Thank you everyone . MORE THOUGHTS. Looked at the threads , no obvious distortion & a fairly clean break.I think this means Brittleness . A file test shows the screw is harder than a standard Alan head cap screw ,music wire ,shank of a H/S drill bit and the adjuster on a Saito. An engineer friend does not like my reliance on "mechanics feel " & has loaned me a torque wrench .I have ordered a new part --Shall I trust it ( Rocker box cover off for a few flights )?
Make an attempt to re temper it ?Or make my own out of a regular Alan screw. I am going to make an attempt to get the jam nut torque specs ,which I think should be included with the instructions for setting the valves. P.S Put my 8 yr old O.S back on the plane & it showed its usual 9600 rpm and worked nicely