W2 econo heads are not limited to .550 due to valve length....if you are limited to .550 it would be the spring height or coil bind on the springs...not the valve length
Was told by my machine shop that if I ran longer valves in those heads, the rockers will not line up with the center of the valve tips. In other words the rockers would ride on one side of the valve tip and not travel correctly on it, causing problems. Longer valve, higher spring height = rockers not riding on the valve tips the way they should. I hope I explained it right.
