whats the difference apart from the obvious cover?

are they interchangable e.g. black top head on silver top block?

any other differences

 

blacktop has solid lifters

 

you can change cam covers, but cams and followers can't be changed without significant cost, pulley setup for cam belt is different, blacktop has a better flowing head from the off though, nothing you can't achieve from a silvertop though with port work.

 

what would be the better head to use on a 2.1 silver top bottom end then?

 

Silver top head, as black top wont fit without a lot of work

 

Ok thanks for that, just save me a few quid there