Nash/Rambler 195.6 ci overhead valve six

and other engines in the family

this site is dedicated to compiling information on the Nash/Rambler 195.6 cubic inch overhead valve six manufactured between 1958 and 1965. in addition to plain old documentation and information my goal is to build modern levels of reliability and power. while this is a very modest design, with the dubious distinction of having no performance parts available for it, other than the factory two-barrel option ("Power Pak") it is proven to be a reliable engine, with forged crankshaft and connecting rods. most of it's shortcomings are easily overcome.

2016 high performance build
2010 rebuild and experimentation

Nash engine nomenclature included the decimal (i would guess as part of some long-forgotten "Nash Precision!" marketing trope), AMC continued it, and that is what appears in service manuals and most internet search results, though enough non-Rambler folk call this engine "the 196" to confound searches and identification.

Thanks to Frank Swygert for much information on this engine and for corrections to these pages.

Brief history

This section by Frank Swygert:

Nash's economy L-head six was fitted with an overhead valve head for the 1956 model year. (No L-heads were sold for 1956 or 1957, but it reappeared again in 1958 and was available through the 1965 model year.) The 1956 model of the OHV still had the side mount water pump. The front mount pump came in 57.

The original L-head was a 172.6 designed specifically for the first unit-body Nash, the 1941 Ambassador 600. This increased to 184 inches in 1950 for the Statesman, and the new Nash Rambler got the 172.6. 195.6 came in 1952, again for the Statesman. The Rambler got the 184 in 1953, Hydramatic Ramblers got the 195.6 (small wonder -- the Hydramatic was heavy and took a lot of power!). 1952 was the last year for the 172.6, 1954 last year for the 184. All three engines used the same 3.125" bore, strokes were different (3.75", 4.00", 4.25" respectively). This was unusual since the crank and rods were forged -- the usual practice was to keep the expensive forgings the same and alter the cheaper to change block casting. I guess pennies didn't need to be pinched as much then as after the "merger" with Hudson.

engine design summary

here's a rough summary of this engine and it's flaws, most of which are dealt with in the sections that follow.

cylinder head

the head has a trough intake, generous and short paths, only one 90 degree turn from carb to valve. combustion chamber is a wedge design, with a shrouded dead zone surrounding the popup. the trough has clever Nash anti-reversion wedges that make for near-perfect fuel distribution. pushrod and rockers on a shaft, 1.5:1 ratio. valve tappet clearance adjustment via threaded pushrod socket.

cylinder head shortcomings include many sharp-edged shapes in the combustion chambers and in the valve pockets, nearby water jacket recludes aggressive smoothing. the piston wedge pops up into the head, with a gap around the circumference that lowers compression i suppose but likely traps unburned fuel. thermostat pod is located far forward of heat signal source. the center exhaust pair, 3 and 4, have a funny upturn to heat the carburetor base, otherwise exhaust ports are a straight out of the exaust valve pocket.

block

the block is fairly ordinary cast iron. four main bearings, siamesed cylinders (eg. no water jacket between paired cylinder walls). the block retains the old side valve adjustment access covers but there's nothing behind them but pushrod side view. cranckcase PCV drawn from front side cover.

camshaft is driven by typical chain and sprockets under a cover on the front of block. harmonic balancer external with pulley groove. fuel pump is driven off the far end of the camshaft. mushroom cam followers install from bottom, requiring crankshaft removal for access. cam followers small diameter limits cam profile regrinding, as does very small base circle. the typical helical gear on the camshaft drives both oil pump and distributor, but each has it's own shaft and driven gear (specifically the distributor does not drive the oil pump.)

lubrication

the oil pump is external but typical gear pump. the pump bolts to the lower side of the block, pulls oil from the pan and pushes directly into the main gallery. an external line from the main gallery feeds the head which pushes oil up through the front rocker shaft support stantion into the rocker shaft, which lubricates the rockers and pushrods via squirt holes. oil filtration is essentially zero, a "tee" bypass from the cylinder head feed line. crankshaft main journals (4) are fed directly from the main galler as is each cam bearing. connecting rod bearings receive oil via drilled crank. the connecting rod big end has a squirt hole that lubricates the cam journals. the cam is also splash lubricated. some years have piston squirt lubrication via conn rod squirt hole. the specifics of the oiling system make it very easy to modify.

ignition

a model-specific Delco Remy distributor and coil and points, mechanical and vacuum advance, inserted on the passenger side of the engine. the distributor is extremely spark-advance-limited, 11 degrees maximum mechanical advance. no other distributors known to "fit in the hole". the L-head version of the engine uses an incompatible Autolite distributor inserted into the drivers side of the block. the ignition is one of the major performance-limiting features.

reliability modifications

lack of information!

there is simply not a lot of knowledge about this engine out there, and much work has been done to address that. i gathered two (or was it three) full motors and pulled the head off a deader in a junkyard. none of the cylinders were standard bore, one engine hinted at a two rebuilds (0.060" over). the junkyard head had cracks professionally repaired (nice work here, i saved it, it's work you don't see much any more). one head had four separate cracks.

solution

first i chopped the severely cracked head into slices to see how the head was constructed and to help the placement of sensors into the head. from this instrumented cylinder head, installed in the current motor, i've gained most of the cooling system knowledge here.

problem: cylinder head sealing failure

If you run any engine long enough, something fails first. on this engine, it is the head gasket. Nash/AMC knew there was a problem from the engine's introduction: the technical service manual specifies a 4000 mile head bolt check/retorque schedule, and in a bizarre manner: with the engine hot. i used to think this was an issue with bolt torque. i now believe it has more to do with 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 located in the head well forward of #1 combustion chamber. with the engine "cold" (first operation of the day) block and head are the same temperature. when the engine is run, combustion heat accumulates mainly in the cylinder head. with no coolant flow the thermostat remains thermally isolated from combustion heat. the thermostat eventually gets a heat signal, either via simple conduction/convection, or around the thermostat gasket. once the thermostat gets this signal, it opens fully within a few seconds. with sudden coolant maximum flow the cylinder head coolant plummets in temperature, which partially closes the thermostat, causing a thermal undershoot. however with the thermostat now open, the system begins to warm up normally.

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 warm 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!

solutions

cold-start heat signal

for all that, a substantial fix is quite trivial: 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 or pass coolant, 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 sealed with Right Stuff.

head sealing

in my engine i replaced all of the typical head bolts with ARP head studs. though i had done this before i had worked out all the oveheating physics above, i still think it's a good idea and i'll do it again. though i checked torque annually they have needed no retorquing. though i consider this problem solved i'll likely continue the annual head bolt check.

problem: inadequate cooling system

the cooling system is adequate for its intended light duty use. it is not adequate for extended modern highway driving, which did not exist when this engine was placed in products. the inadequacy has at least a few separate components, one complete, two of which i'm in the process of solving, having at least identified them. this inadequacy is less surprising when you consider that this engine was introduced in 1941 with 75 hp output; AMC's modifications brought that up to 138 hp, and modern highway (and other performance) applications put a sustained load on block and oil cooling that it simply cannot handle.

basic solution

the first and most obvious fix is to install a large aftermarket aluminum "Ford type" radiator (inlet high on the right, low on the left) from Speedway or Summit Racing. along with some minorly annoying mounting and hose fabrication, solves cooling problems utterly. 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 100F 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.

advanced solution

however i've gone much further with cooling system improvement in line with my demand for extreme reliability under sustained high demand usage at increased power output. for a year i have been driving an electronic closed loop software cooling system with electric pump and multiple sensors. so far this has concentrated on cylinder head cooling. there is no thermostat, temperature is regulated purely by pump speed control. though anecdotal, on a recent endurance rally it held head coolant temperature at the target 188F while climbing at speed up the six percent grade out of Badwater, in Death Valley, in August, in 110F ambient air. there was still a 40F radiator inlet/outlet temperature differential, indicating plenty of headroom.

the new (post-rally) system will incorporate a second pump to circulate coolant between block and head independendently of the main pump to address two problems at once: moving block heat to the head for removal, but also equalize temperatures and eliminate temperature sensor thermal lag. this should also stabilize cylinder bore dimensions, probably a good thing with this long stroke and tall block.

problem: lubricating oil overheat

at this time (september 2016) my knowledge here is provisional, based upon measurements in my engine with the instrumented head and data collection. during a particularly hard event, the LeMons' Hell on Wheels Rally '16, driving 2000 miles in 5 days fron the coast through Nevada and Death Valley in August heat, i ran the engine hot enough and long enough to melt connecting rod bearings. this caused an unusual rearrangement of the oil pressure/rpm profile; lower pressure at higher rpms, but increased at low rpms. i had taken an oil sample before the event, changed oil, ran the rally and took a sample, then tore down the engine. samples were sent off for analysis. though i did not have oil temperature data during the rally (i will next time i drive) upon return and before teardown, i rigged up oil temperature measurement. on a single highway test run, 65 mph for 20 miles, oil temperature rose to 240F within 5 minutes. while not in itself harmful, this was hardly a stressful run, as the rally certainly was. in a few days of surface street driving around Los Angeles, oil temperature took 20 minutes to rise no higher than coolant temperature, approximately 190F. i interpret all of this to mean that the lower block cooling is quite ad hoc, that a lot of heat is being retained in the oil that has no path to escape the block. the crank case and oil probably accumulates heat, coupling to the water jacket is too far away and too little to extract heat in an engine now running under circumstances unthinkable when it was sold.

solution

a large cooler with it's own fan, sensor and software process will extract heat directly from the engine oil, the engine already having been modified full-flow oil filtration with external lines already in place. this work will be documented as it is installed.

other minor fixes and improvements

the valve cover design is pretty good but engine oil flows along the rocker shaft and pours steadily right onto the spot where the cover gasket meets the head, and often develop a seep there, even with a new gasket. any tendency to leak is made worse by the oil dripping off the rocker shaft, onto the back edge of the seal. Typical for front-engine cars, the block tilts to the rear about 5 degrees, causing oil to run down the length of the rocker shaft and exit of the rear end, right onto the bottom of the valve cover and seal.

a simple twist of steel baling wire around the far end of the rocker shaft provides a path for oil to return to the cavity in the head casting. there is now no oil leak or mess even when running with the valve cover off. the same wire twist has been in place for six years. it is tight enough to have a shape, but loose enough that it could never wedge itself between the rocker and washer. Even if it wears into two pieces they'll lay harmlessly on top of the head. here's a brief movie (AVI format) of it in operation.