#1 or #5, there are two #1 TDCs for each 1 revolution of the distributor.
#1 TDC shows when #1 and #5 pistons are at top dead center, they are a "matched pair".
#2 and #6 are also a match pair, as are #3 and #4.
Matched pairs are needed to balance an engine so it spins smoothly, V6 has 3 matched pairs, V8 4 matched pairs, 4 cyl 2 matched pairs.
Because this, like most other engines, is a 4 stroke engine, each piston must go up and down 2 times(4 strokes) for one full cycle.
The Cam controls the Valves and the Valve positions control which stroke each piston is on.
For everything to work the crank must complete 2 full turns(4 strokes) for each one full turn of the Cam.
The distributor is connected to the Cam
So the crank does 2 full turns for each one full turn of distributor, so a timed distributor will point to #1 at TDC and then at #5 on the next TDC, then at #1 again on next TDC, and so on................#1 and #5 alternate at each TDC
#1 TDC just tells you that #1 and #5 are at TDC, it can't tell you Cam position, so if timed the distributor will be pointed at #1 or #5.
If you are reinstalling a distributor then remove #1 spark plug and put a compression gauge in that hole, or your finger.
Rotate engine until you see/feel compression starting to build up, continue rotating until #1 TDC mark is lined up at 0deg, that is the end of the compression stroke for #1 and that is when #1 spark plug should fire.
Install distributor so rotor is pointed at #1 on the cap.
You have a 50/50 chance of getting it right without testing for compression.
So put crank on #1 TDC and install distributor so rotor points at #1
If engine starts then you got it right the first time
If engine doesn't start then you got it wrong, doesn't hurt anything, rotate engine until distributor rotor points at #5(#1 TDC again), pull out distributor and rotate rotor until it points at #1
Last edited by RonD; 02-22-2016 at 10:18 AM.