Distributor Gear Dimple

[Lawrence M.]

I pulled the distributor out of my 1969 350/350 to change the vac can to a VC1765 for full time vacuum. I found the dimple on the distributor drive gear opposite the rotor contact. The engine starts and runs fine. Should I change the gear to the correct side? Will doing so make a difference in getting the car started or timed after reinstalling the distributor? I was planning on sending the dist out to be rebuilt in the late fall. Should I just put it back in and drive the car until I send out the dist? Always a suprise with 40 year old cars.

Larry Merchantz

[Joe L.]

Re: Distributor Gear Dimple

I pulled the distributor out of my 1969 350/350 to change the vac can to a VC1765 for full time vacuum. I found the dimple on the distributor drive gear opposite the rotor contact. The engine starts and runs fine. Should I change the gear to the correct side? Will doing so make a difference in getting the car started or timed after reinstalling the distributor? I was planning on sending the dist out to be rebuilt in the late fall. Should I just put it back in and drive the car until I send out the dist? Always a suprise with 40 year old cars.

Larry Merchantz

Larry------


Having the dimple on the rotor side has nothing to do with proper performance of the engine. It's only purpose is to ensure that the distributor will be properly positioned so that the vacuum control will clear the shield supports and be in the proper orientation for attachment of the tach drive cable.

[Lawrence M.]

Re: Distributor Gear Dimple

Thanks Joe, The distributor apperas to be positioned correctly. The vac can clears the shielding and the tach cable has a straight shot into the distributor. I'll leave the gear as is until I have the unit rebuilt.

Larry

[Roger O.]

Re: Distributor Gear Dimple

Larry------


Having the dimple on the rotor side has nothing to do with proper performance of the engine. It's only purpose is to ensure that the distributor will be properly positioned so that the vacuum control will clear the shield supports and be in the proper orientation for attachment of the tach drive cable.

Interesting , never heard that before.

[John H.]

Re: Distributor Gear Dimple

Interesting , never heard that before.

Roger -

That's because it never appeared in any of the Chevrolet Shop Manuals, but it did in the Delco Distributor Manuals and on all the Delco distributor assembly drawings.

The entire system was very precisely indexed, from the crank and cam sprockets, through the indexing of the helix centers on the distributor drive gear on the camshaft, through the driven gear helix center on the distributor mainshaft, to the rotor position on the autocam assembly, and the indexing of the cap to the distributor housing, so the vacuum advance can was centered in its available arc of travel between the #8 intake runner and the plug wire support and cleared the opening in the top ignition shield when the initial timing was set correctly.

Most aftermarket replacement cams are NOT indexed from the front sprocket relative to the helix center of the gear that drives the distributor, as the cam manufacturers don't understand why all that precision is necessary, so they may or may not orient the distributor correctly with the "dimple" in the driven gear pointing in the same direction as the rotor. Aftermarket driven gears (that go on the distributor mainshaft) don't even have a "dimple".

[Joe L.]

Re: Distributor Gear Dimple

Thanks Joe, The distributor apperas to be positioned correctly. The vac can clears the shielding and the tach cable has a straight shot into the distributor. I'll leave the gear as is until I have the unit rebuilt.

Larry

Larry-----


As long as the distributor positions correctly, the "correct" orientation of the "dimple" and rotor is irrelevant. Most of the time it will be ok even if the "dimple" and the rotor are "out-of-phase". However, having them aligned just ensures that it will be ok. In your case, you've already "ensured" that it works ok the way it is.

[Roger O.]

Re: Distributor Gear Dimple

Larry-----


As long as the distributor positions correctly, the "correct" orientation of the "dimple" and rotor is irrelevant. Most of the time it will be ok even if the "dimple" and the rotor are "out-of-phase". However, having them aligned just ensures that it will be ok. In your case, you've already "ensured" that it works ok the way it is.

Joe,
Was this a production issue with fitment of the split pin ? I've rebuilt my fair share of distributors ( 2 last week ) and have always just lined up the dimple but have (just for fun) flipped the gear 180 and looked through the pin hole. Some seem perfect ,some clearly are off and if you tried driving the pin through would bend the pin.
Joe , when they punched this hole through the shaft did they immediately insert the pin during assembly or were all the holes in the shafts punched ahead of time ? Thanks !

[John H.]

Re: Distributor Gear Dimple

when they punched this hole through the shaft did they immediately insert the pin during assembly or were all the holes in the shafts punched ahead of time ? Thanks !

Roger -

The mainshaft was machined (including drilling the pin hole for the gear) long before the distributor was assembled; the gear was pinned to the bottom end of the shaft after shimming the gap between the top of the gear and the bottom of the housing.

[Joe L.]

Re: Distributor Gear Dimple

Joe,
Was this a production issue with fitment of the split pin ? I've rebuilt my fair share of distributors ( 2 last week ) and have always just lined up the dimple but have (just for fun) flipped the gear 180 and looked through the pin hole. Some seem perfect ,some clearly are off and if you tried driving the pin through would bend the pin.
Joe , when they punched this hole through the shaft did they immediately insert the pin during assembly or were all the holes in the shafts punched ahead of time ? Thanks !

Roger------

I don't think there was any manufacturing issue with the split pin which related to the "dimple". As far as the split pins go, I have no knowledge that they are in any way "tapered" or otherwise "directional".

With respect to the hole in the shaft, it's just as John described.

[Michael B.]

Re: Distributor Gear Dimple

I have pondered this topic before. There are 13 teeth on the distributor gear meaning that the distributor could be dropped in one of 13 different positions or approximately every 27.5 degrees. Since our V8 engines fire every 45 degrees, placement of the distributor in any particular position will index the alignment of the rotor arm and cap terminal at a slightly different position when the timing is set to spec.
Think of it like this. You are going to statically time the engine. You drop the distributor and the rotor tip is in a fixed position. You turn the key on and rotate the housing and cap until you get a spark. At that point there is a specific relationship between the rotor tip and the cap terminal. If you re-drop the distributor in a different position, the alignment and air gap is going to be different at the same trigger point. If you can visualize it, there is a difference.

[Roger O.]

Re: Distributor Gear Dimple

Thanks John and Joe for your answers. Not that it matters but I was born and raised and still live in Kettering. I used to work on Virginia Kettering's Cadillac many many years ago , nice lady sorry to see her pass. I for one am sure glad Boss saved us from those old hand cranks.

Happy 4th to everyone !!!

Ah ... I was waiting to see if someone would bring up the odd number of teeth, thanks Michael.
So if you set your initial timing the same and mark exactly where the vac can nipple is your going to have 13 different spots you can put the vac can. ( We all know where the can goes and why. )
My question is if we flip the gear 180 does that then give us 13 more positions for a total of 26,meaning instead of having to change the vac can 27.5 degrees at one time we now are able to make a much smaller change of 13.8 degrees ?
(There are times with me when I don't want to run stock timing but want to keep the can in the proper position for the sake of the tach cable but also want all the shielding in place.)

[Duke W.]

Re: Distributor Gear Dimple

Roger -


Most aftermarket replacement cams are NOT indexed from the front sprocket relative to the helix center of the gear that drives the distributor, as the cam manufacturers don't understand why all that precision is necessary, so they may or may not orient the distributor correctly with the "dimple" in the driven gear pointing in the same direction as the rotor. Aftermarket driven gears (that go on the distributor mainshaft) don't even have a "dimple".

Here's another anecdote along this line. I was talking to a cam manufacturer about grinding one of my cam designs, but he would not guarantee indexing, only lobe separation angle. ...said I would have to install it and dial it in to where I wanted it.

Needless to say, they did not grind the cam.

Regarding distributor indexing and the dimple, if you set the timing at, say 10 deg., then pull the dist. rotate the dimple 180 deg, reinstall and reset the timing to 10 deg. the VAC will be clocked 13.8 degs different.

Conversely if you rotate the drive gear and reinstall the dist. with VAC at the same point the intial timing would be off by 27.7 deg.

I can't imagine a situation where you would have to reindex the gear to achieve a different timing than OE. At typical 8-12 deg. intial settings the VAC is about halfway between the interference points, so you can run the timing anywhere from several degrees ATC to over 20 BTC without interference

That's why it's critical to get the dimple properly oriented because there isn't enough rotational freedom to install it 180 out and achieve the proper initial timing range due to interference with either the inlet manifold or shielding bracket on C2s and I believe the situation is similar on C3s.

On some Chevy engines, it can probably be done either way, but not on Corvette engines.

Duke