Nash/Rambler 195.6 ci overhead valve six

updated july 2017

you can contact me at tom at sensitiveresearch dot com

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.

SHORTCUTS TO SPECIFICS

2010 engine build
2017 engine build
cooling system modifications
cylinder head dissection
distributor mods (before EDIS)
EDIS-6 crankshaft-timed ignition
center exhaust port equalization
head bolts and cylinder head sealing
induction improvement
full-flow oil pump mods and oil cooling
mechanical valve timing

Nash engine nomenclature included the decimal (i would guess as part of some long-forgotten "Nash Precision" marketing trope, otherwise, it's sort of annoying). AMC continued it, and that is what appears in service manuals and most internet search results, although enough people 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 summary

here's a rough summary of this engine and it's shortcomings, most of which are dealt with in the sections that follow. if you are going to work on these motors you really need to have a legible copy of the factory technical service manual (TSM). a Haynes or Motors manual is no substitute; those are plain crap. the TSM has detailed information you simply won't find elsewhere. for reference, here are the relevant 1961 TSM engine pages, along with the few 1965 TSM pages pertaining to the differences from the earlier motor.

history

this particular engine has been rebuilt at least three times. twice by me. when this engine was in the 1963 Rambler American 440 Twin Stick hardtop i got in 2005, it had a commercially rebuilt engine in it, .030" overbored. within a year i rebuilt the cylinder head due to sticking valves (old gasoline, foolish mistake).

in 2010 the engine was pulled and received a complete and (what i thought was a) careful rebuild. many of the successful modifications i made to this engine were done at this time. on this site i refer to this as the 2010 build. in 2014 i again removed the engine, did a cosmetic freshen then installed it in the current chassis, my 1961 Rambler Roadster. in august 2016 i drove the roadster in the LeMons Hell on Wheels '16 Rally, very hard in very hot weather, 8% grades in Death Valley, which did some unpleasant things to the bottom end. when i got home the engine was once again removed, torn down, and this time, after careful diagnosis of it's various shortcomings and problems, completed what i call here the 2017 build, by a professional engine builder, Pete Fleming. that turned out to be a great (if expensive) decision as his machine work appears to be impeccable, finding and fixing problems previous machinists either neglected or couldn't see.

as of this writing (july 2017) the 2017 build is broken in and has 4000+ fairly hard miles on it (after proper break-in of course) -- at a performance level that the 2010 build was not capable of and certainly would not have survived.

21st century warning on buying replacement parts

in the 2010 build of this engine, described below, i bought a combination of new and reconditioned parts from supposedly reputable suppliers, and six years later, after problems resulting in a full teardown and far more careful (and expensive) build, found many of those expensive parts to be severely lacking in quality, and many were not what they claimed to be. specifically:

there were many other small and not so small problems with other parts (wrong gaskets in sets, etc). a lot of this is simply the current historic moment, ancient cars, consolidation of suppliers and parts catalog collapsing.

some of the previously inadequate machine work done in the 2010 build includes the following. the one caveat here is that i simply didn't know a good machinist at the time, and probably couldn't have afforded better work, then. nonetheless some of this was just sloppy. all of these were very specifically addressed in the 2017 build:

i'm not trying to deflect responsibility for failures that are in fact entirely my own doing (umm, this is an old Nashcan motor, pushed to three times it's output of 50 years ago). given the low cost of the 2010 machine work those are mainly my mis-use of modest machining (though the machinist did state the deck and head would be milled, but sanded it anyway), i pushed the engine past it's oiling and cooling ability, etc. my goal writing this isn't to apportion blame but to highlight problem areas to watch for.

there is simply not a lot of knowledge or lore or performance experience surrounding this engine. i gathered four 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 cracks in four cylinders, which was dissected to see how the head was constructed and to help place temperature sensors into the head. from this instrumented cylinder head i've gained most of the valve pocket cleanup and cooling system knowledge here.

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 popup 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.

contrary to popular belief, the trough intake is not a/the major performance-limiting factor. the camshaft is the first limitation, followed by head sealing. the rest of the design and construction is fairly decent.

cylinder head shortcomings include many sharp-edged shapes in the combustion chambers and in the valve pockets, nearby water jacket precludes aggressive smoothing. the piston wedge pops up into the head, with a gap around the circumference. 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 exhaust valve pocket. this latter "feature" has been fixed in my motor.

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 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 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 initially the same temperature. when the engine is run, combustion heat accumulates in the cylinder head. since the thermostat is initially closed, with no coolant flow the thermostat remains 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; the problem is that this conduction/convection takes so long that coolant elsewhere in the head is boiling, with clearly audible steam hammering. when the thermostat does begin to open, very hot coolant in contact with the thermostat causes it to open very rapidly. with 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 begins to warm 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!

solution: 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.

solution: 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 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.

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.

block

nash/amc/rambler did not serial-number chassis and engine, and continuously made many small and occasionally large engineering changes, often with no change in part number or casting number. as per the rest of the industry at that time car and engine options and features came and went and were forgotten (E-Stick clutch, various incarnations of "heavy duty", ...), repair shops swapped and modified parts seemingly without reason to keep cars on the road, and for a car that was apparently unloved, it is rare to find one with it's engine not overbored. all of this combines to make precise parts identification difficult. luckily, it doesn't matter, all blocks and most parts interchange with only minor warnings and issues.

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. the headbolt pattern sucks. some of the headbolts draw up from vertical walls and some from horizontal webbing. the bolt pattern and other block deck issues make for head sealing issues.

the camshaft is in the typical pushrod OHV configuration, driven by the usual chain and sprockets under a cover on the front of block. harmonic balancer is external with pulley groove. fuel pump is driven off a camshaft lobe. mushroom cam followers install from bottom, requiring engine removal for access. the cam follower's very small diameter limits cam profile regrinding, as does its 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.) all OHV blocks are drilled for the flathead's distributor location on the right side of the engine, filled with a welch plug.

cranckcase ventilation was a simple road draft tube in the early years, PCV first in california then national. the draft source (road or PCV) draws from front valve/pushrod access side cover. air taken in via the long crankcase oil filler tube dipstick vented cap, and many engines have stamped vents in the valve cover, depending on year and carburetor model.

valves and cam

the cam and valve setup is a very ordinary camshaft in block, solid followers, pushrods, rockers on a shaft, 1.5:1 ratio. valves in head. valve seats are cut into the head casting. valve tappet clearance adjustment via threaded pushrod socket. the OHV block retains the old flathead side valve adjustment access cover, whose main purpose in the OHV engine is to leak oil.

the camshaft, and followers, are probably the major performance-limiting feature of the engine. there are of course no cam blanks to be custom ground, and the single OEM cam has it's base circle very close to the rough casting; further the short lift, short duration lobes leave no meat for regrind. lobe accelleration and absolute lift are further limited by the tiny mushroom tappets, which rock faintly in their sockets.

my current engine got a substantial increase in power with a cam grind that introduced overlap (idle vacuum is about 10 InHg) and very careful attention to the valve, seat, and head pockets, to fill the tall skinny undersquare cylinders.

crankshaft, connecting rods, pistons

the crankshaft and connecting rods are all forged parts, with very large and nearly overlapping journals. though only four main bearings the bottom end seems more than adequate. be careful selecting or mixing connecting rods; i have found at least two different parts, fully interchangable, with identical part and casting numbers, that were over 100 grams different mass but within each set, 10's of grams difference. more old world engineering. (the difference seemed to be at the little end.) the pistons are heavy, with thin rings, cast aluminum with steel inserts, of a popup wedge design. aftermarket pistons are often of terrible quality. most of the engines i've disassembled for parts were .030" or .040" over, with one .060" over. the blocks are apparently bored .080" over without problem.

for my 2010 build i got decent quality replacement pistons and rings from Kanter. i static balanced those with a gram scale, and they were not bad to begin with. rods and bearings were fine, probably; though connecting rod bearings failed (leading to the 2017 teardown) it seems it was the combination of my pushing the improperly prepared engine too hard, helped along by some crappy machine work.

lubrication

the oil pump is an external but typical gear pump. the pump inserts into the lower side of the block left, pulls oil from the pan and pushes directly into the main gallery. top end (rocker shaft, etc) is lubricated by an external line that carries oil from a tap on the block, up to the head casting where it flows upward through a rocker shaft support, into the hollow shaft and from there to each rocker. on earlier engines the top-end source is the main gallery, eg. full engine oil supply. on later engines the top end is fed by an intermittent source generated by a flat on the camshaft's front journal that pulses the main gallery feed, to limit oil flow to the top end.

oil filtration was essentially bypass filtration, which apparently works better than you'd think, with running engine oil turnover rate. when an oil filter is present (it was an option in early engines), there is a "tee" bypass in the cylinder head feed line that then feeds both top end and filter. the filter output, via 3/16" steel line, is to the crankcase on the right side of the engine, behind the generator location low on the block.

crankshaft main journals (4) are fed directly from the main gallery 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.

with quality parts in good condition, more difficult to acheive than you'd think, the lubrication system seems more than adequate for higher performance. however, oil heating is a fairly severe problem; see below.

problem: lubricating oil overheat

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 on old iron. there were no freeways when this engine was built.

solution

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.

other minor lubrication 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.

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.

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.

a Pertronix module will solve the limitations of points, but my testing has shown that wear in the distributor itself, which shows up as timing jitter and weak spark, combined with the utter lack of total spark advance (some 22 degrees), is a major performance limiting factor. consider also that insufficient spark timing adds a lot of heat to the cooling and exhaust systems.

solution: EDIS-6 and MegaJolt Lite Junior

i did this upgrade as part of the 2010 build and there is no question that this is superior to anything involving a distributor. the EDIS-6 and MegaJolt Lite Junior combination is superior in every way. timing is adjustable without restriction, it has zero moving parts, is easy to add. in this engine, where the oil pump has it's own drive shaft and gear, you can just pluck the distributor out.

however there are mods and improvements you can make to the stock distributor that are fairly easy and worthwhile if you intend to keep it in place.

induction

intake trought plate and carburetor. 32/36 vs 38/38.

exhaust

exhaust center port equalization. also add wrapping, surfacing and sealing. downpipe, O2 sensor.