Hi again Dave,

I rebuilt the YS as in previous Post , however I ran it up and it runs Ok , but there is a definite metal to metal click while turning the prop , it happens at exactly the same point every 2 turns of the prop , could you tell me what this is likely to be the problem , I cannot remember this clicking pre-crash , it is a definite metal to metal click.
Thanks for any help Dave

Cam gear or crankshaft gear chipped?

definetely not , if i hold down the valves which means there is no compression then the clicking stops and everything turns very smoothly

Any other ideas Dave

Check the rocker arm shaft.