Justinc is on the money. It is lifter preload.

You need to search this forum on how to measure that and then buy or make shims to lift the rocker pedestals. It is done with a digital micrometer.

Just a tip - Be very careful torqueing down the pedestals. If you set your torque wrench incorrectly - say set the Nm value on your imperial foot pound torque wrench then the pedestals will crack as the engine reaches operating temperature, and you will lose oil pressure. This is what happened to me when I was rushing one evening plus distracted by other household matters. Lucky I have an oil pressure gauge and saw it go whilst idling in the garage.