C2 Distributor Position Revisited

[Dave K.]

Confirmed that the plug wires were properly oriented on the cap. Found 6 deg. BTDC , confirmed rotor at #1 cylinder. Removed the distributor and examined the gear dimple vs rotor position. It checked. Rotated gear 180 deg. re installed the distributor. Timed the engine to 6 deg. BTDC -- distributor vacuum can now at mid point between can and intake. Car idles and accelerates great. Original distributor and gear. Service replacement cam. Maybe cam gear incorrectly machined.

Dave K.

[Duke W.]

Re: C2 Distributor Position Revisited

So, you're saying the dimple is now pointing the opposite direction as the rotor tip?

Service replacement cam? GM? Aftermarket? Part number?

Was it advanced or retarded from the OE indexing position?

Does it have a correct pre-'69 balancer and timing tab?

Your situation is anomalous, and you will find a reason if you dig deep enough. A lot cam "happen" to an engine in 40 years.

Duke

[Gerard F.]

Re: C2 Distributor Position Revisited

Dave,

With your original distributor, dimple lined up with rotor, the vacuum can should be pretty close to the coil at 6* BTDC. Reversing the dimple would move the vacuum can 14* CCW at the same initial timing. Don't know anything about the cam and how it would effect the position, but I guess if it works, it works.

My replacement distributor with the dimple reversed to the rotor, was about midway or little bit more towards the manifold at 6* BTDC and worked well for many years that way.

I think the 327/300HP runs a little better at 8* initial.

Jerry Fuccillo
#42179

[Wayne P.]

Re: C2 Distributor Position Revisited

I had to reverse the dimple on my 63, 340hp when I installed an aftermarket LT1 cam last summer. Everything else is original.

[Mike McKown]

You are not alone.

I had a Corvette engine this way. It had a replacement cam in it. Brian McHale has an engine that times like yours as he reported the other day on your thread.

The problem will be more apparent on base engines that have static timing in the 2-6 degree BTC range.

The important thing is to recognize the problem and know how to correct the symptoms. I don't believe it would be worthwhile at all to tear the engine down to find out why it happened. You probably have the answer in the cam gear cut wrong.

[John H.]

Re: You are not alone.

Although the original GM drawings show a VERY specific relationship between the dowel on the cam and the centerpoint of the helix on the gear that drives the distributor, the aftermarket "blueprint" cam manufacturers apparently pay no attention to it, as they don't understand why it's there; those who also manufacture distributor drive gears are also unaware of the VERY specific relationship between the centerpoint of the helix on that gear and the "dimple" machined into original GM gears.

[Dave K.]

Re: C2 Distributor Position Revisited

Yes,

Dimple is pointing 180 from rotor. Engine was overhauled 20 K miles before I bought the car. I believe that it has a GM service replacement cam. I also replaced the distributor shaft due to stripped tach gears but dimple originally faced rotor. Engine has correct balancer and timing tab.

One of my friends in the chapter helped me do the changes. I can assure that he is very experienced and has a distributor machine. He has seen this before as has others. I have no intention of digging deeper.

Do not have further details on cam.

Dave K.

[Rob A.]

Re: C2 Distributor Position Revisited

I had to reverse the dimple as you did on my '66 L79..unrebuilt engine with original distributor and vacuum advance.

[Clem Z.]

my guess that dimple was there so they could

install the dist at the factory with the cap on and they would know where the rotor was pointing. JMHO