SFI 168-Tooth Flexplate Chevy 454 Big Block TH400

This is the PRW 1845400 flexplate for a big block Chevy 454, the externally balanced Mark IV/Gen V 454 built from 1970 to 1990. The 168-tooth ring gear and 10.75-inch converter bolt circle put it behind the common GM automatics, the TH400, TH350, 700R4 and 4L80E, so it is the plate you bolt on when you drop a 454 in front of one of those boxes or when the stock plate is done.

It is externally balanced, which is the part you cannot get wrong on a 454. The counterweight is machined into the plate to match the external balance of the factory 454 rotating assembly. Run an internally balanced plate on that crank and it will shake; this one carries the correct weight for the 1970-1990 454.

The plate is chromoly steel, white zinc plated, and runs 0.035 inch thicker than a stock stamped flexplate, with double-welded ring gears so it holds up behind a high-stall converter and the shock load that comes off a big block at the line. It carries an SFI 29.1 certification, which is what a tech or track requires once you are past a stock combination.

On the bench, mount it to the crank flange, torque the flexplate bolts, then set the converter and check runout with a dial indicator before you button up the bellhousing. A couple of the low reviews trace back to runout, so it is worth checking the plate on the crank and shimming the starter before final assembly.