I live in Houston, am an engineer, and have been intimately involved in maintaining fine auto engines for over 30 years. 2 sibblings and Father were also. So here goes.

IF you have a MBZ aluminum head resurfaced because it was warped--first question is--how much was it warped? These heads are flexible and will easily pull down tight if they are within or near warpage specs (0.030"). THe cams run in precision bearings in towers mounted to the top of the heads. If you remove metal from the bottom surface only to remove the 'warpage' --you end up with a head whose thickness now varies--and the TOP surface., when tightened down, now has the mirror image of the warpage that was removed.

MBZ supplies shims in peel apart stacks to fit UNDER the cam towers to bring the bearings back into perfect alignment. You can easily tell when its right because the bare cam will turn easily in its bearings with all the head bolts torqued down. If the cam does not turn easily--it will overheat the out of line bearing(s) and will eventually break from bending as it rotates--if it does not seize, break the drive chain and destroy the engine before it breaks and destroys the enbgine. Either way it is not a pretty result. It WILL happen if the head has been resurfaced and the cam towers are not shimmed back into perfect alignment. I have never seen a U.S. shop that can resurface the head properly (grind both sides at the same time to ensure absolute parallel surfaces.) This is because it requires removal of the guides and core casting plugs so the top surface can be ground as well.

I KNOW a lot of times the cam towers are not properly shimmed, and the engine runs--and many times the cams do not seize or break--but for how long? When I head about cams breaking, or timing chains breaking, I am pretty confident that a large majority, if not al, of these failures are caused by out of line cam bearings and the resulting bending and increased drag.

LOTS of heads are resurfaced unnecessarily when all they need is a good dressing with a flat edge wrapped with 100 grit wet or dry abrasive paper to uniformly clean up the sealing surface. It may take an hour in a bad case (and you do have to use a good straight bar to span the head crosswise) But its faster to throw the headback and forth on a blanchard grinder and presto-- in 2 minutes you have a nice pretty and clean 'flat' head that will cause your camshafts to break.