You are right. I didn't know that the steel valve seats were casted into the alloy with a interference-angle of about 5°....
And even more, the Ariel 500 cc valve seats were both casted into the alloy as a single spherical cap.

First cut at the new valve seat.
Interference valv seat to alloy head: 0,05 mm.
I hope it works.

I hope it works...

Thank you all for the answers! David, I think your right. After inserting a pilot bolt into the valve-guide bore the rough measurement from vertical coresponded 38 degrees. Next step will be to set up the head on a milling machine table.
I will continue to report.
Peter

Hallo,
is there anybody who knows about the valve angel for Model NH,1957 s/a, with alloy-head ?
After welding I have to re-bore the valve-seat and the valve-guide borings.
Will be glad for any support.