Originally Posted by P10 
Just to throw a bit of light on the shim issue, the shim thickness selection even on a Hydraulic lifter head is still important as having a difference in clearance in total height will cause the rocker to sit on an angle and the cam lobe contact area is reduced.
The reason i bring this up is a head on a semi-race motor i had to fix up a while ago. The head had been removed and an engine shop machined the valve seats and heads but had not ground the ends of the stems to correct for the amount of material removed at the other end.
Now it was only 2 valves but they were a bit higher than the others and they caused the rockers to sit on an angle, which resulted in severe damage to the hardening and excessive wear on the new camshafts. I had to replace the cams again and carefully measure the height of the valves without removing the head.
As it was the valves that were too high were the ones fitted with the flat shim so i just got my engine shop to machine them down to what i needed. Motor is now fine but that was over $1000 to fix due to someone assuming it would be ok
In this situation it was easy to find a cause and im not saying it would be this far out on a stock head, but it's something to consider.
which is the reason why some of us stated that the only time you need to shim a head is WHEN you do a valve job
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