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this section covers mods and improvements to the 195.6 OHV cooling system, including what i believe is a definitive answer to this engine's chronic headgasket failure/overheat problem.
this engien also suffers from an overheated, or more correctly undercooled, oiling system. the "lower end", crank, rods, cam, tappets, oil pump and oil galleries, are nearly unchanged from it's introduction in the 1940's as a modest 75 horsepower engine.
engine oil cooling is covered in the oiling section.
run any engine long enough, something will fail first. on this engine, it is the head gasket. Nash/AMC knew there was a problem right from the engine's introduction: the technical service manual specifies a 4000 mile head bolt check/retorque schedule, and with the engine hot. i used to think this was an issue with bolt torque. i am now convinced it has more to do with head bolt motion.
poor thermal coupling between combustion chamber heat and the thermostat seems to be the root cause of a complex stress mechanism. the thermostat is isolated in a pod in the head well forward of #1 cylinder. with the engine "cold" (first operation of the day) block and head are initially the same temperature. when the engine is run, combustion heat accumulates in the cylinder head. since the thermostat is closed, with no coolant flow the thermostat, some four inches forward, remains isolated from combustion heat.
the isolation of the thermostat in it's pod greatly delays the heat signal from reaching the thermostat. the thermostat eventually gets a heat signal, either via simple conduction/convection, around the thermostat gasket, or via leakage in or around the thermostat itself. once the thermostat gets this signal, it opens immediately; the problem is that this conduction/convection takes so long to begin that coolant elsewhere in the head is well past boiling, with audible steam hammering. when the thermostat does begin to open, very hot coolant in contact with the thermostat causes it to open very rapidly. this sudden coolant flow the cylinder head coolant temperature plummets, which partially closes the thermostat, causing a thermal undershoot. however with the thermostat now open and coolant flowing, the system then warms up normally and within a minute or two stops oscillating.
this thermal cycling is easily measured. i measured coolant temperatures of over 250F, accompanied by audible steam hammering. at the same time that the head is overheated the block remains cool to the touch. i estimate during this time that there is a 150F degree temperature difference between block and head. assuming 150F difference, i calculate 0.024" cylinder head length increase (heating) and decrease (sudden cooling) in these first few minutes. i surmise also that the head gasket is a thermal insulator and "lubricant" between block and head.
given this thermal cycling and expansion/contract it is not hard to visualize the undesirable horizontal motion of the head bolts. when the head grows in length the head bolts splay out in a "V" with the bolt heads moving apart; when the head and block temperatures equalize, they move back to their correct vertical position. i believe this back and forth motion applies rotational torque and backs out the head bolts. the expansion/contraction is likely bad for the sealing surfaces, contributing to leakage. accumulated over time this loosens the head and causes the leaks that are symptomatic of the common end-of-life failures in this engine. if you think this bolt-loosening theory sounds dubious, check out this page at BoltScience.com: the Jost Effect. there's even a video showing transverse motion backing out a bolt!
in contrast to the seriousness of this thermal design flaw, the fix is almost laughably simple: drill a bypass hole in the body of the thermostat, install the thermostat with the hole towards the front, so that it "leaks" coolant past the sensor button. hole-drilling is often done to allow purging air bubbles from the system. many aftermarket thermostats come with a drilled hole and a loose pin so that crud can't block it. i suggest a fairly large hole, eg. 3/16". this slows the infamous "fast warmup" this engine is known for; while inconvenient for cold winter mornings better temperature regulation will have only positive effects. i suspect that many thermostat installations leak slightly, by design or by accident. this might explain the disparity in experiences (some have head failures, many don't). i suspect engines with constant if small coolant flow do not have this thermal-spike issue. my engine obviously had it; and i used a new thermostat that appeared to close completely, it had no hole, and carefully assembled by me with Right Stuff.
this engine was introduced in 1941 as a 75 hp flathead. AMC installed an poorly designed overhead valve head on it in 1958 to squeeze out a few model years. the "Power Pak" option brought power up to a claimed 138 hp, and "modern" highways put additional burden on the cooling system. it should be no surprise that there are cooling system problems.
easy for me to say, i don't care about "stock". Speedway Motors' "Ford type" generic aluminum radiator has twice the cooling at half the cost of repair or replacement. the downside is that you need to fabricate brackets to mount it. i estimate my 18" x 24" two-row radiator has two or three times the capacity of stock, at half the cost. i get a routine 60F temperature drop inlet to outlet at highway speeds. my SPAL 16" 1500 CFM fan is adequate, just. the fan is needed only under 10 MPH even in Los Angeles summer weather; 10 MPH generates far more than the alleged 1500 cubic feet of air.
the water pump is increasingly a problem. though adequate, they are hard to find. there are at least three different variations, two of which have different shaft lengths.
a major part of the 2010 build was to modify the oil pump for full-flow oil filtration. after the failure of the 2010 build (on an admittedly abusive rally) i finally thought to stick a temperature sensor in the main oil gallery. on a quite modest 20-mile Los Angeles freeway run, 65 MPH in 75 F weather, engine oil temperature rose to 230 degrees F. though 230 F isn't itself a problem, that it did so on such an easy drive in 2017 points out the demands we make today on old iron. there were no freeways when this engine was new.
as part of the 2017 build i added a 10" x 10", stacked plate cooler with fan and with that, 70 MPH all day cruising temperatures are in the 200 F range. to do so on this engine requires the full-flow-filtration modification mentioned above. there is simply not enough oil flow through the stock bypass filtration (with it's 3/16" steel line) to remove any real heat.