In-line six-cylinder engines are still popular today and Jeep is proficient at making them perform well and last long. The outstanding virtues of its six-cylinder current 4.2-litre (258-cubic-inch) include smoothness plus an abundance of low-speed torque-just what a Jeep needs to climb over the boulder-infested Rubicon Trail.

The engine's 258 cubic inches is a direct descendent of AMC's 232 by virtue of a .40-inch stroke increase. This results in the 258's 3.75x3.895-inch bore and stroke dimensions, making the engine undersquare.

Riding in seven main bearings for maximum support, the cast nodular iron crank has four counterweights for smoothness, with two-bolt main bearing caps holding it in place. The block's weight is kept low because the block skirt stops at the crank.

The valvetrain begins with the chain-driven cam, located on the right side of the block. It rides in four bearings and drives the electronic ignition distributor. Cam timing is 262 degrees duration intake and exhaust, which is optimum for torque production. Hydraulic valve lifters provide quiet, maintenance-free operation. The pushrods operate stamped steel rocker arms, which ride on powdered iron pivots with stamped steel bridges as locators. A retaining bolt screws into the threaded rocker pedestal, securing the rocker assembly.

Originally, the pistons generated an 8.6:1 squeeze, but the ratio was bumped up to 9.2:1 in 1983. They're cast aluminium alloy with steel struts, which helps control expansion and adds strength. Three rings-two compression and one oil-contain combustion and control oil. Cast iron connecting rods rely on press-fit piston pins to connect the rod. The pin boss is offset from the piston centerline toward the thrust side.

A two-barrel carb sits on an aluminium intake manifold, which is separate from the head. Feeding the carb,is a mechanical fuel pump, driven by an eccentric off the cam.

Fourteen bolts hold the cylinder head in place. It has a dual-quench-style combustion chamber, which creates turbulence for thorough air/fuel mixing and promotes quick burning of the mixture. The head combustion chamber volume is 64-67 cc.

The 4.2-litre is a great engine that's standard in the Grand Wagoneer and J10, and optional in the Jeep Wrangler. But after driving the 2.5, we can't help but wonder how much better it would be if equipped with TBI. If Jeep asked our opinion for engine improvements, that's what we'd suggest.

SPECIFICATIONS

Displacement 4.2 litres (258 cubic inches) Type: I-6

Bore 3.750 inches Stroke 3.895 inches

Deck height 9.490 inches Compression ratio 9.2:1

Cylinder head volume 64-67 cc

Horsepower 112 @ 3000 rpm Torque 210 @ 2000 rpm

Firing order 1-5-3-6-2-4

Crank cast nodular iron

Main journal diameter 2.499 inches

Rod journal diameter 2.093 inches

Valve diameter Intake 1.787 inches Exhaust 1.406 inches