How can I ID a crank? How do you tell if it is a Mark IV, Gen IV or Gen VI?

Are then interchangeable?

The mark 4 will be a two piece rear main seal crank. Gen5/6 are the same, one piece rear main. get the casting number off of it and look it up online, or post it here and someone will help.

BTW, you can use a two piece rear main seal crank in a one piece rear main block with an adapter. But not the other way around.

post up the casting numbers

Look up the casting number here http://www.mortec.com/

Where on the crank is the casting #'s?