What's the end-play spec for the tach drive cross gear?

[Mark E.]

1970 454 June car

Symptom: Tach needle "dips" or drops a couple hundred RPM every few seconds. It's more frequent when RPM is increasing and sometimes doesn't occur at idle or when cold(!).

The tach needle works fine when the cable is spun with a drill. The square end of the cable is just a bit worn at the tip.

I have two distributors and this problem happens with both of them. One distributor may be original (it came with the car in 1990, has the proper band and no hole or provision in the housing for a cross gear button); the other distributor has a brass button pressed into a hole drilled through the housing. Main shaft, cross gear, cross gear housing are in good condition in each distributor with no wear in the housing. The tach drives feel smooth when each distributor is turned by hand. The cross gear end-play for each distributor is 0.060" - 0.065"; I can see the cross gear pull in while spinning the distributors.

Could this problem be caused by too much cross gear end play? The end of the cable engages the cross gear securely when the gear is NOT fully pulled into the housing. But it barely engages the cross gear with the gear fully pulled in.

I'm thinking of reducing end play but cannot find a clear spec for this. No spec in either the CSM or COM (I didn't even see a procedure for servicing a tach drive distributor). I found this article by Dave Fieldler of TI Speciality:

http://www.tispecialty.com/articles/article6.htm

He specifies 0.010". This suggests end play in my distributors is way too large.

I did searches here and on CF; most seem to check by feel- if smooth it's okay.

To reduce end play for the distributor with a brass button I am thinking of using a punch to tap the outer end of the button to push it in a bit more. Other ideas? Are buttons of different thickness available?

And how can I adjust end play for the distributor without a button? Fieldler talks about adding a brass button to the end of the cross gear: "...custom make a spacer from bearing bronze to a thickness that will allow .010” endplay in the tach drive gear. Since this spacer is held in place by the tach gear, a hole must be drilled in the end of the gear so that a protrusion on the spacer can be pressed into the gear."

This sounds above my pay grade. Is there a better/easier way other than drilling the cross gear or housing for a button?

[Mike M.]

not familiar with brass buttons but for years, when I rebuilt my distributors, after I'd drilled the necessary hole, I always installed THE PLASTIC BUTTON ON THE INSIDE OF THE DISTRIBUTOR HOUSING. MIKE

[Mark E.]

not familiar with brass buttons but for years, when I rebuilt my distributors, after I'd drilled the necessary hole, I always installed THE PLASTIC BUTTON ON THE INSIDE OF THE DISTRIBUTOR HOUSING. MIKE

Thanks Mike. How do you adjust the end play using a plastic button?

[Mike M.]

mark: I believe the thickness of the plastyic buttons are such that one doesn't need to . io still have some buttons I can send ya if u wish. on the cuff. mike

[Mark E.]

mark: I believe the thickness of the plastyic buttons are such that one doesn't need to . io still have some buttons I can send ya if u wish. on the cuff. mike

Thank you Mike for the generous offer! I have plastic button so I'm good. I'll remove the brass button, install the plastic one and check end play.

I'm still interested in hearing from anyone who's adjusted end play by moving a brass plug in/out.

[Timothy B.]

Mark, The brass button should sit flush with the surrounding surface of the distributor housing. The tach cross gear gets a thrust washer installed before you slip it into the large brass bushing that is screwed into the side of the distributor, this keeps the gear centered and there will be just a very little play in/out of the gear, the thrust is inward when the distributor spins clockwise.

I suspect if all the ends are good the cable has been changed and the ends are to short.

[Mark E.]

Mark, The brass button should sit flush with the surrounding surface of the distributor housing. The tach cross gear gets a thrust washer installed before you slip it into the large brass bushing that is screwed into the side of the distributor, this keeps the gear centered and there will be just a very little play in/out of the gear, the thrust is inward when the distributor spins clockwise.

I suspect if all the ends are good the cable has been changed and the ends are to short.

The hardened thrust washer between the gear and the brass coupler is in place. The issue is I have 0.060" end play which I believe is excessive and allowing the cable to periodically uncouple from the cross gear.

[Timothy B.]

Mark,
I wouldn't think 1/16" play would cause the cable to ride out of the tach gear. Do you think the inner cable has been changed and cut to short.

[Mark E.]

Mark,
I wouldn't think 1/16" play would cause the cable to ride out of the tach gear. Do you think the inner cable has been changed and cut to short.

Timothy,

I hope you're wrong. I loathe the thought of tearing into the dash to replace the cable. Same cable since 1990 when I acquired the car so probably original.

I'm beginning to think I'm the first guy who will try setting end play by adjusting the position of the brass button. I hope it's a good idea. I'll also try the plastic button.

I'm down with Covid now but stay tuned.

[Mark E.]

Update:

I tapped the brass button in to reduce cross gear end play to about 0.010". I only have about an hour of driving into it but the tach is working normally! It seems that little bit of end play was enough to cause the cable to periodically skip.

I'm keeping my fingers crossed this doesn't cause any other problems like excessive wear on the gears. Lars posted on CF he does not recommend a brass button because they can wear leaving metal particles. So after a few miles I may pull the distributor, install a plastic button and see what end play that provides.

[Gary R.]

Hi Mark,
For what it is worth, I was rebuilding a couple of Tach Drive distributors for my own use about 15 years ago. I bought a new cross gear from a well known vendor. It had the brass button on the end. The problem was gear mesh with the main shaft. It was binding and this is by hand. Under power it will have stripped out. I was by the vendor they had to be fit, use a file and remove material. Well I tired that and it didn't work. Then I machined a new button and started over, removing a few thousands at a time in my lathe, no good. I had other main shafts and swapped them , still no good. No matter what I did the gear was binding, it was cut bad. I returned it and the replacement was the same.

I had one of the custom ones with a bearing made from a shop in FL I think. I don't recall the name but I think they're gone now. That worked ok but none of the new vendor gears did. I found a used gear in my box of parts, it was perfect. It worked in every distributor I checked.

I have not had to rebuild a distributor since so I can't say if the gears are still the same or if I got part of a bad batch, regardless they were all bad.

[Mark E.]

Hi Mark,
For what it is worth, I was rebuilding a couple of Tach Drive distributors for my own use about 15 years ago. I bought a new cross gear from a well known vendor. It had the brass button on the end. The problem was gear mesh with the main shaft. It was binding and this is by hand. Under power it will have stripped out. I was by the vendor they had to be fit, use a file and remove material. Well I tired that and it didn't work. Then I machined a new button and started over, removing a few thousands at a time in my lathe, no good. I had other main shafts and swapped them , still no good. No matter what I did the gear was binding, it was cut bad. I returned it and the replacement was the same.

I had one of the custom ones with a bearing made from a shop in FL I think. I don't recall the name but I think they're gone now. That worked ok but none of the new vendor gears did. I found a used gear in my box of parts, it was perfect. It worked in every distributor I checked.

I have not had to rebuild a distributor since so I can't say if the gears are still the same or if I got part of a bad batch, regardless they were all bad.

How frustrating.

How were you checking operation? By spinning by hand and feeling for smooth operation? Is there a better check?

After 100 miles my tach drive seems to be okay- spins smoothly by hand, tach works fine, IR gun reading of drive temperature is normal. Yet I wonder if it will grind up sooner rather than later.

[Gary R.]

How frustrating.

How were you checking operation? By spinning by hand and feeling for smooth operation? Is there a better check?

After 100 miles my tach drive seems to be okay- spins smoothly by hand, tach works fine, IR gun reading of drive temperature is normal. Yet I wonder if it will grind up sooner rather than later.

The gears were binding by hand, no way was I going to install them in the engine. Once I installed the used GM gear I could spin it for the next 82 years without it binding. The new junk I was sent wouldn't make it with a drill driving it.