Mechanical tachometers were standard equipment on Corvettes from their inception through the 1974 Corvette model year. Unfortunately, they’re all prone to the same problem: Eventually, the cross shaft gear in the distributor that drives the tach cable fails due to wear. In order to replace the cross gear assembly, it’s necessary to remove the distributor.
We replaced our Corvette’s distributor tachometer drive gear with the factory style cross shaft and bronze coupler – obtained from Zip Corvette Parts. Zip also has available every part needed to rebuild a Corvette tach drive distributor. These distributors were found in all 1962-1974 Corvettes.Our project car is a 1967 427 coupe with transistorized ignition. Here’s how the repair was done. 1962-1974 Corvette Tachometer Drive Cross Gear Replacement:
01: The Corvette’s distributor shielding must be completely removed. Start by removing the wing nuts on the upper shield, then remove the side shielding retainer bolts. A gear wrench makes things go a bit faster.
02: Since the plug wires will have to be removed in order to lift off the lower distributor shielding, it’s a good idea to make a diagram of the plug wire orientation on the distributor cap. Pull the wires from the cap next.
03: The braided plug wire shielding is secured to the distributor shield mounting posts on both sides. Remove both nuts and pull the braided shielding free. The coil wires also have to be removed, since they will also impede the removal of the lower distributor shielding.
04: Unscrew the tachometer drive cable from the cross gear assembly and pull the cable free.
05: After lifting off the lower distributor shielding, the plug wires were replaced on the distributor cap to make things a bit more orderly. The cap was then removed by using a screwdriver to turn the two “j” latches on either side of the cap, and then pulled forward and rested on the carburetor to give additional access to the distributor.
06: The connector for the transistorized ignition module is unclipped from the distributor next. This is the last of the umbilicals tethering the distributor to your Corvette engine.
07: The distributor retaining bolt is located directly below the rotor platform and is virtually inaccessible using a conventional wrench or socket. A special distributor wrench like this one is well worth the $12-$15 cost. Once the retaining bolt is removed, the “hairpin” clip can be pulled free from the distributor shank.
08: Before lifting out the distributor, the rotor platform was marked with a piece of tape and a corresponding scratch put on the block. This is done so your Corvette’s distributor indexing will be correct upon reinstallation.
09: Lift the Corvette distributor straight up and out of the block. Try not to rotate the main shaft while removing it. As soon as you get the distributor out, put a clean shop rag in the hole in the block to prevent any foreign objects from falling in.
10: A wide-blade screwdriver is used to remove the cross gear assembly from the distributor. When the assembly is removed, inspect the shaft gear to make sure it isn’t worn or has broken teeth. If it does, the shaft will also have to be replaced in the distributor or the new cross gear will be chewed-up in short order.
11: The new (left) and old (right) cross gear assemblies, side by side. Note the wear on the old bronze unit – no wonder the tach ceased to function.
12: The new cross gear assembly is inserted and tightened, again using the wide-blade screwdriver. When the assembly is tightened down, spin the rotor gear back and forth to make sure the cross gear is moving freely. If everything checks out OK, it’s time to begin reassembly.
13: Remove the shop cloth and gently lower the distributor back into the block. If you haven’t disturbed the orientation of the rotor shaft, it should drop back in place without a problem. If it doesn’t, remove the distributor again and check the position of the oil pump drive key in the bottom of the shaft and orient it to match the slot of the oil pump drive, visible by looking into the hole in the block.
14: With the distributor back in place, line up the index marks on the tape, replace the hairpin clip, and tighten down the retaining bolt with the distributor wrench.
15: Replace the distributor cap and tighten down the “j” latch bolts with a screwdriver. Reverse the order of disassembly to replace the coil, shielding, plug cable braided shields, etc. That’s all there is to it. Now you can watch your Corvette’s tach needle swing as you bang through the gearbox.
Tachometer Drive Cross Gear Replacement: Replacing the Tachometer Drive Cross Gear on a 1963-1974 Corvette
Source: Zip Corvette Parts
8067 Fast Lane | Mechanicsville, VA 23111 | (800) 962-9632
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