Ford 351c Cleveland
Before beginning this discussion, a disclaimer: I have little experience with and no interest in Ford motors. The reason this chapter is here is that the Robertshaw thermostat is occasionally discussed as a replacement for sleeve bypass applications in early Jaguars. They can't function for bypass control in Jags because they really aren't sleeve bypass thermostats, but they are still occasionally of interest because they are very high flow. 

In the process of assembling information on Robertshaw thermostats for Jaguar use, I've come across quite a bit of information about Ford cooling systems. It seemed reasonable to deposit that information here, and maybe dispel some of the mythology that has grown up around this particular design. But mostly it's here because I admire the clever design, which has the advantages if high flow, balanced force, simplicity, and low cost to manufacture.

Cutaway View of Robertshaw Thermostat

This design is of relatively recent origin. It was patented 1957 by Louis Puster of Sylphon-Fulton, which at the time was the parent company of Robertshaw (US patent 2872117). For many years it was manufactured and sold under the Robertshaw brand name. For this reason, it's known as a Robertshaw thermostat, although current applications may be supplied by other companies.  If you say "Robertshaw thermostat", people will generally know what you are talking about. It was used for many production vehicles from Ford, AMC and AM General, and is most frequently discussed in the context of the Ford 351 Cleveland engine. Today, Robertshaw style thermostats are available from several US, Chinese and Australian labels (but not Robertshaw, which no longer sells direct to consumer). 

The surprising thing about this thermostat is that it's a controlled bypass design in it's native applications, although it appears otherwise at first glance. When open, the wax capsule doubles as a poppet valve and nestles into the bypass port. Unlike the more common dual poppet thermostat, the bypass valve height isn't self adjusting, and so the dimensions of the wax capsule are critical to making a sufficiently tight seal. In fact, all of the dimensions are critical, including the travel distance of the wax motor. This can pose a challenge to keeping the system working properly.

Because the thermostat is located between left and right transverse flows, a local area of high static pressure is created above the bypass. This will force hot coolant through the bypass if there's any leakage at all. A less critical problem is that the entire poppet cylinder seals against the bypass plate. In this position, there's limited circulation of coolant around the wax capsule, making the system less responsive at extreme temperature. It's no wonder there is so much discussion about bypasses and thermostats in the Ford world.

Several design features make this thermostat unique. First, the return spring is housed in a central pylon, which is welded to the top flange. The poppet is a "can" rather than a flat plate, and has a huge 1 9/16 inch diameter. The reason that the bottom of the "can" is punctured is to allow dynamic pressure to balance on both sides of the poppet. This reduces the force necessary to open the  thermostat and prevents lifting in high flow conditions, a unique advantage of the design. The O-ring seals the poppet against the central pylon: it can deteriorate and allow coolant to leak past the thermostat. If you have difficulty holding temperature, this ring may be worn out. The only fix is to replace the entire thermostat.

These thermostats often have a copper hat or lip attached to the wax capsule, and this acts as the actual bypass valve. The hat has to be sized to closely match the diameter of the bypass passage. For the 351C, there is a removable "water bypass plate" (that's the correct term: not a restrictor, not a washer), which must be matched the thermostat being used. There are other Ford engines where the bypass port is cast in place, and can't be changed. But whatever application you deal with, your thermostat has to match your bypass opening. If the hat is too big, the thermostat will tear itself apart on the restrictor plate. If it's too small, it won't fully block the bypass and the engine may run hot. 

Let me add a few words here about blocking the bypass. I understand that the design has its problems, and that there are a few documented mods for blocking off the bypass. These include soldering a penny to the bypass plate! Keep in mind that the bypass is there to promote faster warm up and to maintain continuous flow of coolant around the block. However, I understand that the reality of things is sometimes different than design theory. So you may find that permanently plugging the passage is the expedient solution to a persistently troublesome system. If you are inclined to do this, there are three things you must keep in mind: First, you need to allow air to bleed through the bypass plate, or you will have trouble with the initial coolant fill. So a tiny hole in your plug will be necessary. Second, you need to allow enough room for the thermostat to move backwards as it opens. Because of this, the plug has to be below the bottom of the bypass plate: a custom plate may be required (there is an E-Bay vendor selling custom plates). Finally, you need to provide for some coolant circulation when the thermostat is closed. The reason for this is that hot spots form when coolant can't circulate. Even if the coolant doesn't boil, extreme damage can be done to the engine by uneven heating. So you will need to drill the pylon of your thermostat, or use Stewart 300 or 301, which are pre drilled. 

A word about non-Robertshaw style thermostats. There are open frame thermostats sold under familiar brand labels which are "Cleveland-specific". They do have the appropriate bypass hats attached to their wax capsules. The problem is that they may adequately address the bypass, but not the main radiator passage. The Robertshaw thermostat has a honkin' big opening. When used in Ford applications, they have a flow cross section of  1.239 sq", which is probably 25% bigger than the next best alternative 54mm open frame thermostat. So if your Cleveland is still running hot after you ran down to Mr Parts and bought a new 'stat, that may be why.  You've cured your bypass issue, but added restriction to the main radiator flow path.


There is a huge amount of confusion about this design, propagated over many years and by many authors, so I'll take a shot at dispelling some of the mythology:

  • "Balanced flow" design. The idea of almost all modern cooling systems is that there is a continuous flow of coolant around the engine block, whether the thermostat is open or closed.  All controlled bypass thermostats are "balanced flow," because they simultaneously control both the main radiator passage and the bypass passage. The Robertshaw when used in a stock 351C is no exception, the only distinction being that the wax capsule doubles as a bypass poppet.
  • "Balanced force" design. Because the bottom of the poppet is perforated, the pressure on either side of the poppet is the same. The significance of this is that the thermostat can't be forced open or closed by dynamic pressure. This reduces the force required to open or close, and also prevents lifting at high RPM's. 
  • How much of the flow goes through the center? None. If you look at the bottom of a Robertshaw, it may appear as if coolant could flow through the center of the poppet. But the openings are only there to allow hot coolant to contact the wax motor and to balance pressure on the poppet. They do not contribute to flow because the pylon is a dead end. Ok, there may be be a pinhole to allow trapped air to bleed, but that's it. 
  • It's spring loaded. In comparing a Robertshaw to a conventional open frame thermostat, the open thermostat is often described as "spring loaded." The Robertshaw is "spring loaded" as well, but the spring is concealed inside the center pylon.
  • It doesn't rotate, it moves backwards. Some have the impression that the poppet rotates to open a passage through the center. This is not the case. There is never any flow through the center.
  • It's a reverse poppet or "balanced pressure" thermostat, not a "balanced sleeve" thermostat. The term "balanced sleeve" creates confusion for the British car world, because British cars actually use sleeved thermostats for bypass control. The cylindrical poppet of a Robertshaw is not intended to control flow through a transverse bypass.
  • If you order a 160F thermostat direct from China, what you're almost certain to get is a 182F thermostat. 

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