Quote:

Use a mic and measure the lower head bolt outside flange area at one of the corners. The range in stock thickness can be from .995 to as much as 1.015 for stock heads that haven't been milled.
Measure both heads and post the readings. You need to measure both heads and if one head does need milled mill the other so you have both heads the same thickness when done.

MLR426

this is correct. i've found the '906 heads to be 1.00 and the '452's maybe 1.1.