Tappets

[Bob Culver]

With no closing ramp as moderns, with increasing gap the closing impact is notably increased. With the weak springs of a standard engine hardly a threat to valve breakage or seat wear, although it is with other makes.
Even with just no measureable gap there is a strong load on the seat.
As far above, most modern valve steels have greter expansion than XB or earlier so some increase over the origianl is appropriate.