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 and to re-torque them every . With modern head gaskets this service interval can be extended to re-torque the bolts every , or every other year. The cause of the headbolts 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 in height during normal operation, this loosens the bolts over time and eventually will cause the seal between 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. The modern era I-6 The company designed an entirely new six-cylinder with a short stroke and seven main bearing crankshaft for 1964. This design was produced in various forms through 2006. The inline six was AMC's first modern six-cylinder engine. To commemorate the engine's May 1964 introduction, a total of 2,520 "Typhoon" versions were made based on the Rambler Classic hardtop body. Each featured the 8.5:1 compression ratio engine, 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. The new engine replaced the Nash OHV I6 in the Classic and Ambassador for the 1965 model year (this was also the first use of a six in the Ambassador since 1956). In 1966, a version finally replaced the aging OHV and L-head engines. Road tests by Consumer Reports described the new powerplant as "a very smooth and quiet engine, which should give good performance." Both the 199 and 232 featured a bore, and either a or stroke. The 199 was discontinued in 1970; the 232 was offered alongside the (made by using a stroke crankshaft and slightly taller block) during the 1970s, but was discontinued in 1979. Increasingly stringent emission control regulations