What did I do today? Put the head on, again. It went on a few weeks back for the first time. I had checked the seal on all the valves with a small vacuum pump and gauge. All good. Put the head on and all the other fruit, checked the clearances. needed a pile of shims, no problem. And some more. grrr. Engine felt ok as I rotated it, one very very small tight spot, once, then fine. mmm. Carry on. A final vacuum test found no seal on no.3 intake, but clearance is good. wtf? Shoot, I must have not had enough clearance originally and bent the valves. GRRRRRR. Pulled it off, pulled out no 3 valves. One tiny bit of crap on one seat. cleaned it off, blued ok, reassemble and now a good seal. No bent valves, happy man. So now the head is back on, back where I was two weeks ago. Carry on.