The tachometer has never worked on my 74. When I manually twist the cable it comes to life so I'm assuming the problem lies with either the cross gear, the drive gear, or the distributor main shaft. I'm hoping the distributor gear isn't hammered... In you all's experience what usually dies with these components ? Do you think I could get away with installing a new cross gear ? Eckler's has two. One duralast with needle bearings between the gear and coupler guaranteed lubed for life ($84.99). The other shows a bronze replacement cross gear ($57.99). Both include the tach drive gear.

Thanks for your help,
Scott

 

Hello
I had a similar problem with my 73 tachometer. I pulled the distributor out and looked at the main distributor shaft. The gears on the shaft were bad and that is what caused the tach drive cross gears to go bad. I would recommend pulling out the distributor first and checking the distributor shaft drive gear. If this is bad, then putting in a $50 cross gear will not help you at all.

I bought my distributor shaft at Ecklers with the cross gear and bronze bushing.

kdf

 

If the gear portion of the main shaft is damaged, you'll just chew up the new cross gear if you replace only that. I ended up rebuilding my distributor because the main shaft was damaged. Maybe it was overkill, but I replaced the cross gear with a roller bearing unit.

 

When the cross gear fails, it will usually take out the mainshaft as well. However, the reason the cross gear fails is usually because the thrust load on the cross gear has caused it to bore itself into the distributor housing. Before you do anything, you have to repair the damage inside the housing. I do ths by fabricating a custom thrust button to install in the hogged-out area of the housing. Only once this is fixed can you install a new mainshaft and a new crossgear. If you don't take care of this problem, the new crossgear will jam itself into the housing and strip out the mainshaft again.

Don't forget to chamfer the ends on your new manishaft: none of the shafts I've seen have the chamfers on the end, making distributor installation very difficult...

 

The cable itself could also be bad. It's a square drive cable and the end can "round" slightly... but enough to cause slippage.

I've had good luck with the "needle bearing" setup and a set screw.

Here is another approach-
They sell a jig that screws into the bushing threaded hole that allows you to accurately drill a hole through the distributor case which you then thread for a set screw. The set screw rides on the end of the cross gear, and holds the cross gear so that it -almost- centers on the mainshaft gear, and does not "wander".

I tried a set screw, but it kept moving. So I installed a stainless hex screw (1/4-28) in place of the set screw, and put a jam-nut on it to hold it in place on the outside of the distributor.

This has been in service for 15-20 years and I have not had any problems with it. It's been so long since I installed the first one, that I can't remember when I did it.

Just be sure to "spot face" (grind flat) the set screw or hex screw at the tip where it rides on the cross gear or you stand a chance of chewing up your new cross gear. The stock setup allows the cross gear to "float" quite a bit even when the bushing is tightened all the way, so I do not set them at "0" endplay.... I leave them just a little loose to account for heat expansion etc.

With the set-screw all the way out, I install the cross gear & bushing, and use a puty knife on edge to tighten the bushing as tight as I can get it... then I slowly turn the set screw in until the cross gear doesn't move end-to-end (0 endplay). Then I back off the set screw whatever I choose as the appropriate endplay, usually 1 turn, and lock the set screw with the jam nut.

Again... the first one I did has been in service for 15-20 years and I have not had any problems with it.

 

There is also an area of correct tach cable routing. You may find that there is not enough cable to route to the tach drive connection. There may be a lot of stress on the cable where it screws into the tach drive gear.

One solution is to reference your distributor to an alternate position as documented by Chevrolet, and the other is to buy a 90 degree adapter gear which will remove some of the stress from the cable to the drive gear.

I think that one problem I had was that the tension on the gear from the cable was too much which caused interference with the proper operation of the coax tach drive cable, causing friction between the tach drive gear and the main shaft gear, which contributed to the grinding of the teeth on both shafts. Sounds good in theory.

Here is the tach drive adapter to help remove the cable strain to the tach drive gear.
kdf

 

Thank you very much guys!