Distributer/Tach Gear

[Bill S.]

After starting up my 66 smallblock a grinding noise was heard from under the hood and immediately after that the tach stopped working. Obviously the gear and coupling need replacement. Do I have to pull the mainshaft from the distributor or can I replace the gear with just pulling out the distributor?

Thanks Bill

[jay]

Re: Distributer/Tach Gear

bill: I just went through the same ordeal and after much consideration I choose to re-build the complete system. The reason is that even though the main shaft seems o.k. even the most minute imperfection will cause the gears to strip and leave you with another rebuild. The reason for the damage is that the gears were not properly lubricated or the distributer was not shimed at the correct height. good luck, hope this helps

jay

1964 roadster

[Rod McCormick]

Re: Distributer/Tach Gear

Bill,

I strongly suggest removing the distributor and pulling the distributor main shaft out. All the pieces that shreaded off the gear fall down into the bottom of the distributor. There is a brass bushing at the bottom (at the top too) through which the main shaft spins. The small fragments of shreaded gear will work their way inbetween the lower bushing and the main shaft. This ruins both the main shaft and the bushing. Once they make it through that barrier, they fall down into the oil pan and could get picked up by the oil pump. Second reason to pull the main shaft is that I suspect that the gear on the main shaft is also damaged and you will have to replace the main shaft as well as the tach drive gear.

Rod McCormick 67 Convertible

[Bill Clupper]

Re: Distributer/Tach Gear

In addition to pulling the mainshaft, check carefully the dist. housing where the crossgear bottoms in the cavity. Improper shimming, lack of lubrication, or a too-long cable will result in a condition where the gear "eats into" the cast iron it rides on, causing the misalignment that breaks the gears. New ears in an unrepaired housing will be short lived at best.

[Joe L.]

Re: Distributer/Tach Gear

Bill----

I absolutely agree with Bill Clupper's advice that you inspect the inside of the distributor cross gear housing which serves as the "thrust surface" for the cross gear. THE MAJOR cause of cross gear failure is mis-alignment of the gears caused by excessive wear on this surface. Bear in mind that the cross gear is a flame-hardened gear and the distributor housing is soft cast iron.

In 1970, GM added a nylon plastic thrust button at this location to all PRODUCTION and SERVICE distributors. Distributors so-equipped can be identified by a 3/16" hole seen in the end of the cross gear housing. This hole, which is a through-hole, retains the tab attached to the rear of the button. This button, GM #1957329, can be retroffited to any earlier distributor. Unfortunately, it is GM-discontinued. A much better solution, anyway, is to use the bronze thrust button available from Hi-Tech Innovations. This button mounts in the same hole used for the GM nylon button. If you wish to add such a button and the necessary hole, you can fill the remaining exterior portion of the hole with epoxy and paint the distributor housing as original. That way, no "eagle-eyed judge" will know that your distributor is modified to later coinfiguration.

Also, I recommend that whenever the cross gear is replaced that the mainshaft be replaced also. In fact, I can practically guarantee you that when you dis-assemble the distributor you will find that the integral mainshaft gear will be damaged significantly. Hi Tech Innovations can supply you with a new mainshaft and a cross gear of either the original or their patented roller bearing variety. Also, and most importantly, they can transfer your existing "auto cam" from the top of the distributor mainshaft to a new shaft. ALL Corvette tach drive distributor mainshafts are the same(except FI, of course)but there are many different "auto cams" afixed to the top. The "auto cam" is one of three components that determine your distributor's centrifugal advance characteristics. So, if you wish to maintain this as original, you need to maintain your existing "auto cam". Replacement mainshafts sold today in the aftermarket have only one or two "universal" auto cams. The GM original replacement mainshafts with the correct "auto cam" for every distributor are long-since discontinued.