Corvette Distributor Gear Replacement

Tachometer Drive Cross Gear Replacement

Get that mechanical tach needle swinging again with a new distributor gear from Zip Products Inc.

Mechanical tachometers were standard equipment on Corvettes from their inception through the 1975 model year. Unfortunately, they're all prone to the same problem: Eventually, the cross 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...

Rather than replacing the cross gear assembly with a stock bronze unit, we opted to go for a stainless steel needle-bearing unit, obtained from Zip by mail-order, for added longevity. Our project car is a 1967 427 coupe with transistorized ignition. Here's how the repair was done.
	01: The 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 the 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 the distributor indexing will be correct upon reinstallation.

	09: Lift the 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: Here's the high-tech, stainless steel, needle-bearing cross gear assembly from Zip that will replace the stock bronze unit. The needle bearings will ensure that this job won't have to be done again - ever!
	11: 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.

	12: 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.
	13: 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.
	14: 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.
	15: 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.

16: 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.

	Tachometer Drive Cross Gear Replacement: Get that mechanical tach needle swinging again with a new distributor gear from Zip Products Inc. SOURCE: Zip Products, Inc. 8067 Fast Lane Mechanicsville, VA 23111 (800) 962-9632 PARTS LIST: M-2791 • 62-74 Distributor Gear w/coupling - Stainless Text and images by Tom Benford