There are currently no tools available for measuring this gap. However, we have found that the best item to use is a length of welding rod with one end bent at 90 degrees. You will need a measuring device, such as the vernier shown, to select/create 2 welding rods; one that measures at 40 thou., and the other that measures 60 thou. in diameter. This will give you an idea as to what shims you need to fit under the pedestals to set the pre-load.
Once you have ascertained the measure of the tappets, pre-load adjustment is achieved by fitting special pre-load shims under the pedestals. But beware, the rocker ratio is 1.6 that of the clearance that needs correcting, so a 16 thou. shim will make approx 25 thou. difference.
You MUST find an average shim size, and fix it to EVERY pedestal on that bank. So, for example, if you find that you have 4 tappets at 70 thou. and 4 at 80 thou. you would put 15 thou. shims on all the pedestals. Making the 70 thou. tappets 46 thou. and the 80 thou. ones 56, thus all the pre-loads are within, or closest possible to, the allowed tolerance.