Phasing distributor gear

[Norris W.]

[Joe L.]

Re: Phasing distributor gear

Norris----

The distributor drive gear is installed with the "dimple" more-or-less aligned with the rotor contact. This has been my understanding for years. Also, just to confirm, I checked several NOS distributors that I have and all are configured like this. Presumably, the factory got it right.

[Norris W.]

Re: Phasing distributor gear

Thanks Joe. I've done some more playing with it today. I've only got the one other distributor at my easy reach over the weekend to compare it to, but anyhow, this is what I kinda think: The distributor would go in the right position before I built the engine. I replaced the camshaft. The obvious conclusion is that the camshaft gear is slightly different in reference to the cam timing. I don't know how or even IF the cam manufacturers phase the distributor gear, or if it's just ground. In any case, rotating the distributor gear put the distributor housing in exactly the right location.

[Gerard F.]

Re: Phasing distributor gear

Joe,

This may seem to be a dumb question, but couldn't one just reverse the position of the rotor, i.e. turn it 180 so that the dimple on the gear collar lines up with the contactor on the rotor. I'm looking at my original 67 distributor and wondering why one would pull the gear and turn it 180. Am I missing something here?

Jerry Fuccillo
#42179

[Joe L.]

Re: Phasing distributor gear

Jerry-----

The rotor can only be installed in one orientation. There are round and square "indexing posts" on the rotor which control this. They fit into corresponding holes in the distributor cam top plate.

[Gerard F.]

Re: Phasing distributor gear

Joe,

As usual, you are correct. Rechecked my original on which I changed the shaft and haven't put back on yet. By sheer luck, the rotor contact lines up with the dimple on the gear collar. Had it been otherwise, and I reinstalled it, I would have wasted a lot of time without your tip. Thanks so much.

Jerry Fuccillo
#42179

[Duke W.]

Re: Phasing distributor gear

This issue has been a nemesis for fifty years.

On all Corvette engines that I am aware of the dimple must be in line with the rotor tip in order to achieve the correct initial timing without the dist. interfering with another component, but to the best of my knowledge this fact is NOT MENTIONED in ANY GM service literature, but it is mentioned on the distributor assembly drawings.

Because the dist. gear has 13 teeth installing it 180 out will alter the radial indexing of the dist 6.5 deg. to achieve the initial timing spec, and on most engines the the vacuum can will interfere with something before the spec can be achieved with the gear on "backwards".

Duke

[Norris W.]

Re: Phasing distributor gear

Agreed, BUT the question remains, how accurate are the cam manufacturers in the positioning of the gear on the camshaft? Again, in the case of my 435 car the distributor went from being all in the back of the intake where the back carb mounts to all in the shielding bracket by moving one tooth. I did it 4 or 5 times to make sure I wasn't moving 2 by accident. Distributor fit in right position BEFORE camshaft change, wouldn't get in position after, so I can only assume the difference is in the phasing or indexing of the camshaft gear in relation to the camshaft itself. (Crane blueprinted factory cam both times) Incidentally, I didn't degree the camshaft either time, just threw it in on factory marks............ new timing set (cam and both gears) also. Obviously retarding or advancing the camshaft would have the same effect on housing position to get the timing right.

[Duke W.]

Re: Phasing distributor gear

The dist. drive gear on the cam better be indexed within a small fraction of a degree or get another cam manufacturer.

I also need to correct an error in my earlier post. Since the dist. driven gear has 13 teeth this is 360/13 = 27.7 degrees per tooth, and the two gear indexing positions are one-half tooth or about 14 degrees. Look at the pin axis. On one side it is right in the center of a tooth and the other side it is exactly between two teeth.

If you set the dist at 12 initial and then remove it and rotate the gear 180 and reinstall, in order to achieve the same 12 initial the dist. housing will be either 14 degrees CW or CCW from its original position, and that will usually result in the vacuum advance interfering with something before you can achieve the 12 deg. initial timing.

Duke

[Norris W.]

[Duke W.]

Re: Phasing distributor gear

If you look at the ends of the drive gear teeth with the roll pin installed (as I described in my previous post), it's pretty obvious that indexing the gear one way or another is going to reorient the shaft 14 degrees.

Duke

[John H.]

Re: Phasing distributor gear

Many of the aftermarket cam manufacturers pay no attention whatsoever to the drawing requirement for centered helix angle orientation of the drive gear on the cam relative to the dowel hole where the sprocket attaches, as they can't understand why it could possibly be important. It's important on a Corvette due to the distributor having to be oriented such that the vacuum advance can fits in the shielding without interfering with either the shielding or the plug wire support bracket or with the #8 intake runner on high-rise aluminum manifolds and the tach drive output must be oriented such that the cable won't be kinked; all of these parameters must be met while at the same time being able to set initial timing - THAT'S why it's important on Corvettes, and the GM cam and distributor drawings are VERY specific about the gear orientation on the cam and on the distributor mainshaft. GM-manufactured cams meet the drawing requirements; many aftermarket-manufactured cams don't.