TH400 SFI 168-Tooth Flexplate, Small Block Chevy Internal Balance

This is the PRW 1835001 flexplate, a 168-tooth plate for internally balanced small block Chevy engines from 1957 to 1985 (the two-piece rear main seal blocks) and the 90-degree Chevy V6. The 168-tooth ring gear is the size used with the large-bellhousing GM automatics, the TH400, 700R4 and big-bell TH350, so this is the plate that goes between a stock-balance 350 or 327 and one of those boxes.

Balance is the thing to check before you order. This plate is neutral, with no counterweight, which is correct for an internally balanced crank. It is not the plate for a 400 small block or a 1986-and-later one-piece rear main engine that uses an externally balanced flexplate; bolting a neutral plate onto an external-balance crank will shake the engine.

The plate is chromoly steel with an iridite finish, runs 0.035 inch thicker than a stock stamped flexplate, and has a double-welded ring gear. That extra thickness and the welds are what keep it from cracking around the crank bolts or shedding the ring gear behind a high-stall converter. It carries SFI 29.1 certification, which most tracks and tech inspectors want once the combination is past stock.

On install, clean the crank flange, torque the flexplate bolts with thread locker, then bolt the converter up and check runout with a dial indicator before the bellhousing goes on. A cracked flexplate usually shows up as a knock or clunk at idle that changes when you shift into gear, so if you are chasing that noise, look at the plate around the bolt holes first.