Distributor Gear Dimple Location

[William V.]

In a previous post it was explained that on C2’s the distributor gear dimple is to point in the same direction as the rotor contact.

The reason being is the limited distributor swing between the intake manifold and coil bracket is less that the 27.7 degree difference between the distributor gear teeth (360 deg /13 teeth = 27.7 deg per tooth.) The odd number of gear teeth results in a possible distributor gear-indexing problem. If the gear is installed 180 deg out of phase, one of the following two problems are experienced when the plug wires are installed per the AIM. (#1 plug connected to the distributor terminal just left of the dwell adjustment door)

1. The engine is too far advanced and cannot be retarded enough because of an interference between the vacuum advance and coil bracket
2. The engine is too far retarded and cannot be advanced enough because of interference between the vacuum advance and the intake manifold.
Moving the distributor one tooth to correct one of the above problems results in the other.

I have determined that the dimple pointing in the same direction as the rotor contact is not a hard rule and that there are situations where the distributor gear dimple must point 180 deg out to achieve correct timing. This is the case with my 64 365. I attribute this exception to the Competition Cam I installed to replace an unknown manufacturer of the after market cam that came with the car.

As part of a cam change I like to replace the distributor drive gear. I noticed upon removal of the distributor, the dimple pointed 180 degs away from the rotor contact. I kept this in mind when installed the new gear with the contact and dimple pointing in the same direction. I encountered the above timing trouble and rotated the gear 180 deg to achieve correct timing.

I have several old distributor gears and they all, including the new gear, have identical gear teeth to roll pin hole and dimple indexing so I suspect the cam shaft gear indexing on some after market cams may be out of phase. My suggestion is to be aware and understand the problem and simply rotate the drive gear 180 deg for correct timing.

I also want to mention that the above trouble is unique to Corvette because a regular Chevy has a lot more room for distributor swing.

[John H.]

Re: Distributor Gear Dimple Location

The swing interference issue may be unique to Corvette, but the gear indexing issue is not - the Delco distributor drawings are VERY specific about the dimple being oriented to the rotor tip, and the Chevrolet camshaft drawings (for all Chevrolet cams) are also VERY specific about the precise location of the distributor drive gear tooth orientation with respect to the dowel pin, which "masters" the grinding of the camshaft. I'd agree that aftermarket camshaft manufacturers may not pay a lot of attention to the latter, which may require "adjusting" the orientation of the drive gear on the distributor.