Engine Factory of Lebanon, New Jersey offers a word of advice on flywheel installation for Bowtie applications using a few different GM automatic transmissions, and from what they tell us it’s of paramount importance to be sure that your flex plate is fully-adaptable to the rest of your drivetrain.

Different bolt hole and counterweight combos are what pose a challenge to the motor install process, and whether or not those patterns match make the difference between being able to match your torque converter and not being able to match your engine and transmission correctly.

The first thing that Engine Factory wants their customers to know is that GM offered two different sized flywheels for auto applications from ’86 and later, a 12 3/4″ diameter and 14 1/8″ plate with the larger flywheel’s converter bolt holes layed-out in two different combinations, closer together and further apart.

GM engineers redesigned the crankshafts used in production small-block V8 and V6/90-degree engines in 1986 to use a leak-resistant one-piece rear seal. This change from a two-piece to a one-piece seal required a change in the bolt pattern on the crankshaft flywheel flange.

The bolt circle on pre-1986 production crankshafts is 3.58″ in diameter. 1986 and later cranks have a 3.00″ bolt circle. Additionally, all cranks which use a one-piece seal require a counterweighted flywheel for proper engine balance.

Even though the smaller automatic flex plate traditionally has 143 teeth while the larger plate has 168, different teeth combinations with both flywheels can be used depending on the starter’s bolt pattern. In the case of a 12 3/4″ plate with 153 teeth, the GM starter that bolts right up to the motor with bolt holes straight across can be installed.

In the case of a 14 1/8″ plate with 168 teeth, the starter with the offset bolt pattern can be mated. Because the larger flex plate has three bolt holes that will match the drivetrain’s torque converter, it will fit on to nearly any GM application while the smaller plate fits specifically on to 200 and 700R4 applications, along with the liter-based TH350.

Flywheel Quick Reference Chart

The final step that Engine Factory recommends is checking to be sure that the flywheel is fully mated to the transmission’s pump. In the case of the smaller-diameter flywheel, bolt hole mating with the converter is recommended prior to engine install. Provided that either plate is fully-engaged with the rest of the running gear, you should be able to turn the converter two times because the converter is matched at this point with the pump.

Installing a flywheel to your GM engine does not have to be a long-drawn game of bolt pattern matching if you know what to look for.