Snapped rocker shaft

long running saga going on, but, after the radiator split in august causing the head gasket to fail ( spectacularly I might add), I found that the old head was best used as a door stop.
so:-
I replaced the head with a new AMC one.
new metal head gasket
new head bolts
and so on
all either G spec or genuine
cost a bloody fortune
I then spent a leisurely few weekends rebuilding it.
and it ran like a box of spanners, took the rocker cover back off, found the valve settings to be MILES off, reset them with much mumbling to a spot on 0.2mm.
engine ticked over like a sewing machine.
went for a test drive, got about 1/2 a mile when I came to a spluttering smokey ( as in black un-burnt fuel) halt.
towed it home and stripped the rocker cover off again and found the rocker shaft snapped off between valve 6 and 7 at the bolt hole.
before i pull the head off for a second time has anyone got any ideas what happened?

Possibly caused by the piston hitting the valves (are pushrods still straight?) Did you measure piston protrusion and valve stand down on the new head to work out what thickness head gasket to fit?

well it turns out that I shouldnt be left in charge of anything shaper than a broom - found the fault and it was entirely my mistake - hadnt fully tightened the pillars down to spec.
Note to self, read the damn workshop manual before starting the job.

You probably did, the studs will have stretched before the camshaft broke.

damn decent of you to offer a poor idiot a lifeline

Years ago in my apprenticeship, I replaced head gasket on six cyl. Perkins, got the torque figures out of manual and bobs your uncle, two days later it went again, and we undid all the head nuts with a speed brace out of a socket set, manual said 70 ft/ lbs, but should have been 100 ft/ lbs for higher compression engine, suspect it was the wrong thickness of head gasket was your down fall