Tachometer drive gear/cable wear problem

[Robert W.]

Hi, My tachometer stopped working after ~100 miles on a completely restored drivetrain. I pulled the tach cable off the distributor and found some brass shavings in the cable housing. The cable end looks perfect (and turns easily), but the drive gear hole appears to be worn to failure. I'm pretty sure the cable collar wasn't completely tight so it was slightly cocked, but is that enough to cause this kind of failure? I have a new drive gear on order already, but I'm wondering if something so simple could have caused this seemingly extreme wear...

[Joe L.]

Re: Tachometer drive gear/cable wear problem

Hi, My tachometer stopped working after ~100 miles on a completely restored drivetrain. I pulled the tach cable off the distributor and found some brass shavings in the cable housing. The cable end looks perfect (and turns easily), but the drive gear hole appears to be worn to failure. I'm pretty sure the cable collar wasn't completely tight so it was slightly cocked, but is that enough to cause this kind of failure? I have a new drive gear on order already, but I'm wondering if something so simple could have caused this seemingly extreme wear...

Robert-------


I'm not sure that I completely understand your problem. However, if you have a failed tachometer drive gear, that is usually caused by the gear "wearing into" the distributor housing and, thus, becoming mis-aligned with the mainshaft gear. Unfortunately, that usually results in the destruction of both the cross gear and the mainshaft gear. In about 1970 GM added a nylon thrust button in the distributor housing to prevent just this sort of problem. The button was added to both PRODUCTION distributors at the time as well as SERVICE distributors for older applications.

Various thrust controlling devices are available in the aftermarket.

[Robert W.]

Re: Tachometer drive gear/cable wear problem

Thanks Joe, I'll pull the distributor tomorrow and get some good pictures of the cross drive gear.

[Robert W.]

Re: Tachometer drive gear/cable wear problem

Here's a pic of the off-center drive gear. It can be moved around in it's bore as well as in/out ~0.15"... I'll post more after I get the parts tomorrow and disassemble it.

[Joe L.]

Re: Tachometer drive gear/cable wear problem

Here's a pic of the off-center drive gear. It can be moved around in it's bore as well as in/out ~0.15"... I'll post more after I get the parts tomorrow and disassemble it.

Robert-----

The coupling (the brass component) may be worn creating the off-center gear. The gear's end play is excessive and probably indicates a worn distributor housing gear cavity end.

[Larry M.]

Re: Tachometer drive gear/cable wear problem

Robert:

That is not pretty. Appears that the tach cross gear is/was binding in the distributor causing difficulty in turning. Could be bad gear mesh or perhaps no end clearance was left during this gear installation.

If a soft thrust button was installed, the gear would likely have survived, but if a metal thrust button was installed and clearance was not provided, this could have caused the problem. Let's see what the cross gear and distributor look like disassembled.

Larry

[Robert W.]

Re: Tachometer drive gear/cable wear problem

Well... The driven gear is shot. I have all the parts arriving today, then I'll take a trip to my engine builder's shop to do the complete disassembly and cleaning. Not sure about the housing wear though...

[John D.]

Re: Tachometer drive gear/cable wear problem

Robert, May I make a suggestion. Send you distributor to Don Baker for restoration. I see you have alignment problems etc. Call me and I will tell you more. 724-832-3786 John

[Joe L.]

Re: Tachometer drive gear/cable wear problem

Well... The driven gear is shot. I have all the parts arriving today, then I'll take a trip to my engine builder's shop to do the complete disassembly and cleaning. Not sure about the housing wear though...

Robert-----


Yes, the driven gear is destroyed. As I mentioned, this usually also takes out the mainshaft drive gear, too. I can't see it well enough in your second photo but you need to slowly turn the shaft and inspect it. I cannot tell, for sure, if the housing is worn because the grease might be interfering. However, if the "tit" in the center is actually raised, it's NOT supposed to be that way and then the housing is very worn and this needs to be corrected. Otherwise a new gear will not last very long.

[Robert W.]

Re: Tachometer drive gear/cable wear problem

I realized I didn't close out this thread properly! The distributor shaft, tach driven gear, retainer and thrust washer were all replaced and the engine has about 150 mi on it now without any apparent tach drive area wear. The tach drive gear thrust washer was in the wrong position (on the end...) and the gear/retainer took the hit. Fortunately the housing had no real wear after the grease was cleaned out. Thanks Everyone!