What causes the valves to rotate?   After pulling my heads and lapping the valves,  I noticed the ends of a couple valves clearly show the rubbing of the rocker arm straight across the end of the stem.   These valves were brand new when I put the heads on my engine 400miles ago.  On those valves I can see a straight up/down pattern on the shaft.   There were all new guides installed in these heads. none seem too tight to prevent rotating.   So are these valves bad,  or are the rockers?