Rebuilding Tach Drive ('62-'74) Distributors

[Christopher R.]

I'm going to build a tach drive distributor over the Winter. I've got parts and pieces from about 3 of them. Right now, I'm inventorying and evaluating what parts I've got, and what I need to order.

1. I've got 3 base plates. They all look fine to me. Looking through the archives, I find a few posts where people advised to make sure the plate doesn't wobble. How do I do that? Without assembling the distributor, is there a way to determine if a base plate is good?

2. How can I tell if the cam is worn? The cam is the part with 8 points that bump the points. Even if the cam is worn, when I adjust the points, won't that compensate for any wear?

3. How can I tell if the drive gear is worn? On both the cams and the drive gears, I can see wear, but not much. Do I care if the drive gear is slightly worn? It's a spiral gear that is driven only one way. When I set the timing, won't that take up any slack?

[Ernie F.]

Re: Rebuilding Tach Drive ('62-'74) Distributors

Christopher,

Damn, two old farts on here at the same time. I got you by 14. Were you in St. Louis in the 75?

The distributor base plate can wobble if the upper shaft bushing is worn. You can check this by installing it over the bushing and installing the snap ring that keeps it from moving upward. If you have new bushings, I doubt it will wobble. Most of the time the bushing will wear out that the plate rides on if there is wobbling in the plate. The plates are usually ok. You can check this with a micrometer to verify it.

You can use a micrometer to check the lobes on the cam. I don’t have the measurements here but I can check this on Monday for you. If the lobe is worn, this will create a longer break point in the point set, and also case you to burn points out quicker. The points will not beak as cleanly if the lobes are worn and will not deliver proper spark.

Setting the timing won’t remove the slack from the cross gear. The most important things to look for in the distributor is wear in the distributor housing wall, the wear on the upper and lower bushing ID, and the upper bushing OD. All of these things can cause you major headaches in rebuilding the distributor.

There was a major change in the distributors in 1968 (I think this is when it changed). The distributors wall was drilled on the later model distributors to allow you to insert a Teflon button in the wall to keep the cross gear from eating in to the housing.

You can tell if you have a late model style distributor by looking for the hole on the outside of the housing. If there is not a hole there, then you need to look inside the distributor where the cross gear would ride on the housing wall. If the wall is worn, you will see a tit sticking outward inside a hole created by wear of the cross gear. If this is present, you will have excess play in the cross gear and eat up the teeth. It can be fixed without drilling the housing and inserting a bushing, and it can also be corrected by buying a different style aftermarket gear. Some of the aftermarket gears have a brass end to them and this will allow you to run the gear as is. The best way to check the gears when installed is to rotate the main shaft forward and backwards. If you can do this without having resistance the gears should work in the car.

Another thing to look at that will cause the gears to eat up is the coupling that holds the cross gear in place. The cross gear has raised places that ride in the coupling and over the years they will eat in to the coupling and allow the cross gear to move when torque is applied. If you look inside the coupling and see wear lines, it needs to be replaced.

You can usually turn the gear around and move it in and out of the coupling and feel for drag. If the gear has eaten in to the coupling you will feel ridges when you pull in and out. This is done from the outside of the coupling and you are only using the gear to feel for these ridges. (just a cheat trick after 200 or so distributors).

Many times my shop has had customer request only the tach drive cross gear. We have told them over and over that you should replace both at the same time. If one gear is worn out, the odds are against you that both teeth have wear on them. When the cross gear takes a tank it will more often than not take out the main shaft gear as well.

I understand that Lars has retired so if you need it, I can rebuild this for you and even set the advance curve if needed. I have a old sun distributor machine that I can do it on. If you need help with this, just email me direct at Ernie@Willcoxcorvette.com

Regards,

Ernie

[William C.]

Re: Rebuilding Tach Drive ('62-'74) Distributors

In addition to the fine advice from Ernie, I would mention that another source of "Slop" in the breaker plate is wear on the hole that the pin of the vacuum advance rod fits into. A light tap with a hammer in a downward direction will usually mend that problem. Also check for slop in the lower bushing in the housing. The upper bushings are not subjected to the same side thrust as the lower bushing so the lowers wear out more quickly.

[Ernie F.]

Re: Rebuilding Tach Drive ('62-'74) Distributors

William,

Thank you! You are correct; he will need to check this too!







Hope this helps you.

Ernie

[Terry M.]

Re: Rebuilding Tach Drive ('62-'74) Distributors

"There was a major change in the distributors in 1968 (I think this is when it changed). The distributors wall was drilled on the later model distributors to allow you to insert a Teflon button in the wall to keep the cross gear from eating in to the housing."

Ernie -- In production we see the distributor housing change you cite during 1970 MY.