I use 5/16" valves whenever possible, but I have run into an issue that has me stumped with them. As some of you know, I entered a Y Block in the EMC competition a couple of times, and on the iron headed engine, I tried three different sets of heads with different valves and spring combinations. On one set with the lightweight 5/16" valves that were +.200" SBC, the lash would go away on a dyno pull with the oil temperature at peak, water temperature high. When the engine cooled off, the lash returned. The stems were growing enough to take all the lash out with heat. No tuliping, no seat erosion, no guide wear, no measurable loss of horsepower unless I did a back to back pull. Tried everything with adjustment, changed heads, problem went away. Still don't understand why that set of valves did that. Inferior metal or not heat treated properly was our guess. Joe-JDC