Well, my guess is that it's a warm up transitional issue. The valves will heat up first but more to the point is that the head of the valve will heat up faster then the stem. That being the case, I would think the clearance gets tighter before getting looser. The 2 valve engines may have enough surface area at the seat to head to dissipate heat quicker so may not need as much clearance as the 4 valvers. Just my guess.
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