The AMC I6
Part 1 - The History
The AMC I6 has a strong history with Jeep during its entire 42-year lifetime. Here is what makes it such an amazing engine.
If there is an engine that has a more storied history and lasting fanbase in American off-roading, it must be the AMC I6. From Baja to the King of the Hammers, you won’t find any other engine that’s powering more Jeeps right now. Well, besides the GM LS or Chevrolet Small Block. That’s a topic for another day, but let’s talk more about the same straight-six that was used by the four owners of Jeep: AMC, Renault Alliance, Chrysler, and DiamlerChrysler.
The AMC I6 didn’t start out as the 4.0-liter overhead valve (OHV) many of us know best today. Instead, it was a flathead I6 that had a displacement of 172.6-CI (2.8-liters) when it first came out in 1952 but would eventually reach 195.6-CI (3.2-liters) before a major redesign in 1956. The flathead would disappear until 1958 when it came back with the new Rambler American and used as an “economy engine.”
That 1956 redesign mentioned earlier turned it into an OHV engine design that we’re more familiar with. This also moved the water pump from the left side and using a drive shaft off the generator to in front of the timing chain cover and driven by the accessory belt drive.
All AMC I6s gained a reputation of durability and dependability as it used a forged crankshaft and connecting rods. These parts made these engines tough and could work though anything, however the flathead would overheat under sustained heavy loads. This was typical of any flathead engine of the era due to its exhaust port design.
The change from flathead to the OHV didn’t change its dependability, but it did come with a cost of added maintenance. The 196 OHV engines require head bolt torque checks at 4,000-miles and retorquing every 8,000-miles.
Aluminum Block I6
In 1961, there was also an aluminum version of the AMC I6. It kept its durability by using cast-iron cylinder liners along with a cast-iron head. It would only be made until 1964. As great as it would be to do it, the aluminum head from the cast-iron block engine won’t work on the aluminum block engine. The aluminum block engine’s head is about 1/8-inch wider and uses a different bolt pattern.
Even with the different head design, owners would still need to check the head bolt torque in the same intervals as the iron block. There is the added concern that the cast-iron liners would shift if the torque isn’t kept in check and the engine allowed to overheat. This, along with the short timeframe they were made, also makes finding good heads and blocks for the aluminum engine a difficult task.
The “Modern” AMC I6
What’s considered the modern AMC straight-six began production in 1964 with the 232-CI (3.8-liters) Torque Command I6. It was a short-stroke engine with seven main bearings on the crankshaft rather than four of the 196. It – the 196 – would eventually be replaced by the 199-CI (3.3-liters) I6 in 1966.
The blocks of the 199 and 232 were both the same and had a 3.75-inch bore. The difference was in the stroke with a three-inch stroke on the 199 and a 3.5-inch stroke in the 232. However, the 199 would be replaced by the 232 as the small engine. The 258-CI (4.2-liters) with its 3.895-inch stroke and taller deck block appeared as the bigger engine choice. Unfortunately, it would eventually stop being sold due to the increase in emissions controls by 1979 while the 232 would be forced into smaller and smaller cars as both emissions and safety regulations made vehicles heavier and less powerful.
In 1971, an undersquare version of the AMC I6 came out as the 258-CI (4.2-liter). It featured the 3.75-inch bore and a 3.895-inch stroke, which made it undersquare. The 199 and 232 are both oversquare with larger bores than strokes. The 258 would remain carbureted until around 1980 to 1982 when it gained the AMC Computerized Engine Control (CEC).
1981 it gained lightweight parts such as the aluminum intake manifold and a plastic rocker arm cover. Its biggest change and weight savings came from the crankshaft. It went from 12 counterweights to four which reduced the engine’s weight by 20-pounds alone.
Mexican AMC I6s
Intertwined with the American produced AMC straight-sixes were two engines built by their Mexican subsidiary, Vehiculos Automotores Mexicanos (VAM). The VAM engines were made for their cars and were usually rebranded AMC vehicles like the VAM Gremlin (AMC Spirit), VAM Classic (AMC Matador), VAM American (AMC Rebel), and others. Theses were the 252 (4.1-liter) – a 3.91-inch bore version of the 232 – and the 282 (4.6-liter) – a 3.917-inch bore version of the 258.
The 252 produced 170-horsepower (gross horsepower, mind you, as they were still using that rating system until 1979 instead of using SAE net ratings) with 240-lb-ft of torque while the 282 would produce 200-GHP/280-torque in 1971 to 1973, 132-NHP (net horsepower)/216-torque in 1979 to 1981, 172-NHP/225-torque 1980 to 1981, and finally 129-NHP/218-torque in 1982 to 1983 engines.
It took only 26 months for AMC to develop the engine that YJ, XJ, ZJ, WJ, and TJ owners covet (well, unless there was a V8 model choice). It was the 242-CI or the 4.0. It would weight about a pound more than the 258 despite using lighter materials used. However, most of this was out of a need to make it as robust as possible – like using 15 bolts to secure the aluminum valve cover.
The 4.0 used a 3.875-inch bore with a 3.414 stroke but used the 6.125-inch connecting rod length like the 199. It was such a good engine from the start that it was the only engine retained by Chrysler when it finally bought out AMC.
This is the better-known six-cylinder engine that most XJ Cherokee buyers look for or plan on swapping in later. You can’t blame them. 1987 to 1990 engines produced 177-horsepower but put out 224-lb-ft of torque out of the box, later models from 1991 to 2006 would produce 190-horsepower and 225- to 235-lb-ft of torque.