Overdrive Transmission - Mopar Overdrive Conversion Kit

With all the custom Passon overdrive gears in their proper places, the parts are ready to be assembled into the spotlessly clean transmission case. For long life, it's always best to give the old transmission parts a thorough scrubbing to prevent contamination of the new parts.

The front bearing retainer must be indexed so the drain hole in the case and gasket is aligned with the matching relief in the retainer. Get this wrong and your transmission may leak gear lube and contaminate the clutch.

Countershaft wear is a classic issue with any four-speed. The pen points to visible wear on the old countershaft from years of bearing contact. It's easily solved with a brand-new replacement from Passon for a whopping 29 bucks (background).

The main shaft installs from the rear of the case. There are several important bushings, thrust washers, and snap rings we can't show you due to space limitations. If you're unsure, have an expert shop such as Icon Engineering handle the swap for you. Do-it-yourself types will benefit greatly from the illustrations and hints contained in factory service manuals. Get 'em on eBay.

One of the most painstaking aspects of the rebuild is the installation of the roller bearings on the countershaft. Schultz uses thick wheel-bearing grease to stick them in place as shown. To ease the process of loading them into the case, he uses a cut-down countershaft that fits into the side-cover opening. Then he drives the actual countershaft (too long to go in from the side) into the case from the back to the front. As it passes through the case and gearsets, it pushes the temporary-use short countershaft out the front. All you four-speed rebuilders should dig this trick.

Passon's gearsets are machined with greater accuracy than factory stuff for precise mesh and maximum load-bearing capacity. The gearset shown here is the heart of the OD feature. All engine power flows through this gear junction when you're running full throttle in Overdrive.

Reassembly starts by driving the new input shaft into the case. The slight interference fit requires a drift pin and light hammer blows.

The main-shaft extension housing bolts to the case using a fresh gasket.

Side-cover installation is a tricky affair for those with only 10 fingers. As always, a fresh gasket (included) prevents leaks.

All buttoned up, it looks like a factory stocker but hides an overdrive surprise. As with the original Chrysler OD four-speed strategy, Passon uses the same inverted 3-4 shift-rod tactic except the Passon rod is a beefy 11/42-inch diameter versus 0.370 for stock parts. Though the custom 3-4 shift rod is included with the overdrive conversion kit, tell Passon if yours is an A-, B-, E-, F-, or C-Body so you get the right one.

Icon Engineering is a Mopar-specific shop that's ready for any and all restoration or high-performance work. Here's a view of just one corner of the shop.