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Originally Posted by hozy
Doesn't the rotor always point in the same direction at tdc ?
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You can install the distributor with the rotor in any position.
Let's say we walk up to a freshly assembled engine installed in a vehicle and install the distributor in the engine and start the engine.
First thing we need to determine is which stroke #1 cylinder is on and where the damper line is in relation to the timing marks. You can do this several ways. One way is to put a paper or your finger in the spark plug hole and have someone crank over the engine (ignition disabled) slowly. When the compression starts to build and blows out the paper, or pushes your finger away, you are on the compression stroke. Simply rotate crank in a clockwise manner until the mark on the damper lines up with the 12 BTDC timing mark. Now #1 cylinder is 12 degrees before top dead center on the compression stroke which is a good position for the plug to fire. You can also pull a valve cover and watch the valves while you rotate the crank. If you rotate the crank clockwise, when the intake valve closes the engine is on the compression stroke. Simply keep slowly turning the crank clockwise until the damper line is at 12 BTDC.
We install the distributor so it is fully seated, regardless of rotor position. The rotor can be installed in any position because we are going to position the wires on the cap according to where the rotor is pointing when installed. Whatever terminal on the cap the rotor points to is going to be #1, since #1 is in position to fire (12 BTDC of the compression stroke). Then just install the wire for the next cylinder in the firing order (18436572) on the next counterclockwise terminal. Once all the wires are on the engine is ready to fire and set the timing with a timing light.
In a different test, the rule is, the wires have to remain installed on the cap in the exact position they are installed at all times. That means the rotor must be aligned with the current #1 terminal on the cap, so when #1 is 12 degrees BTDC of the compression stroke, the rotor must point to the #1 wire on the cap.
Either way will work, but personal preference of the placement of the wires, factory correctness, wire length, wire routing, and the location of the vacuum advance can all are factors in the installation of the distributor and the cap and wires.