Mark Houlahan
Brand Manager, Mustang Monthly
March 13, 2012

Reciprocating Assembly

There are some differences in the connecting rods, crankshaft, and pistons used in the Hi-Po 289's assembly. Let's start with the crankshaft. It has been reported time and again that the Hi-Po's crank was nothing more than a cherry-picked stock crank, inspected and deemed "perfect" for use with the higher revving Hi-Po. While this was true for the block casting, the crankshaft was indeed a revised part for Hi-Po use. According to Bob Mannel's excellent book, the Hi-Po cranks were made in batches with higher levels of nickel and magnesium added to stock 289 crankshafts (they did use the stock casting mold as well). This would give the crankshaft increased nodularity, but it was far from an exact science. As such, each Hi-Po crankshaft from these small runs was checked visually. The rear counterweight was polished and then checked with a microscope. If the crankshaft had enough spherical graphite nodules in a specific measured area, it was deemed to have a "high nodular content" and was used for Hi-Po assembly. Ford also added a small "hatchet" shaped counterweight to the front of the crankshaft to move the counter balancing closer to the core of the engine, which necessitated a thinner timing chain.

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The connecting rods and pistons are much simpler. Ford used the standard 289 connecting rod molds to make the Hi-Po rods. As such, you'll see standard C3AE or C3AE-D markings, but where the difference will be found is in the machining, rod cap, and the fasteners. The Hi-Po rod, due to its larger 3/8-inch retaining bolt, had less material removed from the side of the rod during machining. Ford also used a beefier rod cap for the larger rod nut to seat on and to handle the engine's expected 6,000 rpm. The Hi-Po's pistons were specific to the engine, cast from high-strength aluminum for high compression use and with four valve reliefs to accept the high-lift camshaft. While similar to the later A-code 289 piston, which also had four valve reliefs, the Hi-Po's casting number was different. A Hi-Po piston will have a C4OE-A number while the A-code will have a C5OE-H (both with a 6110 base number).

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Valvetrain

There's certainly a lot going on in the Hi-Po's valvetrain area. Starting with a solid-lifter camshaft, Ford also used mechanical lifters (natch), valve springs with fewer coils than standard 289 springs to prevent coil bind at high lift, a flat-wound damper spring inside the valve spring, special hardened retainers, and thicker camshaft thrust plate (before Level 7 engine change) to reduce flexing/distortion of the cam and its higher loading and rpm. Due to the thicker thrust plate, the cam timing gear had to be reduced in thickness to maintain overall dimensions under the timing cover (just like the crank gear mentioned previously). With the thinner crank and cam timing gears, Ford fitted a thinner timing chain to match.

This timing chain measured 13/32-inch thick versus the regular 289's 1/2-inch wide chain. Using the wrong combination of gears, chain, and thrust plate will cause wear on the “hatchet” counterweight or even timing cover wear/damage. To confuse things further, Ford used two different cam gears in the timing set. Early Hi-Pos (before Level 7 engine change) used an iron gear; while from March 1965 on, the gear was aluminum with nylon teeth. The iron gear used an additional C-shaped spacer to properly locate the gear on the camshaft, while the aluminum gear has the spacer cast in. In a nutshell, use the iron gear, spacer, and thicker thrust plate together, or use the aluminum gear, no spacer, and standard thrust plate together. Do not mix the combinations.

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