SFI 29.1 168-tooth external-balance flexplate for the big block Chevy 454 behind a TH400, TH350, 700R4 or 4L80E, with the 10.75-inch converter bolt circle.
Building a 1970-1990 big block Chevy 454 to run behind a TH400, TH350 or a 4L80E, or replacing a stock flexplate that cracked around the crank bolts? The PRW 1845400 is a Sportsman Series flexplate made for that early 454. It has a 168-tooth ring gear, the external balance weight the 454 crank needs, a thicker-than-stock chromoly center, and SFI 29.1 certification, which most tracks require once a car runs quick enough to need one.
The flexplate bolts to the back of the crankshaft and carries the torque converter. It has three jobs. Its ring gear is what the starter turns. Its center is meant to flex a little front to back so the converter can grow under pressure without loading the crank thrust bearing. And on a 454 it is part of the engine balance. The 454 is externally balanced, so some of the counterweight lives on the damper and the flexplate instead of inside the crank. Bolt a neutral, internally balanced plate to that crank and the engine shakes. The 1845400 has the 454 weight built in. PRW makes it from cold-rolled chromoly that is 0.035 inch thicker than a stock plate, with a forged one-piece steel ring gear welded on to SFI spec, so it holds up to a high-stall converter and hard launches.
Pull the inspection cover and turn the engine by hand with a flashlight on the plate. Look for cracks around the crank bolt circle and the converter lugs, and check every ring gear tooth. Transmission Digest has documented flexplates that cracked and turned on themselves with very little noise, so a visual check is worth it even when the symptom is vague. If the plate looks good and the complaint is a shift or lockup problem, our 4L80E common problems and fixes guide and 4L80E Silverado 2500 shift problems cover what usually causes those instead.
The 1845400 is for the externally balanced 1970-1990 big block Chevy 454 with the early two-piece rear main seal crank. It does not fit the 1991-up Gen V and Gen VI 454 used in factory 7.4L trucks, which have a one-piece seal crank flange and need a different plate. The converter side is drilled for the GM 3-bolt 10.75 inch and 11.50 inch circles, which covers TH350 and TH400 converters, and a 700R4 or 4L80E converter that uses one of those patterns. Check your converter's lug pattern before you order. On the starter side, it needs a starter set up for a 168-tooth ring gear, not one meant for a 153-tooth flywheel. If you are not sure what you have, send us the engine casting and year. Browse the rest of our 4L80E parts for the transmission side.
| Fits | Big block Chevy 454, externally balanced, 1970-1990 |
| Main transmission | GM TH400 (also TH350, 700R4, 4L80E) |
| Ring gear | 168 tooth |
| Balance | External |
| Converter bolt circle | 10.75 in |
| Certification | SFI 29.1 |
| Material | Chromoly steel, white zinc plated |
| Thickness | 0.035 in thicker than stock |
| Part number | 1845400 |
| Ships from | USA |
Doing this repair right usually means replacing these at the same time. Same transmission, ships together.
No. Factory 7.4L truck engines from 1991 up are Gen V and Gen VI 454s with a one-piece rear main seal crank flange. This plate is for the 1970-1990 454 with the two-piece seal.
PRW drills it for the GM 3-bolt 10.75 inch and 11.50 inch circles. That covers TH350 and TH400 converters and any 700R4 or 4L80E converter built with one of those lug patterns.
The 454 crank does not carry all of its own counterweight. Part of it is on the damper and the flexplate. A neutral, internally balanced plate will leave the engine out of balance and cause vibration.
If your car runs at a track that requires one, yes. SFI 29.1 is the standard most tech inspectors look for. On the street it is also a stronger plate for a high-stall converter.
A starter set up for a 168-tooth ring gear. A starter meant for a 153-tooth flywheel will not mesh correctly.
