GMC used both conventional and rotating or free valves. Back when we ran our GMC's wide open all day we used rotating valves. But for normal driving rotating valves aren't necessary. As soon as I get my fleet of trucks auctioned off I am going to build some GMC engines when I have the shop to myself. I'm not going to mess with rotating valves. The valve with the taper in the stem is for a rotating cap. The others are conventional valves. Y-Blocks use the same stem diameter as GMC and I will have to figure what is what.
