light and cheap: 

http://www.merlincycles.co.uk/?fn=product&productId=1441&categoryId=12

What BB would you recommend?

It's not just BB size you need to be aware of but axle length depending on chainline.  Most bikes these days seem to come with a recommended chainline of 50mm so axle length will differ fromm crank to crank & BB to BB.

If you remove the old BB it'll give you the width & axle length on it - in theory you'll just need to match that up to retain the original chainset.  To change to the FSA I'd suggest you get the BB width (63/73) and speak to Merlins & ask them which BB is suited for those cranks & chainline,

Shimano publish tables for theirs as there are so many combinations!

Thanks.

Sorry, what is 'chainline'?

I can get the drive side off, and the axle, but not the non-drive side.  I've a Park BB spanner and I'm turning it the right-way (according to the tighten up arrow)...  any ideas/tips?

I'll get the FSA on monday, so will try with the current BB first, and then call Merlin with 'measurements' - and ask for help.

Hmm, seems that the Truvativ BB is a different standard to everybody else's ISIS BB

So I called Merlin, and the guy I spoke with basically didn't believe me that it was a different standard, nor did he give me any real confidence to buy their ISIS BB. 

So after an extended session in my LBS, and a selection of BB's to take home and try, I ended up buying a new ISIS standard BB - and 2 new Park tools

And the Truvativ BB and crankset is consigned to the spares box.

And the little lad is happy, even without me mentioning that cranks were shorter, yesterday after a day at Chicksands he commented how his legs were making smaller circles and not catching the pedals - so a result.