OK- - - -two pistons come to the top of the stroke at once. One is on the compression stroke with both valves closed. The other cylinder is on "overlap" with the intake valve opening and the exhaust valve closing. The "companion" cylinders are 1 and 6, 2 and 5, and 3 and 4. The simplest way to determine which cylinder is ready for the spark plug to fire is to watch the action of the valves.
Take the valve cover off, turn the engine clockwise from the front, and watch the #6 cylinder valves. The exhaust valve is the last one in line back at the firewall. The valve will open (spring compressed) and begin to close. As the exhaust valve closes the intake (second from the rear) will begin to open. At that point, the #1 cylinder is at top dead center of the compression stroke with both valves closed and ready for the spark plug to fire. Forget about timing marks- - - -get the #6 valves rocking, wire the distributor cap with the rotor pointing toward the #1 plug wire, and complete the firing order 5-3-6-2-4. Turn the distributor housing clockwise until the points close, then turn it CCW until they just barely open. Button everything up, lock the distributor adjustment down, and crank it up. Works every time!
Jerry
