I'm doing my valves for the first time and getting all the measurements needed to order new shims. In doing this I found the exhaust valve opener clearance to be low (around .001"). I installed a new shim and re-measured, but I noticed that as the valve itself is rotated by taking off the opening shim and rotating the valve stem, the readings change. The opener clearance varied between .005" and .008" as the valve was rotated, but the closer clearance remained the same. What gives? Is the valve and or/seat out of round so I will need to remove the heads and lap it in?