I believe my tach drive needs to be replaced. If I spin the cable the tach will move so I believe the little gear in the distributor is worn. Looks like a failry simple thing to repair. My question is can I just remove the bronze piece and put in a new replacemnet gear with out having to remove the distributor from the engine? I realize I should look at the shaft too but can I just pop out the old gear and put in a new one? Am I correct that the drive gear will come out once the bronze piece is unscrewed?
This is more of a logistics question I guess because I want to know if it can be done while still in the engine compartment.

 

Most time simply replacing just the drive gear will only work a short while. The gear on the dist. shaft is damaged also, so both will need to be replaced.

 

In addition to what Mike correctly stated above, the reason the gears get damaged is that the crossgear digs and wears itself into the side of the distributor housing thrust surface over time. Once it digs in to the point that the gears start touching, the crossgear locks up and shears the teeth off itself and the mainshaft. If you just replace the crossgear and mainshaft, the failure will re-occur. You have to repair the distributor housing first, then replace all the parts. I custom machine new thrust surfaces that I press into the housings to do the repair - it's the only permanent fix I've been able to come up with.

Lars

 

I'm sure both of you are correct. I really didn't think about wear inside of the distributor. Is there an average mileage on them when this becomes a common thing?

 

Have you checked the end of the cable that fits into the distributor gear?
Before you tear the distributor apart, I'd just make sure you still have the square end on the cable that fits into the square hole in the distributor gear. Sometimes these get rounded off from lack of cable lubrication and folks will just grease up the cable again but it still won't spin because the end has already been rounded off.

Check the obvious before you go for the major surgery-
Just my .02
Elm

 

The ones that get sent to me with complete failure of the thrust surface and gears seem to be in the 100,000 range.

Lars

 

Yep, here is 96,000 miles.





When metal fragments let loose major damage can occur (see bottom bushing and shaft). This unit has been rebuilt with a new parts.

 

You could always swap in an electric tach with the correct face and run an HEI ignition, if your car isn't in the NCRS game.

 

I do have an after market HEI with tach drive but I really want to keep this car stock. Although I could always put it in and change it back later when I plan on selling it. I really think it would run better with the HEI. If I do this I would have plenty of time to rebuild the distributor the right way.
Thanks everyone for the help.

 

So if I read between the lines I can just remove the bronze piece and put in a new replacemnet gear with out having to remove the distributor from the engine and take the chance that it just happens again. Correct?

 

Why would you do that...? If your crossgear is damaged, the mainshaft is damaged. If you just replace the crossgear, you are 100% guaranteed that it will fail again, and you'll keep putting metal through your engine. There are no "between the lines" recommendations in this thread to just replace the crossgear. Quite the opposite: We have clearly stated you cannot do that without trashing the distributor again.

Lars

 

Hey Sportscar 01 take my advice and listen to Lars, believe me he knows what he is talking about. Lars has taken the time to not only give me great information in the past but he also fixed my carb!

 

If I was you I would but side gears in bulk, you are going to need them.

 

All distributors built prior to mid 1969 were very susceptible to distributor side wall wear because the gear wore right on the wall. Mid 1969 Delco started putting Teflon bushings in the wall of the distributor to prevent this. The easiest way to tell which distributor you have aside from looking up the number is to look at the side wall. You can see the difference in the picture below.



If you have a hole there, then you have the 69 mid to 1974 style distributor. If this is the case there is a chance that replacing the Teflon button will fix your issue. But that is doing to depend on how badly worn the distributor wall is in.

Teflon button:


You can see the wear in the wall of this pre mid 69 distributor. (if it were a late you'd see a hole where the nipple is).


Another thing that will cause the cross gears to wear out is the coupler. The coupler can wear on the inside and allow the side gear to **** sideways enough to eat out the teeth. You can see the grooves in the coupler bellow.


What everyone has told you above is correct, replacing one gear is a band-aide that will not last you.

The best advice I can give you is to pull the distributor and check the condition of all the items. Then either have it rebuilt or rebuild it yourself.

When we have a distributor with the wall work, we machine out the side wall and insert a brass bushing into the wall. This replaces the worn area and allows the gears to have a fresh start.

 

Hi, I realize this thread is a few years old but hoping someone might see this. I have a '73 with a tach prob also. It works sometimes and sometimes not. I checked the cable as suggested and it looks fine. I tested the tach with a drill, also fine. I did remove the gear and it looks fine...ugg! Whats this brass button? It didn't come out when I removed the gear. Could this be my problem or is it more likely in the distributor itself. the car has 37k miles...yes pretty sure it has only 37k . thanks. Al

 

There is no mention of a "brass" button in this thread, only a Teflon button which is located in the wall of the distributor near the far end of the gear you removed. This replacement is bronze but requires machining
https://willcoxcorvette.com/corvette...e-button-62-74

 

A few years ago, my original dist. (1970), stripped both gears. It did not have any button, the gear just ran against the casting wall. There was minimal wear.
I used a bronze button/gear setup for the replacement. I drilled a "partial" hole in the casting for the button. (Didn't drill completely thru to keep original look from the outside).
With the button in place (without machining the casting wall) the fit was a bit too tight. I "thinned" the button just enough to get the proper end play. It didn't take much.

Last year there were some postings here that indicated the bronze button would wear quickly. I removed the dist last summer and checked the wear on the button. There was none, just a shiny spot where the gear was rubbing.

It was an easy repair, so far it's been working well.
Note: To check the end play, insert a 1/8" x 2" wooden dowel in the cross shaft.(Where the cable goes). Set up a dial indicator on the other end of the dowel. To check end play, just grasp the wooden dowel and force back and forth.

 

What I did as a alternative to the thrust plate was to drill and tap the housing and use a long 5/16" set screw with the tip ground flat and polished and a jam nut to control thrust. Gives you completely adjustable end play.

 

Thanks for the reply. Sorry I did mean the Teflon button. Could this be the reason my tach works on and off? Also I did feel around in the opening after I took the gear out and didnt feel anthing else that seemed it could be removed. Could it have just worn away? thanks so much for the help. Al