Mercury Mariner 4 and 5HP 4-Stroke Outboards Service Shop Manual 1999, Page 118Get this manual
CYLINDER HEAD
4Check the margin th ickness (a) of the valves after the valves have been groundAny valve with margin thickness of less than the specification, should be replaced
Margin Thicknessa"
Intake Valve Exhaust Valve
0.0280.047 in(0.7 1.2 mm)