The dreaded distributor dimple

[Ed S.]

I (almost) hate to even ask this question but....... alignment of the distributor dimple - assuming we are talking about original parts here, 64 L76 engine - all advice previously given is that the dimple should "line up" with the rotor contact.

My question is, should it be a perfect alignment - or is it off a bit when properly installed. My dimple is on the same side as the rotor contact but..... it is about 10 degrees off dead center alignment. It is not on the back side - looking at the dist with the rotor installed - when the rotor points to the 12 o'clock position the dimple is closer to the 1 o'clock position - they are not "perfectly" aligned. Is this alignment correct? BTW - the hole / roll pin is directly between 2 gear teeth. See the attached pic.

Thanks for the answer.

[Ed S.]

Re: The dreaded distributor dimple

Well...... if being off by about 10 degrees (rotor to dimple alignment)...... is wrong...... .it doesn't make a difference in my car. Carefully installed the dist. following instructions in Lars' how-to document and a tips from Duke...... the dist clocked perfectly when seated, Window was pointing straight ahead and the vac can was mid way between the manifold and coil bracket. Pumped it a few times and it fired right up. I am pretty confident now that I was off by a tooth - had the dist turned to the max towards the coil and it still backfired through the carb. Not now. Fires right up. Is this a great website and club or what! Fine tune the dist, timing and carb tomorrow.

[Don H.]

Re: The dreaded distributor dimple

We're here to help Ed.

[Ed S.]

Re: The dreaded distributor dimple

Yes, I know --- in more ways than one. Besides responses to threads I always first do a thorough search for a previous thread on the problem at hand and do a quick review of the data base of restoration documents. I'm good for now.

[Duke W.]

Re: The dreaded distributor dimple

Sounds like you got it right, but I'll add that once the distributor is properly assembled and installed, static time it.

This starts by setting the balancer notch on whatever initial timing you want on the timing tab BEFORE you remove the distributor.

Then upon installation when it seats (after tweaking the oil pump drive shaft, if necessary) rotate the dist. housing CW to the limit, then either using an ohmmeter or observing the points rotate the housing CCW until you points just start to open or the resistance between the coil negative terminal and ground goes open circuit.

If it's a TI car rotate the dist. CCW until the stationary pole piece tips line up with the rotating pole piece tips.

Duke

[Dan D.]

Re: The dreaded distributor dimple

Well...... if being off by about 10 degrees (rotor to dimple alignment)...... is wrong...... .it doesn't make a difference in my car. Carefully installed the dist. following instructions in Lars' how-to document and a tips from Duke...... the dist clocked perfectly when seated, Window was pointing straight ahead and the vac can was mid way between the manifold and coil bracket. Pumped it a few times and it fired right up. I am pretty confident now that I was off by a tooth - had the dist turned to the max towards the coil and it still backfired through the carb. Not now. Fires right up. Is this a great website and club or what! Fine tune the dist, timing and carb tomorrow.

Ed, it you had the distributor one tooth off it will throw off rotor phasing. The points have to open when the rotor tip is pointing directly at a cap contact. If the distributor is one tooth off the points will open with the rotor tip half way between two adjacent cap contacts. This would explain your backfiring, as the spark would not know which cap contact to jump too. (The closest one).

I did this experiment about a month ago, I did not try to start the engine as I knew this would not work.

Dannn

[Ed S.]

Re: The dreaded distributor dimple

Dan..... where were you when I needed you! Just kidding..... I did the same "experiment" but didn't know I was doing it at the time. Intuition told me "something is not right" - it was either the dimple (low probability) or off a tooth. Confirmed the dimple alignment was correct - reinstalled it the correct way. Runs GREAT. If you don't do a procedure on a regular basis or very rarely, it tends to make you (me in this case) nervous - don't want to screw up more than what already is. Thanks again for the comment.

[David H.]

Re: The dreaded distributor dimple

Ed

A couple of slides from a tech session a few months back. Note also, that aftermarket replacement gears DO NOT have "dimples".

Align those aftermarket gears with "gap".

"Dimple" photo, small dimple is from a 1957 (I think) distributor. Large dimple from a later year.

Dave

PS: Believe it or not, I DID clean those gears off for my photos. You would appreciate if you saw "before".



Dist Gap S1.jpgDist Gap S2.jpg

[Ed S.]

Re: The dreaded distributor dimple

Should have took a pic of my dimple when I had the dist out (that sounds obscene doesn't it!). Oh, well...... you know what I mean. Anyway.... thanks for the pics - I have exactly what you show in your pics and all is oriented like yours as well. I must have it right..... it fires right up. This issue is raised so often - it is good to have these pics in the archives - others will benefit.

[Mark E.]

Re: The dreaded distributor dimple

Ed, it you had the distributor one tooth off it will throw off rotor phasing. The points have to open when the rotor tip is pointing directly at a cap contact. If the distributor is one tooth off the points will open with the rotor tip half way between two adjacent cap contacts. This would explain your backfiring, as the spark would not know which cap contact to jump too. (The closest one).

I did this experiment about a month ago, I did not try to start the engine as I knew this would not work.

Dannn

Dan,

Reorienting the distributor gear with the cam gear will not "throw off rotor phasing". This will only change how the distributor is clocked on the manifold. The engine will run normally with the distributor clocked in any orientation. But practically, interference with brackets, bulkhead and other things limit the positions it will fit, hence the recommendation in the service manual.

[Duke W.]

Re: The dreaded distributor dimple

Reindexing the spark plug wires WILL result in misalignment of the rotor tip and cap terminal. One tooth on the gear is 27 degrees and one position on the cap is 45 degrees.

"Move the wires" was the advice I got from the "experts" back in 1965 when I couldn't get the timing right without the VAC hitting the manifold or coil bracket when I removed the distributor to investigate an ignition breakup problem. It took me three R & I tries before I saw the dimple. It was a Eureka moment. The is nothing about proper dimple orientation in the 1963 Corvette Shop Manual.

Years ago I asked John Hinckley if he had ever seen any mention of it in any service literature. He said the only reference to the dimple he had ever seen was on a distributor assembly drawing.

Duke

[Dan D.]

Re: The dreaded distributor dimple

Dan,

Reorienting the distributor gear with the cam gear will not "throw off rotor phasing". This will only change how the distributor is clocked on the manifold. The engine will run normally with the distributor clocked in any orientation. But practically, interference with brackets, bulkhead and other things limit the positions it will fit, hence the recommendation in the service manual.

Mark,

Re-indexing the gear one tooth will move the rotor tip and points cam 27*. The points will open and a spark generated at 27*, or about half way between cap contacts which are 45* apart.

But this is not the same as re-indexing the dimple 180*.

I missed this in the discussion. My bad.

Dannnn
.

[Dan D.]

Re: The dreaded distributor dimple

No it won't. Both of these are false.

The engine fires every 45 degrees of crankshaft rotation. Everything; firing positions, breaker points cam lobes, plug towers, are always 45 deg (distributor) apart. There is no physical way to change the relationship of one to any of the others. They are always 45 deg apart.

The breaker fixed contact is fastened to the body of the distributor just like the cap is. When one is moved, so is the other. The relationship between the cap and the points opening can not change. When the points open, the rotor will be in the same position relative to the opening regardless of dimple, which of the "13 teeth", plug wire indexing, etc.

Yes, the tower contacts are 45 degrees apart. The crank moves 90 degrees while the distributor moves 45 degrees to the next firing position.

The distributor can be installed on any of the 13 teeth. The firing order of the wires in the cap must be correct, and the rotor must be pointing at #1 when the #1 piston is at firing position. When the breaker points open, the rotor will be pointing at the corresponding cyl.

What is frequently overlooked in these discussions is that the rotor to cap phasing changes with timing advance changes. 35deg of crank timing is 17.5 deg of rotor arc. Depending on timing advance, the spark occurs anywhere within this arc.

Dave,

The engine fires every 90* of crankshaft rotation - not 45*.

I'm still trying to get my arms around everything else you are saying.

As long as we keep this discussion friendly (so far we are), then it is going to be very educational.

Dannn

[Mark E.]

Re: The dreaded distributor dimple

Dave,

The engine fires every 90* of crankshaft rotation - not 45*.

I'm still trying to get my arms around everything else you are saying.

As long as we keep this discussion friendly (so far we are), then it is going to be very educational.

Dannn

Dave meant 45 degrees camshaft rotation, as he explains later in the post.

Also, the engine will run fine with the distributor gear oriented either way. I believe Duke's point (correct me as needed, Duke), is the distributor will only physically fit without interference with the stuff around it if it is clocked in a specific manner, as shown in the manual. And to do that requires the dimple to be aligned and the ignition wires to be in their factory specified tower on the cap.
If space allowed the distributor to be clocked in any orientation then it does not matter which way the gear or cables are installed (as long as the proper firing order is used, of course).

[Duke W.]

Re: The dreaded distributor dimple

The rotor tip does not move relative to the cap terminal as the centrifugal advance is deployed. It is easy to demonstrate with a real distributor, but I'll try to explain in writing as best I can. The rotor tip-cap terminal clocking relationship DOES change as initial timing changes and as vacuum advance is added, but I'll get to that later.

Ignoring vacuum advance (It remains zero) let's say we set the rotor tip exactly centered on the cap terminal. Now move the shaft 15 degrees CCW to show the firing point with a full 30 degrees centrifugal, and, of course the rotor tip will move 15 degrees CCW off its centering on the cap terminal. Now you have to manually pull the centrifugal weights out to the fully advanced position and as you do this the rotor tip will move back into alignment with the cap terminal.

It's more obvious that changing initial timing alters the clocking relationship between the rotor tip and cap terminal because the distributor base rotates while the shaft and cam assembly (or stationary pole piece on a TI or HEI) remains fixed when engine speed is less than the centrifugal start point

Likewise as we add vacuum advance the breaker plate (or stationary pole piece) will move CCW while the rotor remains fixed at any given initial timing and amount of centrifugal advance.

Please, before you argue with the above get a loose distributor and see for yourself.

On a C2 small block the #1 plug wire must be installed in the cap terminal next to the passenger side of the cap window. A real GM gear must have the dimple pointing the same direction as he rotor tip, and the shaft gear must be installed on the correct cam tooth gear. Otherwise, especially on SHP engines that need 10-18 degrees initial, you can barely get 10 degrees before the VAC hits the inlet manifold, and then if you move the shaft one tooth CW the VAC hits the coil bracket with still too much initial.

A screwed up distributor assembly or installation is easy to spot. If all is correct the plane of the cap window should be near normal to engine centerline and the VAC should be about halfway between the manifold and coil bracket interference points. If the plane of the cap window is clearly not near normal to engine centerline or the VAC is very close to or against the inlet manifold or coil bracket, there's a problem.

Symptoms are rough running, jerking, backfiring... the car can be damned near undriveable. I've lost track of how many screwed up distributor assembly or installations I've corrected in the last 55 years, but it began with my own SWC back in 1965. That's when I discovered the dimple that is nowhere mentioned in Chevrolet service literature, and that moving the plug wires that so-called "experts' said I needed to do just made the poor engine operation situation worse.

Duke