GM 621 Bellhousing for Muncie 4-Speed 11-Inch Clutch

Cast bellhousing in the GM 621 pattern (casting 3899621), the standard small block and big block Chevy housing for an 11 inch clutch disc. It bolts a Muncie four-speed to a Chevy V8 in the cars GM built the combination for from 1959 through 1988, and it is the piece most people are hunting when an original 621 turns up cracked at the fork boss or with a stripped starter bolt hole.

Transmission fit is set by the front bearing retainer: this housing is machined for a GM box with a 4.675 in. outer diameter retainer, which is the Muncie M20, M21 and M22 four-speed and the Saginaw that shares that retainer size. Clutch and flywheel side, it clears a 168-tooth 14 in. flywheel running a factory 11 in. disc. The 1/2-13 threaded hole in the housing is there for the Z-bar mechanical clutch linkage rod, so a stock pedal-and-Z-bar setup hooks straight up with no fabrication.

Vehicle coverage the manufacturer lists: Chevelle, Malibu and Monte Carlo 1964-1988; El Camino 1959-1987; Impala, Caprice, Bel Air, Biscayne, Brookwood, Kingswood, Nomad and Sedan Delivery 1959-1988; Corvette 1963-1981; Camaro 1967-1981; and Chevy II and Nova 1968-1974. Two exceptions worth reading before you buy: it does not fit 1962-1967 Chevy II and Nova, because the transmission tunnel on those cars is too narrow for this housing, and it does not fit Monza or Vega, whose clutch fork enters at a 45 degree angle instead of straight through.

Check your existing retainer diameter before ordering. A Ford or Borg-Warner box, or a GM unit with a different retainer OD, will not pilot correctly in this bore no matter how the bolt pattern looks, and a transmission that is not concentric with the crank chews up the input shaft bearing and the pilot bushing in short order.