On another forum, a fellow enthusiast indicated that the dimple on the distributor gear should line up with the rotor tip. Huh? What's the relationship there? Am I missing something? The gear teeth are the same all around the circumference and the rotor wants to point to the plug that is to fire when rotated. Anyone know the secret I'm missing? Thanks.
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Re: Huh? Distributor gear dimple alignment?
Mark,
If you look at the gear, you will see that it is not symmetrical as far as tooth position to roll pin location. One side has the roll pin pointing directly at a tooth and the other side will be pointing between two teeth. That means there are two ways the gear can be installed, as far as it's clock position. Aligning the rotor tip with the dimple in the gear will be the correct position. -
Re: Huh? Distributor gear dimple alignment?
Let me try to explain. Looking at the gear, dimple, and the hole that the roll pin goes into. The key is that the hole(s) (One on each side of the gear.) If you notice one hole opening lines up with a tooth on the gear. The opening on the other side of the gear lines up between two gears. If you place the gear with the dimple 180 degrees off, it will be 1/2 of tooth off. I think there is an odd number of teeths on the gear. I'm sure someone else can explain it better, but thats it in a nut shell.Comment
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Re: Huh? Distributor gear dimple alignment?
The dist. gear has 13 teeth, and if you look at the role pin hole it is aligned with the end of a tooth on one side and between the end of two teeth on the other side.
Thus, the two possible indexings are different by 14 degrees (28 at the crank).
On an engine configuration that allows the distributor a large amount of rotational freedom, it doesn't necessarily matter which way it's installed, however, on a Corvette, distributor rotation freedom is very limited between the points that the vacuum can interferes the manifold or coil bracket.
If the gear is indexed with the dimple pointing 180 degrees away from the rotor tip you cannot rotate the distributor enough to achieve the correct timing. The
"fix" is then to advance or retard the distributor one tooth and reindex the wires. This will allow proper timing, but rotor-cap misalignment results in misfires and poor performance. This drive gear indexing issue is a source of no end of problems for the last 40+ years, and they continue to this day.
There is NOTHING in any Chevrolet service manuals (that I am aware of) about proper indexing of the drive gear. It's "system knowledge" you learn from others or from you own mistakes.
In my case I figured it out circa 1966 after having no end of problems the first time I removed and disassembled my distributor. I did all the usually bubba fixes - tried every combination of advancing and retarding the tooth mesh on installation and reindexing of the wires, and the engine ran like sh...
Once I finally saw that little dimple was pointing 180 degrees from the rotor tip I realized that gear indexing to the shaft was an issue. I rotated it 180 degrees to point the same direction as the rotor tip, reinstalled distributor, reindexed #1 wire to the correct cap terminal as shown in the '63 Corvette Shop manual and achieved the correct initial timing with the housing about halfway between the interference points and the window approximately normal to engine centerline, which is how it should be. In fact, it's the ONLY way everything will go back together and work properly.
"Problem" solved!
DukeComment
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Usually *NM*
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Re: Huh? Distributor gear dimple alignment?
Thanks to all who responded. It makes sense now. I'll double-check the gear installation before dropping the distributor in the engine.Comment
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Re: Huh? Distributor gear dimple alignment-Duke
Duke,
There is NOTHING in any Chevrolet service manuals (that I am aware of) about proper indexing of the drive gear. It's "system knowledge" you learn from others or from you own mistakes.
Due you think this was overlooked once The drive gears were being rol pin to the mainshaft? The '56 distributor gear is staked (pinned)to the main shaft. '55 Service Manual states to grind off the head of the pin then drive it out. The pin they are referring to has a head on both ends about 1/4" dia with a cross hatched pattern. LOoks like a larage nail head. It would be impossiblein '55/56 to reposition the gear without first removing this pin. For this reason maybe the later roll pin assembly came along but reassembly was overlooked in the service manualthe possibility of resetting the gear a tooth off.? Just a thought. Regards, JimComment
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Re: Usually
We put a vaccuum advance distributor in a friends 270 horse car to make it more "street friendly", and found that we had to intentionally index the distributor gear incorrectly to get it to work. There just is no room on a 2 X 4 car when you use a vac advance distributor!
Regards, John McGrawComment
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Re: Huh? Distributor gear dimple alignment-Duke
The '55 and '56 SBs have a lot of unique features, many of which I'm not familiar with at a low level of detail, but by '57 the design had pretty much stabilized, and the single point distributor continued with no major changes through 1974.
By having two different indexings for the drive gear it gave the designers additional flexibility for clocking the distributor. It's quite possible that on some configurations the dimple must be pointing 180 out from the rotor.
I recall someone posting that after installing a single point distributor on a 283/245 he had to install the drive gear "backwards" to get the engine properly timed without the vacuum can interfering with something. I always thought the the 245s didn't have the single point vacuum can distributor because there wasn't room for it, so they used the dual point non vacuum can distributor. There would be no other logical reason, since the 245 was just the base 220/230 HP engine with the 2x4 manifold and carbs.
Each distributor should have an "assembly drawing" or instruction sheet that told the plant how to assemble them. Proper indexing of the drive gear would have to be on these documents or about half the C2s engines built could not have been properly timed at Flint, and this may apply to C1s, too.
The fact that proper distributor drive gear indexing never made it into any of the service manuals is a total mystery to me.
DukeComment
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PS
There was a major design change - addition of the tach drive provision and, of course the FI pump drive, but most of the other internal parts are interchangeable among all single point distributors Of couse, a proper shaft is required for tach drive and FI distributors and FIs use a different cap.
DukeComment
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Re: Usually
I mentioned that in my 10:44 post.
DukeComment
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Re: Usually
Duke,
You are correct, I just had not gone down and opened that post yet!
Regards, John McGrawComment
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Re: Usually
Oh hell, what am I thinking, it wasn't there yet!Comment
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Re: Huh? Distributor gear dimple alignment-Duke
The the dimple-to-rotor-tip alignment requirement is called out VERY clearly on the Delco Corvette distributor assembly drawings and in the Delco Distributor Service publications, but it never appeared in any of the Chevrolet Chassis Service manuals. The Delco assembly drawings also call out VERY clearly the precise relationship between the pin hole in the gear, the dimple, and the center point of the helix on the drive gear, and the mainshaft drawing also calls out the precise relationship of the pin hole in the shaft relative to the flat at the top that orients the rotor to the shaft. This whole "system" is completely indexed, top to bottom, but for whatever reason, Chevrolet never noted it in the Service Manuals. Didn't matter much on a passenger car, but it's important on a Corvette with a tach drive, ignition shielding, and tall #8 intake runners.Comment
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Re: Huh? Distributor gear dimple alignment-Duke
John - do you know of any distributors for other vehicle applications of the era where the dimple indexing was either not specified or specified in the orientation 180 degrees out from the Corvette engine indexing?
DukeComment
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