Look in your manual under Valve Clearance Adjustment
In the 1972 140 series manual it is on page 2:12 for initial setup and
0:5 for final adjustment - 0.016 - 0.018" for B20A, E and F, and 0.022 - 0.024"
for B20B.
I prefer to err on the side of larger clearance due to the possibility of
the valve not closing tightly enough and losing its valve-seat cooling.
I burned a bunch of B30 exhaust valves when set at 0.018" in a long drive
across the desert. They even had several cracks in each valve face.