How to get number 1 cylinder top dead center when motor is not running on a gmc 4.3 1991

It should point to the contact just before number 1 cylinder, once motor running you should set ignition timing with a strobe

Sir. The only other way to know your on no one cylinder top dead center is to take the numbers of the firing order,
White them down with the first 3 in one colum and the second three firing numbers in a colum next to the first three. The numbers across from each other are called " mating cylinders".
If #1 cylinder is ( example ) next to #6 on your paper, then those two are mating cylinders. Meaning one ( with valve cover/s off) will , when engine in turned over slowly, will be on valve overlap, while its mating cylinder is on
Top Dead Center to fire. In valve overlap, when the motor is turned slowly one way, the exhaust valve will open and when engine is slowly turned the other way, the intake valve starts to open. At that point when either exhaust or intake valve is moving on #6 cylinder valve trane, #1 is at top dead center which will be not even 180degrees out, but top dead center on #1 cylinder. Now your at the firing point of #1, which will coincide with the rotor pointing at #1 plug wire on the distributor cap. In essence, static ( not
engine running ) the rotor bug should be about 17 degrees early in the cap. This makes up for slack in any timing chain and gear slack between the distributor gear to cam gear. This when running put you about 4-6 degrees advanced. Computor models will time it for itself. See example below.
1 - 6
5 - 2
3 - 4
If your timing order is 153624 then use the chart. As you list your order on paper, remember one mating number is firing and it's mating cylinder is on valve overlap. 180 deg out would be
#1 valves " rocking " and no 6 is on compression. When a cylinder is on the compression stroke, valves will not move. ( both closed ). So, write down your timing order, v6 or inline 6, or 4 cyl or 8 cylinder. It don't matter. If you look at the manifold , it will give the timing order, maybe on the head or Internet. Two columns, walla.
Hope that helps get you to true #1 top dead center. This system is the only true way, along with a cardboard timing wheel for the crank if needed , to adjust valves, lifters and know where the engine is at all times.
Hope that helps. Sorry for the detail, engines are really very simple, once you " get the picture ". Don't make it to difficult. Crank turns two times to one revolution of the cam.
Terry

pull both plugs and rotate motor till you get timing mark to top dead center on marker, feel for compression as you rotate motor BY HAND ONLY that will be #1

put number 1 cylinder at top dead center and drop in distributor so the rotor points to number 1 on cap. may have to try several times as gear moves when lowering dist.

Sounds to me like your oil sending unit is out. Might want to replace that and that should fix you up.

The timing is not adjustable and it is computer controlled, but the distributor must still be clocked in properly and static timed, and here is how it is done.

There is a mark or notch on the distributor housing that the rotor should be pointing to when the engine is on top dead center. This static timing is all that matters and the computer will be able to control the timing as long as the ignition rotor is in that position when the engine is at top dead center.

1. Place the engine at top dead center.

2. Look under the distributor cap and find where the number one terminal runs under the distributor cap, and where that position on the distributor cap corresponds with the distributor housing, and it should match up to a mark or a notch on the distributor housing.

3. With the engine on top dead center the ignition rotor should be pointing to the mark or notch that is on the distributor housing, if it is not then loosen up the distributor and turn the distributor until the ignition rotor is pointing to the mark or notch on the distributor housing, the engine should now be static timed.

click the ignition so #1 cylinder is at top dead center then pull youre distributer cap off make sure the rotor is pointing towards # 1 cylinder if not you re timing is 180 deg off pull the distributer out a little and turn the rotor to # 1 cyl you probaly dont have the timing marks lined up correct if the timing is off it will back fire either out the air filter or exhaulst ..

On the first cylinder (first closes one of right side). After plug is removed, turn motor until piston is at very top of cylinder.

Ok that's pretty simple Put cylinder # 1 on TDC= top dead center on the (compressionstroke piston all the way up). Mark on metal part of distributor (with marker) where the number one is on the cap( that's where the number 1 wire plugs in) Then in-plant distributor make sure that rotor point is pointed to #1 on the cap, turn distributor shaft until this is achieved or as close as possible try to center shaft so you can adjust... ****** or advance to fine tune after you in-plant distributor if needed. Contact if you need more help . Good Luck

Make sure you have number one cylinder at top dead center, do this by removing the spark plug and puttin a screw driver in it, when it gets to the top stop, you turn the motor by using a socket on the crank shaft, if you do not get the cylinder to top dead center this will cause the problem, how ever this means you have to remove the chain again and realign every thing.