SM420 4-Speed Transmission Bearing Kit ZMBK108 Chevy GMC

This is the bearing kit for a full teardown and rebuild of the SM420, the cast-iron Muncie-built 4-speed with the 7.05:1 granny first gear that GM put behind inline sixes and small blocks in 1948 through 1967 Chevrolet and GMC pickups, Suburbans, and medium-duty trucks. It is USA Standard Gear part ZMBK108. These boxes are still in service under swapped rock crawlers and restored trucks precisely because nothing else off the shelf gives you that low a first gear, so when one starts howling or leaking it gets rebuilt rather than replaced.

The kit contains the rolling stock and sealing parts, not the gears: front (input) bearing, rear mainshaft bearing, countershaft needle bearings and thrust washers, reverse idler bearing, input and output shaft seals, the case and cover gaskets, and the snap rings and small parts you throw away on the bench. On an SM420 the usual complaint is a growl in neutral with the clutch out, which is the input bearing, or gear oil weeping past the tailshaft seal onto the driveshaft. Both are covered here.

Bench procedure is straightforward: pull the top cover and shift forks, drive out the countershaft with a dummy shaft to keep the needle rollers captive, drop the cluster, then press the old bearings off the input and mainshaft and press the new ones on. Reload the cluster needles with heavy grease before you set it back in the case. Inspect the gear teeth and the sliding clutch dogs while the box is apart, because worn dogs cause the jump-out-of-gear complaint and this kit will not fix that.

Fits 1948-1967 SM420 units. Gears, shafts, shift forks and the top cover assembly are not included.