Bewy said

The problem is the shim itself. The open end of the U is allowing the oil to pump straight out of that gap.....rather than entering the shaft where you want it go. The shim needs to have a round or oval hole so that it seals all the gaps.
Every hyd lifter I have worked with has had well over 0.100" of travel, with the vast majority going to 0.150" or more. The pre-load can be anywhere within this range. So most likely the shims are not needed if they are being used to set pre-load. The lower range of pre-load might be to reduce the effects of lifter pump up, should that occur, but the correct fix is to use stronger springs to control the valvetrain.

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d55dave said

So….. you had not enough lifter preload, so you put shims under the rocker shafts. Would that not raise the shafts and decrease the preload even more? Also, decking the heads would increase not decrease the preload.

Am I missing something in your explanation?

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