If you come across the problem where you can't turn the distributor enough to set the timing properly because it hits either the plug wire support or the side of the #8 intake runner first, the distributor drive gear has been installed wrong.

There's a machined "dimple" on an OEM Delco drive gear, and it's supposed to be assembled on the shaft such that the dimple faces the same direction as the tip of the rotor.

There are 13 teeth on the gear, so each tooth is 27* of distributor shaft rotation; with the odd number of teeth, the gear is either on correctly or it will rotate the distributor body 13.5* from where it belongs if it's installed wrong. The entire OEM system, from the pin on the front of the cam to the drive gear on the cam to the drive gear on the distributor shaft with the "dimple" on it is very precisely indexed in machining so the distributor body ends up in the center of its travel, with the same clearance on each side, when the timing is set correctly.

If you have an aftermarket cam (or distributor), they pay no attention whatsoever to the precise indexing on the factory cam and distributor drawings, as they don't understand why it's necessary, so it's the luck of the draw whether the distributor ends up centered; if it isn't, knock the rollpin out of the gear, turn it 180*, and re-install the rollpin, and it'll move the distributor housing 13.5* so you can set the timing with the distributor properly oriented.

:beers:

John I think this post merits it's own thread here in tips and tricks. I think I have identified a way to index the aftermarket gear without the dimple. Look at this series of pictures. The first is an oem gear indexed so the dimple is lined up with the rotor contact. The next picture is an aftermarket gear I installed matching the position of the set pin between the 2 teeth of the gear to that of the oem. The next two pictures are oem and aftermarker gears (in that order) rotated 180 showing the set pin lined up with 1 tooth of the gear on each.

That said if you position the rotor contact facing you and align the hole between the 2 teeth also facing you and set the pin, it should be a home run.

I believe this has to hold true on any Chevrolet drive gear and should allow any aftermarket gear to be installed indexed the same as the oem with the dimple the first time.

Great tip to know. Thanks John.

GM installed the Dizzy with #1 Cylinder at TDC Firing position such that with the Oil Pump Shaft Clocked correctly the Dizzy would Seat with the Rotor Pointing directly at the Rear of the Carb. In this position any Distributor Option like the Vacuum Pod would not inter fear with surrounding equipment when adjusting the timing. You will note that the #1 Tower of the Distributor Cap for #1 Plug Wire will point almost at #1 Cylinder. The trick is Clocking the Oil Pump with a long Screw Driver in a position where the Distributor will Seat and the Rotor is pointing at the rear of the Carb.

Like above.. And the VA pionts to just behind the passenger headlight....
Even so some inlet manifold designs still cuase interference as the stock timing is around 4 to 8 degs off TDC and many aftermarket cams require more....and simple 'fix' is just move the dizzy body around another tooth.

Many dizzies have a tapered gear pin so only goes in 1 way.

GM installed the Dizzy with #1 Cylinder at TDC Firing position such that with the Oil Pump Shaft Clocked correctly the Dizzy would Seat with the Rotor Pointing directly at the Rear of the Carb. In this position any Distributor Option like the Vacuum Pod would not inter fear with surrounding equipment when adjusting the timing. You will note that the #1 Tower of the Distributor Cap for #1 Plug Wire will point almost at #1 Cylinder. The trick is Clocking the Oil Pump with a long Screw Driver in a position where the Distributor will Seat and the Rotor is pointing at the rear of the Carb.

Like above.. And the VA pionts to just behind the passenger headlight....
Even so some inlet manifold designs still cuase interference as the stock timing is around 4 to 8 degs off TDC and many aftermarket cams require more....and simple 'fix' is just move the dizzy body around another tooth.

Many dizzies have a tapered gear pin so only goes in 1 way.

The point is it's easy to install the the dist gear 180 out! In the pictures I posted the stock gear with the dimple is on a dist from a '65 283 and it can go on either way, no tapered roll pin or tapered holes to install the pin. The other is a modern aftermarket billit dist and it can go on either way as well.

There are many ways to make a dist fit so the vacuum can is not acting as an advance limiting device. Not too many realize after replacing the dist gear for what ever reason putting it on 180 out will impact the dist position by 13.5 degs. As John said "so you can set the timing with the distributor properly oriented".