What radiator/thermostat caps do I use? Why?

I broke this out of the coolant fill article hoping more people would be made aware of the Stant catalog error.
Warning! There is an error in Stant listings, which means all catalogs listing Stant parts share the same error. The correct caps for Fiero are Stant part numbers 11230 or 10230 non-vented caps.
You want to use a 15-16psi cap as recommended by OE specs. There are 2 very important reasons for this. (We’ll get to vented vs non-vented in a minute.)
The radiator cap along with the coolant product determines the boiling point of the filled system. A 15psi cap increases boiling point about 45oF vs no cap.
Lower system pressure and the coolant will boil at a lower temperature.
The cap effects water pump efficiency and life by controlling cavitation. When you spin the water pump it forms areas of low pressure around the impeller.
This low pressure makes air dissolved in the coolant, and even the coolant itself, explode into bubbles. These bubbles cause turbulence in the water so it’s harder to pump. Cavitation also damages the water pump by giving the impeller a rough surface that reduces its efficiency permanently. Coolant under pressure is less likely to cavitate and is therefore easier to pump around the system. Life of the water pump is also increased.
There are low pressure, usually 13#, caps out there. They often get installed on old weak systems to prevent popping the radiator tubes, etc. These caps often cause other problems, like easy over heating, and should only be used as a temporary item. Long term use of these caps can cause damage to the water pump impeller. They are only a little better than a bad cap.

GM & AC Delco caps.

Thanks to “Two88GTs,” the current GM and AC Delco cap numbers for Fiero are:
Rad Cap
AC Delco: RC27
GM : 10409635

Thermo Cap
AC Delco: RC40
GM: 6410941

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