[MikeP]

Thanks for your input guys. Plenty more to think about there then! The motor is for Martin Gough's new Southern GT. He has a seventies seasoned block which is being line honed, decked etc. This will help in the dreaded SVA test as it will only need to not produce visible smoke to pass. He has an Eagle stroker kit for this block. The idea behind going for the internal balance is that we were told that the external balance over 6,500 rpm causes crank flex which will knock out the centre main and taper the end mains. The internal balance is "sweeter" running and much more reliable at high rpm and we all love motors which rev. The info came from two reputable race engine builders of long experience who say that all their vrace motors are internal balance. The cost of the internal balance could be about £600 plus depending on the number of heavy metal slugs (tungsten or Mallory metal?) which would be needed. The info we were given is that certain aftermarket steel cranks are more suitable for internal balancing as they have more material in the counterbalance weights and therefore need less slugs put in and that the crank we have may not be suitable.
Thanks for your info Chas as I am interested in exactly the kind of motor you describe for my car. Please continue to market away as much as you like - your ali motors look spot on.
I am uncertain about the mains girdle as the advice seems to be conflicting about the value. I do know however that a similar device on a racing Imp one litre engine raised the rev limit from 8,500rpm to 10,000rpm on a tuftrided standard crank with 125 bhp per litre. So I guess the jury is still out on that one. I take note of Chas's point that swapping dampers and flywheels is easier on a neutral balance set up.
Keep the input coming as my brain is now only doing 6,000rpm!
Thanks again
Mike