351M/400 thermostats

The 335 series engines are a bit unusual in that they don't have the typical external bypass hose that moves coolant through the block when the thermostat is closed.  Instead they have an internal bypass system that provides the same function, and it has the advantage of not having a hose that can go bad.

But, it requires a different thermostat than other Ford engines.  How do I know this?  On the right there's a snippet from Ford's Master Parts Catalog.  If you click on it you'll get a bigger picture where you can see that the 351M/400 takes a different thermostat than the other V8's - an RT-139.
Picture
So, what is an RT-139 and why is it different?  In the pic at right (thanks, FilthyBeast), you can see that it has a "skirt" or "bell" on the bottom.  That's because it is a "bottom-bypass" thermostat, and that skirt blocks the hole you can see in the engine, thereby stopping the flow of coolant into the block that is bypassing the radiator.  The whole process of thermostat opening and coolant bypass is well-described on this ARE page, but here's Reader's Digest version:
  • Cold Engine: The skirt is up, as in the pic, and the thermostat is closed so the coolant cannot go to the radiator.  Instead it goes through that hole to the engine, bypassing the radiator.
  • Warm Engine: The thermostat opens and the skirt drops down into that hole.  So coolant flows to the radiator and is blocked from going directly back to the engine.
However, many people don't know about the bypass and don't use the special thermostat.  By my calculations that means they'll be recirculating over 10% of the coolant back into the block without sending it through the radiator.  In other words, that immediately reduces the cooling capacity of the system by at least 10%.

​So, you might ask what thermostats are correct for these engines.  The list isn't all that long, but here are the ones I know of:
  • Motorcraft RT-139  This one is said to obsolete but they do turn up from time to time
  • Robert-Shaw 333-195  This thermostat appears to no longer be available.  But, if you have an old one you can pull the bell off of it and place it on a 303-195 as that seems to be the only difference
  • Gates 33129
  • Napa 297
  • Stant 13469
​Now, if you really like to research things, here are just four of the threads on FTE that discuss thermostats in more detail.  (But wait, there's more!)