that is the main reason why OS Speeds are so fast compared to most others...having the head tight like that makes a massive difference in performance.. An engine can have the best port timing, the fanciest crankshaft, all the bells and whistles and if the head isn't tight enough it will all add up to nothing...Its amazing having a dyno and being able to see how much effect the shims have on power...egnines that are known to be mild can simply be over shimmed and can turn into total beasts just by lowering the button by a shim or two.... conversely add 0.3 mm of shims to a OS Speed and watch what happens to it ! will go from wild to mild instantly......