Small Block Distributor Alignment

[Al C.]

I noted with interest the Dec. 1967 Chevrolet Service News reprint on page 17 of the Summer 2004 Restorer. Does the reclocking of the plug wires make up for the asynchronism of the number of gear teeth (is it 13?) vs. the number of plugs, to align the rotor at #1? If so, does it do that exactly, or approximately?
It seems that this could be a solution for using a vacuum advance distributor with a 2x4.
Thanks, ... Al '58 245hp

[Michael H.]

Re: Small Block Distributor Alignment

Al,

I hope I understand your question correctly. There are eight teeth on the distributor gear so there would be one gear position for each cylinder. Moving the spark plug wires one position in the distributor cap would equal one gear tooth position. If you moved the distributor gear and shaft one tooth CW and repositioned all the plug wires one position CW, nothing would have changed, other than the routing of the plug wires in their holders.

If you want to reposition your distributor so you can fit a vac adv unit, you would have to move the distributor one way and then move all of the wires one location in the opposite direction. If you only need one half of one position, then you would have to remove and rotate the drive gear 180 degrees on the shaft but make no other changes.

Michael

[John H.]

Re: Small Block Distributor Alignment

There are 13 teeth on the distributor drive gear, so each tooth rotates the shaft 27.7 degrees; re-positioning the drive gear 180 degrees on the shaft will re-clock it by 13.8 degrees.

[Michael H.]

Re: Oop's, John is correct....

That would be a very strange looking gear with only eight teeth. My brain must have been "off" late last night but the fingers were still typing.

[Tom Wander]

Re: Small Block Distributor Alignment

I installed a vacuum advance dist on my 2x4 by rotating the drive gear 180. It works perfectly with room to time correctly. No other changes were made.

[Al C.]

Re: Small Block Distributor Alignment

Tom - Thanks, that's good to know.
I still wonder about the configuration shown for the 1968 327, as to whether the rotor would be lined up accurately...
Regards, ... Al '58 245hp

[Al C.]

Re: Small Block Distributor Alignment - Again

John - I'm trying to understand the distributor geometry. Are these statements all true?
1a. With the dimple-to-rotor original gear orientation, engine at #1 TDC, inserting distributor at proper tooth, rotor points to 22.5 degrees right of center ("1:30 o'clock").
1b. Body is rotated so that when cap is mounted, lead #1 is also at 22.5deg, This is zero advance, points just opening. Rotor-to-cap lineup is dead on.
1c. Point timing is adjusted, relative to that reference, by rotation of the cap-and-body. 10 deg advance on crank requires 5 deg CCW on distributor.
1d. Rotor-to-cap lineup becomes offset as timing/RPM advances, being misaligned by 20 degrees (dist) when crank is at 40 degrees advanced.
1e. With the setup of 1a, the dist body could be rotated to any multiple of 45 degrees from 1b provided that #1 wire is always inserted into the cap at 22.5 degrees right.
1f. Situation 1abcde can be repeated with 27.7 degree increments in 12 other variants; #1 will NOT be at 1:30 (lets not go any further with this one ...).
...pause for breath...
2a. If the gear is reset at 180, then with engine at #1 TDC, the rotor will point to 36.3 degrees right (about 2:24 o'clock") by inserting distributor at proper tooth.
2b. Body is rotated so that when cap is mounted, lead #1 is also at 36.3 deg, and zero advance. Rotor-to-cap lineup is dead on.
2c. Timing away from zero behaves as 1c.
2d. Rotor-to-cap lineup becomes offset as timing/RPM advances, being misaligned by 20 degrees (dist) when crank is at 40 degrees advanced.
2e. With the setup of 2a, the dist body could be rotated to any multiple of 45 degrees from 36.3 provided that #1 wire is inserted into cap at 36.3 degrees right.
2f. Situation 2abcde can also be repeated with 27.7 degree increments in 12 other variants etc. The mind boggles, just don't go there.

A. I am still mystified how the 1967 Chevrolet Service News configuration reporduced in the recent Restorer can work, with the #1 wire moved? Statement 1e above seems to show that the vacuum can could be rotated to the back side yet the #1 wire should still be at 1:30 (22.5 deg right).
B. And is perhaps too much made of the rotor-to-cap lineup being offset when the gear is "backwards", considering that timing/RPM advances, misalign it anyway? It seems that an initial offset *in the proper direction* that would be cancelled by the advances would result in best alignment when running!

...Al '58 245hp

[Timothy B.]

Re: Small Block Distributor Alignment - Again

Al,

That's a mouthfull and I'm not sure I understand all of your post. If you place your balancer mark to line up at the 10 degree advanced mark on the timing cover, that is where you want to fire the spark plug. The next thing is to install the distributor in the correct tooth so that when the distributor body is turned to just open the points (fire the plug), the rotor alignment is pointing to # 1 terminal on the cap. At the same time make sure the vacuum advance is in the center of its travel (between the intake manifold and coil bracket), and the distributor cap hold down screws are perpendicular to the car as mentened in the 63 assembly manual.

When the centrifical advance moves the rotor it also moves the cam that opens the points so the rotor alignment is not changed. When the vacuum advance moves, the points and point plate move so there is also no change in the rotor alignment. I think this is where you are getting confused and I hope I didn't confuse you even more but I believe this is how it all works. The key is to install the distributor so all can be achieved (rotor to #1 on cap, points just opening, vacuum advance near center of travel, and on midyears, distributor cap screws perpendicular to car).

Good luck, Timothy Barbieri

[Al C.]

Re: Small Block Distributor Alignment - Again

Timothy - I don't think it's correct that "the centrifugal ... moves the cam..."; I believe that the centrifugal weights cause the plate bearing the points to move relative to the cam. My questions are attempting to more fully understand the subject - including the referenced unique 68 setup, as well as accomodation of a vacuum can on 2x4's where such does not "normally" fit. Thanks, ...Al '58 245hp

[Timothy B.]

Re: Small Block Distributor Alignment - Again

Al,

I will have to read the article as I just received my copy of the restorer today. Just for your information, in order to get all these things lined up as I stated in my previous post, I had to turn the bottom gear 180 degrees on my 63 vette.

[Duke W.]

Re: Small Block Distributor Alignment - Again

The vacuum advance, not the centrifugal advance mechanism, causes the breaker plate to rotate.

The orientation of the cap to the rotor does not change as the centrifugal advance mechanism adds advance because the rotor tip advances with the cam lobe.

Since the vacuum advance rotates the breaker plate, the rotor/cap terminal angular orientation will change as the points open depending on the amount of vacuum advance added.

The rotor tip and cap terminal are wide enough to operate over a range of about 20 crankshaft degrees (10 distributor degrees).

Duke

[John H.]

Re: Small Block Distributor Alignment - Again

Al -

Centrifugal advance moves the points cam relative to the breaker plate (the weight base is part of the autocam unit, which includes the point cam and rotates relative to the distributor mainshaft), and vacuum advance moves the breaker plate relative to the points cam. The only thing permanently affixed to the mainshaft is the small plate carrying the pins to which the inboard ends of the weight springs are attached.

[Al C.]

Re: Small Block Distributor Alignment - Again

Thanks Duke and John - Well, I see I was really off base in understanding the internals of the distributor. Makes more sense as you two have described it! ... Al '58 245hp
P.S> Still wondering about that 68 configuration ...