What is it about the straight-six engine design that makes it almost completely bulletproof? Take a look through the history of American automotive design, and you'll discover a trio of motors that have a hard-won reputation as being unkillable, able to keep running for hundreds of thousands of miles with only the most minimal maintenance required. Some of these engines have even been known to survive catastrophic conditions that would have felled more delicate power plants, requiring only a fluid top-off and a fresh set of plugs to put in another tour of duty. //

AMC/Jeep Straight Six
The 4.0-liter inline-six cylinder engine originally found in the ultra-popular Jeep Cherokee XJ was the last AMC motor to be sold in America. It had been developed in the early 1980s, and was based on the brand's previous experience with straight-sixes. Featuring the same excellent crankshaft balance that is gifted to all inline engines, as well as seven main bearings, the lack of vibration, excellent oiling, and strong build quality of the 4.0 would see it offered from 1986 all the way to 2006, giving it a whopping 20 years of continuous production. //

Ford 300 Straight Six
Way back in 1965 Ford debuted an engine that would serve for more than 30 years as the backbone of its entry-level, task-focused trucks and vans—not to mention pull duty in a long list of agricultural, industrial and commercial vehicles, too. //

Chrysler Slant Six
Chrysler's famous 'slant' six (so named because its block leans 30 degrees to the right side in the engine bay of the original 60s-era econo cars it was installed in, in order to clear the hood) came in three distinct flavors. There were 170, 198, and 225 cubic inch versions of the motor available, but the 225 is the one that has garnered the strongest reputation for never saying die.