It always annoyed me that flatheads weren't flat. Recently I read someone moaning that they should be called 'under-head valves' which I liked. Anyway I took mine down to Barry at precision engineering and played with his mill.
Barry, the master.
Before & After
The heads are both similar number 5's but one had a lot more flaws in the casting. The aluminium was also thinner where the bolt passes through: 30.5mm compared with 34.5mm on the better head which meant the bolts stuck out at different heights, another thing to fix. I later found that one of the bolt holes on this cursed casting didn't even line up with the cylinder hole, about 1mm out. Odd as they looked like a perfectly matching pair.
It also annoys me how the alloy heads are bigger than the cylinders. The top profile of the cylinder has these beautiful curves and scalloped sides but the alloy heads have straight sides so they overlap and look like an oversized mushroom head. The iron-heads are curved to match the cylinder.
So, getting a bit carried away, we cut some scallops out of the sides to give a more curved profile.
Filed the corners a bit.
Polished the cut edge with a flap disc. Parkarised head bolts.