Hi guys,
I've just removed my distributor in order to fix tach that doesnt work.
This is what I've found: tach gear and main shaft completely worn!
Comparing the old tach gear with the new one they are different, the new one has a "spacer" at the end.

 

Also... if I look at the hole there's a shiny surface, where the gear is supposed to rotate... inside the replacement kit there was also a plastic button, you can see from this image:



this is the image of my distributor:

 

Finally... the main shaft has to be replaced unfortunatelly.... is it hard? any guide??? thank you guys!

 

Other thing I have noticed: I can move up and down the main shaft, in fact there's space between main gear and distributor housing, see this picture, is it ok?

 

Lower bushing likely trashed as well, due to bits of the tach drive...

 

really? where?

 

When you tap the roll pin out of the drive gear, the mainshaft will slide right out.
If there is much side play with the new shaft, you will have to replace the bushings.
You want to get the end play down to maybe 0.005 or 0.008 inch. There are shim washers available.
You can put it the gear back on two ways. Do it the right way. Temporarily install the rotor, and notice the dimple on the drive gear lines up with the electrode on the rotor, when the gear is correctly located.
Not a bad idea to upgrade to some sort of breakerless ignition, while you have everything apart. I used a Pertronix.
Pack the tach drive gearset with grease when you are done.

 

Thank you! I've already upgraded with Pertronix 3, great unit! right now the dimple is in the opposite side but everything works OK except the tach drive gear... I think I will install everything back as now because I dont want to mess with the timing...

 

not quite, the lower bushings wear because of the side load put on it from the oil pump, it`s actually quite common.

 

a factory 74 distributor should already have the plastic button, does the back side of the housing have a hole in it? opposite side of the tach drive gear?

 

I think that "spacer" is just worn off the old gear.

 

Yes there is a hole and I can see the brass....

 

Wow, I think you are right... Impressive...

 

The gear you purchased is made as a fit all... gear. On the earlier distributors there wasn't a Teflon bushing and the cross gear would eat right into the side housing of the distributor. This changed with the addition of the bushing I think in 1969. The only issue with this gear is if you have everything stock it can bind up. So.. one way to test the gear when it is installed is to make sure it will travel freely both forward and backward. If you have installed a new teflon bushing in the side wall then my guess is you'll have to shave some off the end of that small gear or it will bind. Put it together, see how it feels and if you can rotate it both ways go with it.. if not then shave some off and try again. (we chuck them in the lather so it is a smooth cut).

It looks to me like you have multiple issues going on. I would replace both the upper and the lower bushings, both shafts and then re-shim. If you have excessive thrust in the main shaft you'll eat bushings and cause premature wear of the two gears as well.

I just noticed this too.... the gear you have marked as an old gear is not an old gear.. In fact it is a reproduction. If you look at the gear in my top picture.... that is an original GM gear... So, with this you know that someone has already tried to solve the problem and failed.

When you shim the main shaft you should try to have between .002-.007 end play. I tend to lean around .002-.004.

If you look at the first pic you'll see both design's on the distributor. If you look at the middle you'll see the wear that can happen in the coupling. (just threw that one up in case someone else needed it). And if you look at the one at the very bottom you'll see why extra meat was added to the new style shafts.

The meat is there to make up for the groove that has been cut in the housing on the 62-69 distributors. When we build the early distributors we machine this out and insert a brass bushing to make the wall back to normal.

 

Thank you Will, many info here!
So I figure somebody tried to fix the problem.
It has been installed a brass bushing so I'm thinking of leaving it in place.
I've noticed that when I install the coupling without any gear I can do 2 1/2 turns but when I put the gear I can do only 2 1/8 turns of the coupling. This means to me that the gear is pushing against the brass bushing. Is it correct? Or the gear should have some sort of clearance with the coupling?

 

The housing must be machined out so that the brass bushing is flush with the inner wall... If they just pushed the brass bushing in place without removing material then it will bind the gear up. I can't tell from your picture if the bushing is raised off the wall or not.

If you have a Teflon bushing as shown in your picture above, then you should be able to knock the brass out and just pop the Teflon in place creating a flush wall... assuming they didn't enlarge the hole for the bushing.

What people have to understand is that the brass bushing is a great solution to an age old problem.. you can't just pop them in place and run the stock length gear. For most people it easy to install the brass bushing without doing any other changes... they just don't know changes must be made.... (see the note on the bushing on our site) http://willcoxcorvette.com/product_i...oducts_id=3480

So to run the brass bushing without removing any wall thickness requires one to shorten the cross gear the thickness of the bushing side wall...



When this part (above) is installed in the wall without machining some material out, it will increase the wall thickness and something has to change on the other side to assure the gears mesh properly.

So if you feel good about the bushing being in there then you must remove some of the brass off the end of that distributor shaft... again the quick test is the make sure you can rotate the main shaft forward and backwards smoothly without any binding. If you get this you'll be set to go.

Willcox

 

Dave Fiedler has an excellent article on rebuilding/restoring a distributor.

http://www.tispecialty.com/articles/article6.htm

I rebuilt mine last year, (1970 with no nylon button), and the housing was worn by the gear.

I purchased a gear/brass button "set" that was designed to have the brass button sit in place without recessing it into the housing.
The gear end was made to compensate for this.

With a very minor surfacing of the brass button, the correct end play of the shaft was easily done.

Note: My distributor is the original with 98K miles and the upper and lower main shaft bushings were still within spec.

Check the specs on the bushings before replacing, they may not need to be replaced.

Replacing the bushings is not as easy as pressings out the old and pressing in the new. When new bushings are installed the new bushings need to be align honed to ensure the perfect alignment between upper and lower bushings.

If this is not done, then the main shaft could bind/wear due to the ms-alignment.

 

Thank you guys, this is what I was thinking to do.
As soon as the main shaft arrives I will make the job!

 

Hello everybody, I've just finished to rebuild the distributor, reinstalled main shaft, fixed tach gear... I've also bought a new rotor because the one installed looked very bad... when I install the cap and I move by hand the main shaft, I notice the rotor hits one column of the cap (only that one). How is it supposed to work the transmission of the energy between rotor and distributor cap? Does the rotor has to touch every carbon bush of the cap??? thanks

 

This article seems interesting...

http://www.masacc.org/Documents/Dist..._Anomalies.pdf