thanx iv got the rocker ass. here but the head is, in the s,hop, but there appears, to be wear on the faces, of the rockers, so that may be causing the same effect

Check the tips of your rocker arms. They are probably messed up. Instead of gliding across the stem of the valve as they push down, they're getting caught and pushing the valve stem to the side.
The same thing happend to me. When I assembled the head and turned it by hand, noticed that the spring was compressing a little more on one side. I didn't know any better. So i ran it and those are the valve guides that broke.
Compare the tips of your rocker arms with the broken guides with the ones that are not broken and tells us what you find.
I'm curious.

Howdy, about six months, ago I had the head on my datto fully rebuilt (new springs, guide,seats, and some welding work) anyway recently iv noticed i was a bit smokey on start up but thought nothing of it (iv been runnig it rich for big boost) ihad the rocker cover off last weekend and noticed that 3 or 4 of the guides were broken and were half way up the valve . i checked the lift and its pretty much standard 7.6 mm a friend sugested the 0.8mm head shim i fitted was, to blame but i dont knoww. any ideas?