Nash Rambler straight-six engine

The Nash Rambler engine is a family of straight-six engines that were produced by Nash Motors and then American Motors Corporation (AMC), and used in Nash, Rambler, and AMC passenger cars from 1940 through 1965. It was succeeded by the AMC straight-6 engine in 1964, a completely new design. The engine evolved in several displacements and was built with two different valvetrains. The engine was originally of a flathead layout; an OHV version was introduced in 1956 although the flathead continued to be available. A version of the OHV engine with an aluminum block was introduced in 1961.

173 (Nash 600)


Introduced in the autumn of 1940 for the 1941 Nash 600, it was originally designed to be an economy engine by Nash Motors, who initially called it the "Flying Scot" engine. Nash went on to merge with Hudson to form AMC in 1954. The initial design had a 3+1/8 in bore and a 3+3/4 in stroke, for a displacement of 2828 cc. This bore was to be kept for all the variants of the engine, which were enlarged by increasing the stroke – an unusual practice given that changing forging is more expensive than changing the block casting. The intake and exhaust manifolds were integrated into the cylinder block casting to reduce both weight and cost; Nash drew attention to the increased reliability of the design by marketing it as "Monitor Sealed". To help keep costs down, the 600 engine had four main bearings, unlike the 7-main-bearing 234 flathead engine it replaced. The new, smaller flathead design was initially rated at 75 hp at 3,600 rpm. The engine combined with the new Ambassador's advanced "unitized" construction design provided better than 25 mpgus.

When the war ended, Nash updated the 600 for 1946, increasing power to 80 hp at 3,800 rpm. Updates included larger intake valves, reworked manifold and valve passages, and a revised head which provided a somewhat higher compression ratio. A triple-venturi, down-draft carburettor replaced the earlier type, and a number of minor changes to increase durability were also made. The 173 continued to be used for the Nash Rambler until 1952.

184
Upping the stroke to 4 in bore in 1950 increased the displacement to 3016.49 cc for the Nash Statesman. This L-head engine was also installed on the Nash Rambler until the end of the 1955 model year.

196
The 3205 cc model was produced from 1952 through 1965, initially as a flathead (L-head) side-valve, and later in an overhead valve (OHV) version. It became the new American Motors Corporation's first straight-six engine.

This engine was redesigned as an OHV for the 1956 model year, being marketed as the "Typhoon" engine. The flathead version was discontinued between 1956 and 1957, but reemerged in 1958 as the economy engine for the "new" Rambler American and was available through 1965. The new OHV configuration for 1956 continued with the water pump mounted on the left side of the engine (driven by a shaft extending from the back of the generator). This was changed for 1957 with the water pump mounted on the front of the engine above the timing chain. When the flathead model was reintroduced, it received a new water pump. Featuring high-quality parts such as forged crankshafts and connecting rods, these engines earned a reputation for durability. However, the flathead was prone to the typical problem of overheating under sustained heavy loads. The excessive heating was caused by the hot exhaust going through the cylinder block to the exhaust manifold.

American Motors introduced a die-cast aluminum block version of the OHV 195.6 CID engine in 1961. It was produced through 1964. This engine used cast-iron cylinder liners and a cast-iron head. The cylinder heads for the two types of block (aluminum and cast iron) have similar designs but are not interchangeable. The cylinder head for the aluminum block is roughly 1/8" wider than that for the cast-iron block and uses a slightly different head bolt pattern.

The 196 OHV requires more periodic maintenance than newer-model engines. The head bolts must be re-torqued (retightening process) regularly. Factory service manuals recommend that head bolt torque be checked every 4000 mi and to re-torque them every 8000 mi. Modern head gaskets can extend this service interval to re-torque the bolts every 12000 mi, or every other year. The cause of the head bolts loosening over time is believed to be thermal expansion and contraction of the head due to the rather heavy design. The head expands and contracts by as much as 0.02 in in height during normal operation; this loosens the bolts over time and eventually will cause the seal between the head and block to fail. This maintenance is also imperative to prevent the engine from overheating (the first sign of a blown head gasket) and warping or cracking the head. The cast-iron liners in the aluminum block version can also shift if the head bolts are not properly torqued and the engine is allowed to overheat. Good replacement heads and aluminum blocks for these engines are now difficult to find.