Hey people,

I'm very novice at this vette ownership stuff and have run into my first big project (big to me at least).

My Tachometer doesn't work, and I have deduced that the major problem is the cross shaft gear in the distribitor. I bought a replacement, and took the old one out. The old one was in REALLY bad shape. I mean, metal scraps came out in the grease. There was also no set screw, which I have come to discover might be a necessary piece. Anyway, this was before i knew about the set screw even existing, so I just replaced the old cross shaft gear with a new one and put the distributor back in the car. I plugged in the tach and it still didn't work. I went to unplug the tach from the distributor to do some more debugging and HALF THE CROSS SHAFT GEAR CAME OUT WITH IT. The thing had broken in half in less than 30 seconds of running the engine.

So now I am at a total loss. Do I try to replace it again, this time maybe with a set screw. What is a set screw anyway? Would it be a better idea to just replace the whole distribitor at this point? Just looking for some clues, and I appreciate your responses.

Thanks again,

-carson

 

Ping;

Unfortunately, when your cross shaft gear is toast, so is your mainshaft gear, which necessitates replacing the mainshaft. You also probably need to replace the upper and lower bushings, which should then be align honed. The set screw you describe is an aftermarket modification that may render your housing unusable. I speak from personal experience. When I bought my '73, it had the wrong distributor, a non-tach-drive unit. I found a used tach-drive and began the process of rebuilding it. I've been advised to have a machinist spot face the inside of the housing where the set screw hole is, then have a button fabricated to fill the hole and position the cross shaft gear. I have yet to install the tach-drive unit because I haven't resolved how to position the cross shaft correctly to provide smooth engagement of the cross shaft and main shaft gears. I've seen rebuilt tach-drive distrubutors for sale for about $260, but I can't attest to their quality.

 

Well this is a set screw



They almost always have allen heads, but every once in a while you find slotted ones.

 

Obviously, Lars would be the absolute expert on this topic, but I will chime in anyways. If the new cross shaft snapped in two, I would say that the gears which mesh with the main shaft were somehow lined up incorrectly. This can be caused by the old cross shaft grinding a groove inside of the Brass Coupling. The coupling needs to be replaced when the cross shaft is replaced. As far as the set screw is concerned, not all Tachs came with one. Mine for example has a little plastic button which takes up any play in the cross shaft.

Hope this helps.....
Stew

 

Thanks for the info. That helps a lot. Mine also had a little plastic button which is so small that at first I thought was just a piece of the old cross shaft gear that had been torn off. I hadn't even thought about replacing the brass coupling as well.

I guess I need to take out the other half of the cross shaft gear before I try to drive anywhere. Will it hurt to drive it without that piece? I assume not since it has been running fine with a shattered one for at least 6 months now.

 

Check with the Last Detail .....They have the best prices on cross gears main shafts and Bearings...I just rebuilt mine two months ago for like 60 bucks or so.....Works like new now

 

also shim the dist main shaft endplay ( up and down)to .005/.006 to prevent this from happening again. :chevy

 

I got a very informative email from Lars on this subject and he asked me to post it here as well. Here goes....


Your issue is very common. There are several configurations of the cross
gear system in various year distributors. But here's the basic deal:

When the cross gear turns, the torque required to turn the tach drive
cable/system pushes the cross gear into the side of the distributor housing.
To accept this thrust load, stock distributors often had a nylon thrust
button located by a small (about 3/16" diameter) hole in the side of the
housing. Other distributors had no thrust button at all, and these
distributors have no hole in them. The non-button distrubutors simply use
the inside flat surface of the housing to take the cross gear thrust. No
distributors were ever built with a "set screw." The set screw deal is a
modification that I have seen on numerous occasions, and it does not work
well: The tip of the set screw does not have a large enough diameter to
create a good thrust surface for the cross gear to ride against - the load
is not well distributed. What happens is that the set screw fairly quickly
eats itself into the end of the cross gear, destroying either itself or the
gear. Not a good system.

Under normal operation and wear, the cross gear eventually either wears out
the nylon thrust button or eats its way into the side of the distributor
housing. When this happens, torque loads on the gear increase, and the gear
strips itself out. It usually also strips out the distributor mainshaft, so
both components must be replaced. As you have noticed, you cannot simply
replace the cross gear without tending to the other matters...

If the distributor was originally equipped with a thrust button, and if the
inside of the housing has not been damaged, a new brass thrust button can be
installed along with the new cross gear and a new main shaft. This will
provide good service as long as everything is lubricated well with some good
wheel bearing grease.

If the inside of the distributor housing has been chewed up from the cross
gear eating itself into the housing, some machine work is required to do a
good repair. I like to go in and do a spot face of the housing where the
gear dug in. I then fabricate a custom steel thrust button with a polished
thrust face to go into this spot faced area. This re-establishes the
correct amount of end play for the cross gear. If you have access to a mill
and a lathe you can easily do this yourself. Whatever you do, it is
imperative that you inspect the thrust surface/situation inside the housing
and correct it in some way other than shoving a set screw through the side
of the housing.

-Lars

 

ping -
Thanks for posting that up for me - I can't post to the Forum from work, so I have to e-mail tech responses when I see something in a post...

I'm going to be machining one of the steel repair thrust surfaces tomorrow in order to repair a Forum member's distributor. If I can get my hands on the company digital camera I'll shoot a few pics of the part and the repair process for fixing a damaged distributor housing and re-establishing the cross gear position without trashing the distributor housing by using a bolt or set screw through the housing.

 

Gentlemen...

I have the "set screw" installed on my 66 and I have not had any of the above problems with it.

However...

I did not use the set-screw... it was too loose (wouldn't stay put) and the tip was not faced.
I used a stainless steel machine scew which I cut to length and spot faced myself, and a stainless jam nut between the head of the machine screw and the distributor housing to lock the screw at the desired setting.

The set screw kit that I used came with a drill guide which is used to drill the housing from the inside instead of the outside... this assures that the hole/threads are aligned properly with the cross-gear.
I also opted to use the needle bearing bushing instead of the standard bushing.

This combination has proved very reliable.

I suspect the damaged ones (referred to above) were due to over-tightening of the set screw as well as a lack of spot facing etc.
Mine works fine and has not worn as described (I have it out right now during a hone & re-ring).

Addendum... Some cross-gears do not have a perfectly smooth face... they are bored for grease retention etc. These will have a tendency to ride on the outer edge of the cross-gear, and this can cause premature failure (per above). The cross-gears I use have a flat, unbroken surface which I polish, and I also polish the tip of the set screw. So.. you have to have your wits about you when you make mods like these.

Tom


[Modified by Tom454, 8:25 PM 4/20/2002]