The idea of all automotive cooling systems is continuous flow. All of them have bypass systems to ensure that there are no stagnant pockets in the head as the engine warms. Sealing or severely restricting the bypass does your engine no favors. Has nothing to do with ambient temperatures, as even engines in tropical climates require warm up time.
Most period American engines had open bypass systems. The bypass was arranged orthogonal to flow, so that bypass flow would be minimal unless the thermostat was closed or partially closed. The dual poppet thermostat was a bit of an improvement, since it could positively block flow in high stress situations. This particular thermostat was a simpler, alternative design that paralleled the development of the dual poppet. I think the last production vehicle that used it was Hummer H1.
The thermostat you illustrate isn’t a GM design, it’s probably due to Ford, although AMC began using it around the same time…maybe 1962 or 63. It’s one of my favorites, because it packs a lot of original thinking into a very small package. I’m actually devoting a full chapter to it. It’s usually called a Robertshaw thermostat, because that company was the primary source for many years. Although it’s not obvious, it’s a two-valve bypass thermostat, designed to work with a specific type of engine that used a backing plate which restricted the bypass and interfaced with the wax pellet:
Ford Bypass Plate
The plate restricted the bypass passage to a diameter that exactly matched the wax capsule. When the thermostat opened, the capsule would back into the bypass, blocking flow. Ford used various copper “hats” to re-dimension the wax capsules when more bypass flow was required. As a solution to a damaged bypass sealing surface, it would be interesting to use one of these and add a suitable hat to the bottom which allowed it to seal the Jaguar bypass at full extension:
Bypass Adapters
The design has considerably more flow cross section than a conventional poppet thermostat, but getting the full advantage of that area requires a specific water rail, where water enters from the sides rather than the bottom. I’m not sure how the XJS manifolds are constructed, but dual poppet water rails are always designed to flow from the bottom, and are usually rather constricted, so you wouldn’t get the full benefit. OTH, you wouldn’t get the full pressure drop of that huge opening, either:
Robertshaw Thermostat Flow Paths
The thermostat is a “balanced force” design, which equalizes pressure on the top and bottom of the valve. This allowed certain economies in production, but I’m not sure if it really translates to better reliability. These thermostats can rip themselves apart in service if the wax capsule missed the bypass plate due to wear, because there’s no bypass spring to relieve mismatches. There’s also an internal o-ring between the valve and pylon that can cause leakage, but can’t be inspected or replaced. There’s absolutely no reason to drill it to allow the pellet to heat up, as the balanced force design allows full flow around the inside of the pylon. But drilling it obviates the need for the bypass, so that’s probably why you aren’t blowing up engines:
Robertshaw Disassembled
