What direction do I take? I have a stock 1972 350/200 small block with a 4 speed, the tach has been rebuilt 3 times (new main shaft and tach gear). Each time I install it the tach cross gear breaks right in the center after 30 seconds. The shop doing the rebuild restores corvettes and have rebuilt lots of tachs and are scatching their heads (everything appears in spec). Do I dump the unit for a new one (the car is bone stock with 56k on the odo), retro unit or upgrade to a after-market with electronics vs the points? :banghead:

 

Sorry can't help but heres a TTT for you

 

There was a post a while back about reproduction distributor parts (mainshafts & side gears) being sub-standard you might consider doing a search!! ...redvetracr

 

I just went through this with my 72 distributor and (2)- 74 distributors. I bought the replacement main shafts and cross gears from two different well known and respected vendors. I got the same cross gear from both and after the 3rd cross gear sent them back for credit. They just plain didn't work. Of the 2 main shafts 1 was good the other was bad. The gears I got were black with a brass shoulder on the gear face. I was told to fit the gears by facing off the brass shoulder. Well, I faced them off in a lathe to the exact dimension of a good GM gear I had and the new gear bound up. I worked on them for a while,even making up a new brass shoulder - still didn't work. I could replace the new gears with the old one and it worked fine. I turned them by hand-never in an engine. It would have wrecked the main shaft and cross gear in seconds under power. If your guy just replaced the gear and shaft without checking the fit then that could be the reason you're having a problem. I got a needle bearing coupling and gear from High Tech Innovations in FL and it fit pretty good, I didn't get the thrust washer for it though so I'm still waiting for that to come in the mail. The gear fit was great just too much end play. I'll probably endup making the washer.
Hope this helps, good luck.
Gary

 

I would suggest installing an electronic tach from a 75-77 model. Works like a dream.

 

Stratos -
A couple of things can be happening:

First, it's quite common for older distributors to get a bit of wear in the inside of the distributor housing in the cross gear thrust surface area. This will look like a slight counterbore inside the housing, and it looks like it's supposed to be there... but it's not. When you put a new gear into the housing, the new gear will drop into this counterbore formed by the previous gear, it will jam into the side of the housing and break.

The solution to this is to fabricate a custom thrust button for the inside of the housing to eliminate this counterbore problem. By doing this, you can also set up the cross gear end play properly.

The second problem is wth the cable itself - especially if you're running one of the 90-degree adapters. If your distributor is installed in the "correct" orientation, the cable will have a very sharp bend to it, increasing the torque requirement to drive the cable. The 90-degree adapatrs are also real torque hogs. This can casue the gear to snap. To correct this, the distributor in a C3 chassis should be installed in the alternate orientation as allowed by the 1969 Service Bulletin. This straightens out the tach cable and reduces torque load.

If you need any help with this, or if you'd like me to fix that distributor correctly, let me know...

Lars
V8FastCars@msn.com

 

I tried something similar to this, where I rotated the dist 45 degrees CW (and also moved the plug wires CCW one space on the cap), and permanently removed the right angle adapter. I got a new tach cable made; it has been working great.

I also upgraded the points with the Crane XR-i pointless conversion. It uses the existing lobes on the shaft for timing detection. So, you can retrofit back to the old points in minutes should the need arise.