Flexplate (Automatic Transmission)

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Flexplates (used with automatic transmissions) and flywheels (used with manual transmissions) are both identified with basic part number 6375. All 428 Cobra Jet and Super Cobra Jet engines were originally externally balanced by Ford. That is, external components like the harmonic balancer and flexplate play an integral role in balancing the engine's reciprocating assembly. The CJ and SCJ flexplate balance weights are not identical: the SCJ weight is shorter, and the 1975 Ford master parts catalog says that the SCJ flexplate's diameter (15 1/2") is about 1/32" less than the CJ flexplate's diameter (15 17/32"). All of this means that they don't interchange directly without balance problems. The balance weight on a CJ flexplate extends for approximately 20 teeth on the ring gear; the balance weight on an SCJ flexplate extends for approximately 16 teeth on the ring gear. The ring gear used for both flexplates has 184 teeth.

CJ FlexplateSCJ Flexplate
CJ FlexplateSCJ Flexplate

Flexplates are bolted to the end of the crankshaft, with a circular plate (part number B9AZ-6A366-B) used around the innermost six bolt holes on a 12-inch bolt circle to provide additional rigidity. The 184-tooth ring gear attached around the edge of the flexplate is engaged by the starter to get the engine's internals spinning at startup time. A torque converter is bolted to the flexplate to connect the engine to the automatic transmission, physically linking the engine to the rest of the drive line. I've heard that the eight triangular cut-outs are unique to the 428 flexplate, but I can't confirm that. I've never seen a flexplate with stamped or cast identification numbers on the flexplate disk itself, but a FoMoCo logo is often present. Here's a sample:

FoMoCo stamping

Notice the welded-on balance weight in the upper left arc? If your flexplate is missing this weight you may have some balance problems! If yours is present, it should have a stamped identification number that looks something like this:

Balance weight

This particular flexplate was found in a 1968 Shelby GT-500KR, though the "C6SP-6375-A" number stamped here has also been identified for use on earlier base 428 and 428 Police Interceptor engines.

YearService Part NumberEngineering Number
Early 1968 CJTBDC6SP-6375-A
1968, 1969, 1970 CJC6AZ-6375-BC6VP-6375-B or C7AP-6375-A
1969, 1970 SCJC9OZ-6375-BC9OP-6375-B

Flexplate Service Part and Engineering Numbers

Many thanks to Greg Sali for the CJ flexplate pictures used here!