Welcome to TRS
Most vehicle engines are 4-stroke engines, this means that each piston has 4 strokes for one complete cycle, 2 down strokes and 2 up strokes
1. Intake stroke is piston going down with intake valve open, this sucks in the air:fuel mix
2. Compression stroke is piston going up with both valves closed, this compresses the air:fuel mix, heating it up and vaporizing the fuel so it can be ignited by a spark
3. Power stroke is piston going down, spark plug has fired and fuel is exploding pushing the piston down adding power to the crank shaft
4. Exhaust stroke is piston going up, exhaust valve is open, exhaust is pushed out into exhaust manifold.
Back to 1.
4-strokes means that the crank shaft must turn 2 full rotations for each full cycle
The crank shaft pulley has a mark that denotes #1 piston being at TDC(Top Dead Center).
But #1 piston is at TDC TWO TIMES during 1 full cycle and you want spark timed to TDC on Compression stroke not Exhaust stroke
So the #1 TDC timing mark is only correct for distributor timing 1/2 the time
There are 2 ways to do distributor timing without pulling off valve cover
1. Pull spark plug in #1 cylinder, put in a pressure gauge(or your finger
) then rotate crank shaft manually until pressure starts to build up in #1 cylinder, thats the start of #1 compression stroke, continue rotating crank until TDC mark is lined up, and that's Compression stroke TDC
2. Line up crank pulley TDC mark and install distributor with rotor on #1
Try to start the engine, if it starts then all is well, fine tune timing as described
If it doesn't start then timing at distributor is 180deg off, doesn't hurt anything
Put crank back on TDC mark, pull off distributor cap
Rotor will be pointed at #1 OR #5
Pull up distributor and reverse rotor, if it was on #5 then rotate to #1, if it was on #1 then rotate it to #5
Reinstall, fine tune
And just a heads up the distributor shaft also drives the oil pump shaft so it does need to line up with that shaft inside the engine
After installing distributor it may not sit flat against the engine, rotate crank 90deg one way and then the other, then back to TDC, that should line up the shafts and distributor will drop into place