Hi,
Thanks for all the advice. I'm going to ask them to sonic check to see if I have room for 4.13 bore.
The machine shop I'm using can balance for me. But the question of going from interal to external balancing was something I wasn't too sure of.
As far as I know the 390 crank has a sort of 'nodule' to balance and the 428 balances against an off-balance flywheel
I've asked George if he has any 428 cranks but if I need to replace the fly wheel as well that pushes the cost to far I think (since I also have to add on the cost of transporting from US to UK and pay import tax on the cost including carrage!)
Bruce:
In answer to your question, The external balance is on the fly wheel. Unless the engine is not in a car you will not be able to see that! Even if you did see external balancing it dosn't mean its a 428 and not having external balancing dosn't mean its not a 428! Most of the block markings are nearly useless for definately identifying an FE as a 352, 390, 406, 428, 427 etc. So unless you can do some bore and stroke measuring you can never be sure.
I don't think i'm the first person to find out his '428' is a 390 and I bet I will not be the last!
Thanks everyone
Paul