AMC Straight-6 engine

The American Motors (AMC) Straight-6 family of engines was used by a number of AMC and Jeep vehicles from 1964 through 2006.


 * For an outline of all engines used by AMC see

195.6
American Motors' first straight-six engine was the 195.6 cubic inch (3.2 L). It was produced from 1958 through 1965 in both Overhead valve (OHV) and "flathead" (L-head) side-valve versions.

Sometimes referred to as the 196 engine, this engine was originally designed by Nash in the 1930s as a flathead, and redesigned as an OHV engine in 1956. The flathead version was discontinued for 1956 and 1957, but reemerged in 1958 as the economy engine for the "new" Rambler American. When the engine was changed to an OHV configuration the water pump was moved from the left side of the engine (driven by a shaft extending from the back of the generator) to the front above the timing chain. When the flathead model was reintroduced it also received the new water pump. Equipped with such high quality parts as Forged Crankshafts and Connecting rods, these engines earned a reputation for remarkable durability. The flathead, however, was prone to the typical flathead problem of overheating under sustained heavy load. This was from the hot exhaust traveling through the Cylinder block to the Exhaust manifold.

American Motor introduced an Aluminum block version of the OHV 195.6 cubic inch (3.2 L) engine in 1961. It was produced through 1964. This engine used Cast iron cylinder liners and a cast iron head. The head is slightly wider on the right side than the cast iron block head. The heads will physically interchange, but the cast iron block head will need material added to the right side to properly seal on an aluminum block.

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 and to re-torque them every. With modern head gaskets this service interval can be extended to re-torque the bolts every miles, or every other year. The exact cause of the headbolts loosening over time is unknown, but it is believed that thermal expansion and contraction of the block, head, and bolts eventually causes the seal between the head and block to loosen. This maintenance is imperative to prevent the engine from running hot (the first sign of a blown head gasket) and thus 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 run hot. If this process is not followed, then repairs will be necessary and replacement heads and aluminum blocks for these engines are now more difficult to find.

In compensation, this required maintenance is quite easy. Re-torquing the head and adjusting the valves typically takes under 45 minutes to complete, start to finish, and is done with only two open-end wrenches and a feeler gauge. Some skills are required, but it is not challenging, and adjusting the valves on a running engine (as is required) is today a rare pleasure.

Modern head gaskets usually come with a statement that "no re-torque or sealant is required". This "no re-torque" refers to the process when the early gaskets had to be retightened upon installation after the engine had been run for a short period at full operating temperature. This procedure was necessitated by the type of material used for the gaskets. This process was usually called "bedding in" the gasket. Today, all replacement 196 head gaskets are made of modern materials and do not need to be retightened after the initial installation. However, they still require re-torquing every 8,000-12,000 miles or every other year as noted previously.

It is imperative to use a sealant on all 196 head gaskets. Long-term experience of owners of cars with the 196 engine is that the gaskets need an additional sealant regardless of the gasket manufacturers recommendation. A brush-on Shellac (such as "Indian Head" brand) or spray-on copper or aluminum sealer is all that is needed. Without the additional sealer the head gasket will seep a small amount oil into the water jacket and down the sides of the engine. Nevertheless, there seems to be a small amount of seepage on the right side even with sealer that is normal for this engine. This is likely due to the number of head bolts (15 - four around each cylinder plus one extra) and just adequate sealing surfaces - at least wide in all areas, but not much more in most. On the L-head engines, the head also seals the intake tract on the right side of the engine.

The modern era I-6
The company designed an entirely new six cylinder for 1964, and was produced, albeit in a modified form, up until 2006.

First produced in 1964 and introduced as an option in the Classic and Ambassador in the middle of the 1964 model year, the 232 cubic inch (3.8 L) "Typhoon" in-line six was AMC's first modern six cylinder engine. It replaced the Nash 196 cubic inch (3.2 L) OHV I-6 in the Classic and Ambassador in 1965 (the first use of a six in the Ambassador since 1956). In 1966 a 199 cubic inch (3.3 L) version finally replaced the aging 196 cubic inch (3.2 L) OHV and L-head engines. To commemorate the engine's May 1964 introduction, 2,520 "Typhoon" cars were made on the Classic hardtop body. Each featured the new 232 six, Solar Yellow body paint, a Classic Black roof, and a distinctive "Typhoon" script in place of the usual "Classic" name. All other AMC options (except engine options) were available.

Both the 199 and 232 featured a 3.75 inch (95.25 mm) bore, and either a 3.0 inch (76.2 mm) or 3.5 inch (88.9 mm) stroke (respectively). The 199 was discontinued in 1970; the 232 was offered alongside the 258 (made by using a 3.895 inch (98.9 mm) stroke crankshaft and slightly taller block) during the 1970s, but was discontinued in 1979. Increasing emissions controls continued to rob the engine of power, making the smaller version inadequate as safety regulations also added weight to the vehicles it was required to push around.

AMC changed the deck height of the six in 71, increasing it 1/8-inch to allow the longer stroke of the 258. This alters pushrod length on all engines built after 71. The 232 was continued on the taller deck block by using the previous crankshaft with the slightly longer 199 rod.

199
The 199 cubic inch (3.3 L) 199 was produced from 1965 through 1970.

Applications:
 * Rambler Classic (1965-66, fleet cars only)
 * Rambler American (1965-69)
 * AMC Hornet (1970)
 * AMC Gremlin (1970)

232
The 232 cubic inch (3.8 L) 232 was produced from 1964 through 1979.

Applications:
 * Rambler Classic (1964-1966)
 * Rambler American (1964-1969)
 * Rambler/AMC Marlin (1965-1967)
 * Rambler/AMC Ambassador (1965-1970)
 * Rambler/AMC Rebel (1967-70)
 * AMC Javelin (1968-74)
 * AMC Hornet (1970-77)
 * AMC Gremlin (1970-78)
 * AMC Matador (1971-74)
 * AMC Pacer (1975-79)
 * AMC Concord (1978-79)
 * AMC Spirit (1979)
 * Jeep (1972-79)
 * Jeep (1974-79)
 * Jeep (1965-1971)
 * Jeep (1965-1970)
 * Jeep (1972-1973)
 * International Harvester Scout (1969-1971)

258
The 258 cubic inch (4.2 L) "258" in-line six was produced from 1971 until 1990. It featured an Undersquare 3.75 inch (95.25 mm) bore and 3.895 inch (98.93 mm) stroke; it was otherwise similar to the 199 and 232. Later 258 models (starting with the 1980 model year for California AMCs, 1981 for California Jeeps and 1982 for all other applications) are equipped with AMC's CEC system.

Applications:
 * AMC AMC Hornet/AMC Spirit/Eagle (1971–1988)
 * AMC AMC Pacer
 * AMC AMC Matador (1971–1978)
 * AMC AMC Gremlin and AMC Spirit (1971–1983)
 * International Harvester Scout (1972-1974)
 * Jeep (1972–1986)
 * Jeep and Jeep (1972–1986)
 * Jeep (1971-1988)
 * Jeep (1987–1990)
 * Jeep (1972-1973)

4.0
The 242 cubic inch "242 or 4.0 L or 4.0" Straight-6 was an evolution of the 258 and 2.5 and appeared in 1987. It had the same 3.875 inch (98.4 mm) bore as the 2.5 with a longer 3.414 inch (86.7 mm) stroke giving it a displacement of 241.573 CID (3,959 cc).

The 4.0 was discontinued at the end of the 2006 model year as the redesigned Jeep uses Chrysler's 3.8 L OHV V6.

The first 4.0 engines in 1987 had RENIX (Renault/Bendix) engine control systems, which were quite advanced for their time, but are now handicapped because there are very few scan tools which can be "plugged in" to a RENIX system for diagnosis. The Renix also used a Engine knocking sensor, which allowed the computer to know if detonation was occurring, thus allowing the computer to make the appropriate changes to prevent this. Renix systems have no "permanent" memory for the diagnostics system. Once the ignition switch is turned off any codes generated are lost, thus making it difficult to detect intermittent problems.

The 1987 RENIX 4.0 made 173 hp (129 kW) and 220 ft·lbf (298 N·m) of torque. In 1988 the 4.0 received higher flowing fuel injectors, raising output to 177 hp (132 kW) and 224 ft·lbf (304 N·m).

In 1991 Chrysler Corporation, having bought AMC, were the new owners of the Jeep brand. Chrysler redesigned the RENIX engine control computer and raised the intake ports approximately 1/8-inch (3.175 mm) for a better entry radius. Chrysler also enlarged the Throttle body and redesigned the intake and exhaust manifolds for more efficiency, and the fuel injectors were once again replaced with higher flowing units. The camshaft profile was also changed. The net result of all these changes was an engine that made 190 hp (142 kW) and 225 ft·lbf (305 N·m) of torque. Badging on most Jeeps equipped with this engine read "4.0 Litre HIGH OUTPUT." The new cam profile combined with altered computer programming eliminated the need for an EGR valve and knock sensor, but made the engine more sensitive to alterations, especially where emissions are concerned.

Small changes were made to the cylinder head for the 1995 model year. In 1996, the engine block was redesigned, and a new strengthened unit was then used. The new block made use of more webbing cast into the block, and a stud girdle for added rigidity of the crankshaft main bearings. The cylinder head was again changed around 1998 to a lower flowing, more emissions-friendly design. Engines installed in 1999 Grand Cherokees carried the Power Tech name, which had been used intermittently in prior years. The name was subsequently passed on to 4.0s in all Jeep models.

The 4.0 engine was in production in North America until 2006, when the Jeep TJ was replaced by the new JK design that uses Chrysler's OHV 3.8 L V6. It is foreseeable that this engine may be made for many more years in the People's Republic of China, where a slightly modified version of the Jeep with 2.5 and 4.0 engines is still being produced.

Output as of 2004 was 190 hp (142 kW) at 4600 rpm with 230 ft·lbf (312 N·m) of torque at 3000 rpm.

This engine has performance features suited for off-road use. The ample low end torque is ideal for trailing and rock crawling. The only downfall to the engine is the relatively low power output, especially at high rpm, for mudding applications but this can be fixed with various aftermarket options including a supercharger for the engine. Interchangeability within the late AMC/Jeep engine family is superb. The better flowing 4.0 L heads can easily be modified to fit earlier engines right along with the fuel injection system. The longer stroke 258 crankshaft and rods will drop right in the slightly larger bore 4.0 L block, easily creating an even higher torque 4.5 L engine with the stock bore (4.6 L bored 0.030 inches over, and 4.7 L with a 0.060 inches overbore). Blocks should be sonic checked for adequate cylinder wall thickness before boring 0.060 inches over.

This engine was used in the following vehicles:
 * 1987–2001 Jeep
 * 1993–2004 Jeep
 * 1987–1990 Jeep
 * 1987–1992 Jeep
 * 1991–2006 Jeep

Rod Lengths
1964-71: 1972-06:
 * 199 CID (3.3 L) -
 * 232 CID (3.8 L) -
 * 232 CID (3.8 L) -
 * 242 CID (4.0 L) -
 * 258 CID (4.2 L) -