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post #1 of 15 (permalink) Old 10-06-2019, 04:33 PM Thread Starter
 
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Distributor Parts ID

OK, seems there are questions on rebuilding a points-type distributor for those who want to keep the points and keep the car original - or just rather not go though the changeover to electronic.

Pulled out my factory 1968 points-type distributor that was on its original 2-Bbl 350CI. I did not see any readily visible numbers to ID the distributor. But, it does have the large vacuum can having both the vacuum advance nipple on the front side, and the vacuum retard nipple on the lower back side. Emissions crap.

Photo #1 shows the distributor and specific items pointed out. The question came up about the gear orientation to the rotor. You can see that there is a dimple in the lower area of the gear and it lines up with the rotor tip. There is a roll pin that holds the gear on to the shaft and gets knocked out to remove the gear.

Note the clearance gap between the top of the gear and the base of the housing - .053". There was no side-to-side play at all which means the bushings within the housing and the shaft rides on were good - not bad for 120,000 miles.

Photo #2 is a close-up. There is a shim already installed. If you look closely, you will see that the base of the housing has a couple very small square looking blocks. The movement of the gear rides up to these blocks, and not flush up against what you might think is the flat of the the housing. So this is where you get your gap measurement - top of the gear to the underside of those small blocks.

Photo #3. Hmmm. What the heck limits the mechanical advance and what is this "bushing" thing I hear about to limit the mechanical advance. I never looked that close at a distributor and never knew that there was a "bushing" that can limit the mechanical advance OR wear out and let the mechanical advance advance too much. So there it is looking under the rotor. You can actually see the black colored bushing on the advance pin which moves within the slot to provide a specific amount of travel to tailor the mechanical advance to whatever the factory felt it should be for any specific engine. Remove the bushing, and you would get additional mechanical advance timing. Stick a thicker bushing on the pin and you would remove some of the mechanical advance. Same goes for the slot the pin/bushing rides in. Longer slot gives more advance, shorter slot gives less advance. With the longer slot, you will hear of things like brazing up the hole a little and filing on the braze until you get the advance you want or some will insert a stop screw. So you have options, but between the bushing and the slot, that is how you can adjust your mechanical advance to work with your initial timing at the balancer to get the total mechanical advance you want for your engine combo.

Photo #4 is with the rotor removed and looking straight on at the bushing/pin, along with the advance weight above it and the weight spring that you will hear of that can be changed out to adjust the timing curve - or the rate at which the mechanical advance pulls in.

Photo #5 is looking at the top of the distributor with the rotor removed. You can see the advance weights at the top and bottom sitting on the plate. This plate is the plate having the slot that the pin/bushing rides in and your rotor attaches to. Each weight slides over a fixed pin on this plate. The center plate is the movable assembly that holds the rotor. The springs are connected between shaft posts and the rotor posts, and the springs try to hold the two weights inward, but when the engine is running, the centrifugal force generated by the rotation of the weights pull the weights outward, which advances the moveable plate and rotorĖand therefore the ignition timing.

You can also see in the photo the vacuum set-up. The vacuum advance canister is attached to the distributor by 2 screws. Note the screw at the right has a grounding wire under it. This can be a source of problems that will have you creating new curse words because it was not making a good enough ground, got broke, or got left off. So check to see if it is there and in good condition. The attachment is under the moveable plate that the points mount to. You can see the pull-rod ( a little rusty at the end) that comes out of the canister and then hooks into a hole on the points plate. It is hidden just on the other side of the blue wire. When vacuum is applied to the diaphragm in the canister, it pulls the rod (shortens it) and the rod pulls on the points plate and changes the opening/closing timing of the points.

So those are the two forms of advance found within the distributor - mechanical advance that changes the position of the rotor and the vacuum advance which changes the position of the points.

Photo #6 is an example of the assorted bushings you can use and install on the pin to custom tailor the amount of mechanical advance.

The question has come up as to how much play you should have in the up & down movement of the shaft. This is the gap between the distributor gear and base of the housing shaft. In my photo, I have .053". It is said that you want .010" or even less. The .010" or less reference can be found in the HO engine blueprint booklet. This booklet is aimed at performance upgrades and racing and they claim "anything more than .010" of play is unacceptable." The tighter gap is to improve distributor timing accuracy. You have to take into account heat expansion, and even variations in machining of components. It would be a real shame to shim the distributor gear space down to .010" only to find that when the distributor was tightened down it jammed right into the cam gear and cause premature wear and destruction of the gears.

I read another article that said to test the play when you shim it down that tight of a clearance, to quickly pop the cap of the hot engine and grab the rotor and see if you can work it up and down a slight amount. If you can feel some play, then it is OK. If not, I guess you are in trouble?

Personally, I think I prefer to err on the side of caution and go for something like .015". The 350CI in my Lemans ran just fine with the .053" gap. Probably kept the gear cooler as well. You can close up the gap on the gear to improve timing accuracy of the distributor, but to what extent if you have some slop in the timing chain and gears?

Just my thoughts on it all.
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post #2 of 15 (permalink) Old 10-06-2019, 05:01 PM
 
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What a great write up PJ and photos as well! Nice work, you can see the rubber bushing on the stem in the slot, those sometimes completely wear off,...use a new one the brass ones I like.

You may want to get a rebuilt one, and change out the vac can etc. most of those pulled way too much timing for todayís gasoline.

Those springs you should save for your suspension as they look pretty stout!

A little cash and effort tuning that up will pay tremendous dividends in your cars performance. I would shim that 053 a bit, but you are correct not too tight.

Nice examples to help everyone!
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post #3 of 15 (permalink) Old 10-06-2019, 09:58 PM Thread Starter
 
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Originally Posted by Lemans guy View Post
What a great write up PJ and photos as well! Nice work, you can see the rubber bushing on the stem in the slot, those sometimes completely wear off,...use a new one the brass ones I like.

You may want to get a rebuilt one, and change out the vac can etc. most of those pulled way too much timing for todayís gasoline.

Those springs you should save for your suspension as they look pretty stout!

A little cash and effort tuning that up will pay tremendous dividends in your cars performance. I would shim that 053 a bit, but you are correct not too tight.

Nice examples to help everyone!

Thanks for the compliments. I don't plan on rebuilding the distributor, just used it as a demo. I myself have never gone to such lengths as to either rebuild or even dial in an advance timing curve. I have learned a lot about that from you and pulled out a number of my technical books along with the internet in better understanding the value in knowing and applying it to our cars.

I can find photos, diagrams, drawings, and descriptions, but they still seem to lack in their presentation - and few are ever a Pontiac unless you get into the HEI. So using some of the questions by our members, I did my best to answer them with a practical example, my actual points-style distributor, and then point out those items fitting of the questions asked. So I too am learning as we go along with everyone else. Now I know exactly where the advance pin bushing is and goes and how it all works! LOL Makes a lot more sense when you start talking about adding more initial advance at the balancer, but now the mechanical advance in the distributor is too much and needs to be reduced by using a larger bushing or fashioning a stop of some sort. In the same token, if I needed more mechanical advance from the distributor, I know I can swap out the bushing, remove the bushing, or file the slot a little longer - all really simple stuff once you know what to look for, how to adjust, and the parts you might need.

The rate at which the distributor's advance comes in at "X" RPM is the "advance curve" and this is controlled by either the small springs that provide the tension on the weights, or by changing out the weights (as some kits offer). So if wanting the total advance to be all in a 2,500 RPM's, or 2,800, or 3,200, etc., you can do this by selecting and trying a mixture of different spring tensions as supplied in many of the spring kits.

So your total mechanical advance is a combination of your initial timing down at the balancer at idle, and the addition of the distributor's advance through the weights. If the total is too much, or not enough once you have the initial timing where your engine wants it, you then make your adjustment at the distributor by either limiting the advance of the weights or extending the advance of the weights.

Once you have the values for the initial + distributor = total mechanical advance where you want them (generally anywhere from 30-36 degrees), you can then create an advance curve by changing out the springs (or weights in some cases) that will cause your total advance to be all in by "X" RPM's.

Then the last thing to dial in is the vacuum advance which you want to match up to your engines vacuum so it operates within the vacuum range your new bigger cam may have changed (generally less vacuum than stock), and the number of additional degrees needed to maximize the total mechanical advance at "X" RPM's + the full vacuum advance in degrees so it does not exceed a combined advance of about 50-52 degrees when you let off the gas, or tool along on light throttle. If too much vacuum advance, then this too can be adjusted with a stop to limit the vacuum rods travel in pulling on the points plate and advancing the points (or Pertonix set-up).

This is where a dial-back timing light would really come in handy, or the use of a standard timing light and a timing tape or balancer having a fully degreed hub.

I think that should cover it so that all can understand. Again, this is for those who have points-type distributors, or if you do a Pertronix conversion on the points, and want to know more of its operation and adjustment. They are not too complicated and do provide reliability and performance if you know how to work on them and enjoy doing so.
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post #4 of 15 (permalink) Old 10-07-2019, 08:53 AM
 
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Such a great summary Jim it really encapsulates what one is trying to do. Just building on your write up I would say donít get hung up on getting ďAll InĒ by 2500, or 2400. Your car will run great between 2600 to 3600 all in. And the lower numbers sometimes can cause detonation on acceleration.

60ís Corvettes are designed with a lower number and I have set them that way, but not all cars need it or should have it. A nice curve starts above idle speed say starts at 1000 and goes to 3000, or 1100 to 3100,..those are nice for street performance and no detonation.

On todayís reformulated gasoline 46 to 48 degrees advance with the vacumn in on top of your 36 to 38 total. In the 60ís .....52 to 54...with vac

And that is from Larís and Henry Olsen, the guys who really know.....so that is where I set them.

Remember gang if running points set your dwell first!.......

Super write up Jim!
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post #5 of 15 (permalink) Old 10-07-2019, 10:23 AM
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Very good write up Jim.

I rebuilt a point type distributor for my El Camino using parts from various sources and information from David Ray and Lars. It's not that hard to do and the results can be very satisfying. I went from 8 to 13 mpg by removing the MSD distributor with a bad vacuum advance to the original numbers matching distributor.

https://www.summitracing.com/parts/crn-99601-1

https://www.davessmallbodyheis.com/s...te-information

Ed


68 GTO (Thanks Mom)
70 El Camino SS
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post #6 of 15 (permalink) Old 10-07-2019, 03:38 PM
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Excellent, Jim. 1.Where did you source those mechanical advance bushings? I could use some.
2. The distributor number is on an aluminum tag that wraps around the distributor shaft under the points plate. Yours is missing. (they fall off easily, and repros are available for the anally inclined)
3. That distributor looks to be in very good condition. I have seen MUCH worse. I would use it.
4. The heavy stock springs: I seem to have much better luck with them on my cars than when I screw around with the light 'racing' springs. I would clean them up and re-use them. But that's just me.
5. Love the new avatar photo. Shows that Pontiac Jim is deeply immersed in the zen of old car ownership and repair with much inner peace and unending patience for us grasshoppers.
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post #7 of 15 (permalink) Old 10-07-2019, 06:35 PM Thread Starter
 
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Originally Posted by Lemans guy View Post
Such a great summary Jim it really encapsulates what one is trying to do. Just building on your write up I would say donít get hung up on getting ďAll InĒ by 2500, or 2400. Your car will run great between 2600 to 3600 all in. And the lower numbers sometimes can cause detonation on acceleration.

60ís Corvettes are designed with a lower number and I have set them that way, but not all cars need it or should have it. A nice curve starts above idle speed say starts at 1000 and goes to 3000, or 1100 to 3100,..those are nice for street performance and no detonation.

On todayís reformulated gasoline 46 to 48 degrees advance with the vacumn in on top of your 36 to 38 total. In the 60ís .....52 to 54...with vac

And that is from Larís and Henry Olsen, the guys who really know.....so that is where I set them.

Remember gang if running points set your dwell first!.......

Super write up Jim!

On the "all in" RPM's, I was just using examples. I am on board with 3,000 - 3,200 RPM's. During the days of Sunoco 260, you could pull "all in" at 2,500 and 3.90/4.33 rear gearing. Detonation needs to be avoided at all costs whether going for that "all in" number or setting up your timing curve.

You also know I am a little conservative on the total advance, that being about 32 degrees. I have read, and you have said, that you can go a little more with the ethanol laced gas. Again, I think 32 is a good compromise for the average engine, but nothing wrong with moving up towards 36 degrees, or dropping down to 30 degrees - each build is different and the best we can do here is suggest a start point and let the owner fine tune things.

I read that the 50-52 degrees total was with the vacuum advance and about maximum - and you are probably correct, using that leaded 260 Sunoco gas. But I agree more with the 46-48 range as practical.

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post #8 of 15 (permalink) Old 10-07-2019, 07:25 PM Thread Starter
 
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Originally Posted by geeteeohguy View Post
Excellent, Jim. 1.Where did you source those mechanical advance bushings? I could use some.
2. The distributor number is on an aluminum tag that wraps around the distributor shaft under the points plate. Yours is missing. (they fall off easily, and repros are available for the anally inclined)
3. That distributor looks to be in very good condition. I have seen MUCH worse. I would use it.
4. The heavy stock springs: I seem to have much better luck with them on my cars than when I screw around with the light 'racing' springs. I would clean them up and re-use them. But that's just me.
5. Love the new avatar photo. Shows that Pontiac Jim is deeply immersed in the zen of old car ownership and repair with much inner peace and unending patience for us grasshoppers.


The photo of the bushings was grabbed off the internet as an example for our readers to see the assorted types. It is probably the bushing kit you can purchase for the MSD distributor, but I cannot say if it would work on a factory Pontiac points distributor, but probably not. I see several kits for the HEI, but probably won't work.

The only kits with a bushing for a GM points distributor are made by Moroso and Mr. Gasket. It includes a brass bushing, the one Lemans guy mentioned in his post?

I have read in other forums that you can use a piece of rubber vacuum hose, plastic tubing, or even a section of a plastic "T" cut down to fit. You may have to get creative - look through Home Depot, hobby store, Tractor Supply, etc.. And then, trial & error.

Enjoy the "Zen" photo. Right now with my hair grown out a little and the beard longer, I'm looking more "mountain man" or "homeless." Just not sure. I like to change things up. Thinking of going ponytail again. Never know until I wake up and look in that mirror what I am going to do. Have not tried "skin head" yet. LOL
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post #9 of 15 (permalink) Old 10-07-2019, 08:04 PM
 
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Spot on Jim and totally agree,....!

You look great,...just keep on Trucking!....
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post #10 of 15 (permalink) Old 10-08-2019, 10:29 AM
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Jim, if you go ponytail, you're gonna have to wear tie-died shirts and start driving a VW Microbus. Just sayin'.....You're too clean looking for 'homeless'....at least so far!
I have been doing 'skin head' for the past 9 or 10 years....do not have many other options other than 'Homer Simpson'. So it's the Walter White look for me. And plenty of sunblock for the scalp!!
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  Pontiac GTO Forum > The 1964-1974 Pontiac Tempest, Lemans & GTO > 1964-1974 Tempest, LeMans & GTO Engine Tuning and High Performance

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