[DeTomaso] Interesting Robertshaw 333-180 thermostat design change

[Mike Drew]

All,

The Robertshaw 333-180 is the ultimate thermostat for the 351C—or at least it used to be. 

There has been an undocumented design change with the same part number. Now, instead of supplying a thermostat with the 351C ‘hat’ which partially (but not fully) blocks off the orifice in the center of a disc pressed into the block, instead they are supplying a conventional Windsor thermostat (ordinarily bad news) along with a replacement disc with a smaller (too-small) orifice. In order to use this thermostat the stock disc needs to be removed from the block and replaced with this new one. That then means that going forward only NON-Cleveland thermostats can be used. If a proper 351C thermostat is installed, it will be prevented from fully opening because the hat will strike the now-too-small disc.

With this new setup, when the thermostat opens it will fully block off the orifice. However, the engine is supposed to have at least some water circulating past the disc, with an area equivalent to that of of a 5/16ths bypass hose as used in other Ford engines. (If you compare the size of the stock ‘hat’ with the hole in the stock disc, you will see the hole is measurably larger). If I was to use this new setup, I would therefore be inclined to modify the disc, either by drilling bypass holes or (preferably) enlarging the center hole.

Too, the stock setup allows for some misalignment between thermostat and disc. If the thermostat wasn’t perfectly centered, it would still have room to fully open. With this new deal, if things aren’t perfectly aligned, it might not fully open, or worse, it might open and get stuck open, leading to very slow warmup problems. 

Here is a pic of a ‘real’ Robertshaw 333-180 compared to the new one:

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Here is the new one with its included disc:

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And the disc laid over the thermostat:

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I’m sorry I don’t have a stock disc handy to compare, and show the clearance between the orifice and the hat. You will just have to take my word for it....

Mike (hoarding his one remaining good thermostat!)

Sent from my iPad