Way back in the day vehicles didn't have very efficient heaters. Henry Ford took note of this so when the flattie was being designed he insisted that they run the exhaust ports through the block. This would make the motor run hotter and hence a more efficient heater. The rest is history!
Any motors from the fifties and earlier that I've had anything to do with seem to cool better with no thermostats. Those old water pumps don't pump that well at the best of times so you don't want any more restrictions that you can avoid.
Another thing if a motor's running too hot try playing with the spark timing. A few degrees here and there can make quite a difference but on a Cab 11 it's a pain in the butt to do. Sometimes a size bigger on the carbs main jet helps too.
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1940 Cab 11 C8 Wireless with 1A2 box & 11 set
1940 Cab 11 C8 cab and chassis
1940 Cab 11 C15 with 2A1 & Motley mount & Lewis gun
1940 Cab 11 F15A w/ Chev rear ends
1941 Cab 12 F15A
1942-44 Cab 13 F15A x 5
1942 cab 13 F15A with 2B1 box
1943 cab 13 F15A with 2H1 box
1943 Cab 13 C8A HUP
1944 Cab 13 C15A with 2C1 box
1943 Cletrac M2 High Speed Tractor
MkII Bren gun carrier chassis x 2
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