To correct the pre-load, install (or remove) shims as needed between the head and the rocker pedestals (equal amounts under each pedestal on a head). Typically shims are between 0.3mm and 0.7mm thick depending on supplier.
It is IMPERATIVE that the smaller hole in the shim be fitted over the oil feed hole in the head!! Otherwise no oil will find its way to the rockers and the rocker shaft will wear very quickly! Click on picture to the right for better view