Slide out the rubber cap [A].

Starter Motor InstallationThermostat Installation
Replace the O-rings [A] with new ones, and apply grease
to them.
Assemble:
O-rings
Thermostat Housing [B]
Thermostat [C]
Thermostat Cap [D]
Install the thermostat [A] in the housing so that the air
bleeder hole [B] is on top.
Tighten:
Torque - Thermostat Cap Bolts: 5.9 N ...
Cooling System Flushing
Over a period of time, the cooling system accumulates
rust, scale, and lime in the water jacket and radiator. When
this accumulation is suspected or observed, flush the cooling
system. If this accumulation is not removed, it will clog
up the water passage and considerable reduce the efficiency
...
Oil Pressure Relief Valve Inspection
Remove the oil pressure relief valve (see Oil Pressure
Relief Valve Removal).
Check to see if the valve [A] slides smoothly when pushing
it in with a wooden or other soft rod, and see if it comes
back to its seat by spring [B] pressure.
NOTE
Inspect the valve in its assembled stat ...