this time it is not for my own vette, but trying to help out a friend with his distributor..... atleast that is what we think his problem is about.
the vette doesn't start anymore.... a complete rebuild v8 and after the rebuild the engine was tested and did run ok, but now.. monthsss later he tried it again but no go .. after tested everything the conclusion is that it must  be the timing/distributor... ( think engine was tested with another distributor back then)

the question is   how can you see the rotor is in right position when putting the rotor/shaft in the block.
i did find an article but it was not clear to me what points out ( how can you see)  the rotor is in right position (what is the reference point ??)

quote " 2. Install the rotor to the distributor. 3. Hold the distributor body in the orientation show in figure 1 relative to the engine/block and drop the distributor straight down into the block. Pay no attention to rotor orientation at this time. If the rotor is aligned with the oil pump driveshaft, the distributor will drop all the way down and seat. If the rotor does NOT line up, the distributor will not drop all the way down. 4. If the distributor does NOT drop all the way down (chances are best that it won’t), pull the distributor up out of the block just enough to disengage the rotor from the camshaft gear, and turn the rotor a little bit. Drop it down again. Repeat this until the distributor drops all the way down and the rotor engages with the oil pump. 5. The distributor will now be all the way into the block, but the rotor will not be properly aligned. You can now pull the distributor up until the cam gear disengages, turn the rotor JUST A HAIR (half a cam tooth), and drop it straight back down again. The rotor will now move one tooth over, and the chamfer on the oil pump shaft will allow the oil pump to line back up. The distributor will drop all the way back in again, with the rotor moved over one tooth. (If it doesn’t work, try rotating the rotor the opposite direction.) Repeat this operation (I call it “walking the distributor”) by lifting the distributor up, slightly moving the rotor, and dropping it back in until you’ve “walked” the rotor around to its correct position as shown in the figure below. Once you get the technique down, you can do this very quickly – much quicker than trying to align the oil pump driveshaft with a screwdriver while looking down the hole. The screwdriver technique also requires that you pull the distributor ALL THE WAY OUT to fiddle around with the screwdriver several times until you get it right. So try my “walking” technique: it’s quick and accurate. 6. Once you have “walked” the rotor into position, you should be able to obtain the orientation of the distributor body and the rotor as shown in Figure 1.  "
/end quote

that figure 1 is a dark picture that doesn't say much to me

getting the crankshaft/harmonic balancer etc in right position is quite clear to us....... but the rotor 

thank you
art-corvette

Art, here's the easiest way to index the rotor.
With the engine on TDC(Top Dead Center) on the #1 cylinder, and the balancer mark lined up at "0", the rotor should be lined up with the #1 plug wire on the dist. cap.

Trace the #1 plug wire up to the cap, and mark it somehow. Tape works well. Then, mark a line or other ID on the base of the distributor in line with the #1 plug wire you just marked on the cap. Now you'll have a reference point on the dist. Now, loosen the dist. hold down, and move it away from the bottom of the dist. so you can raise the dist. enough to turn the rotor freely. Turn the rotor to line up with the mark you made on the base of the dist, and then go a little more counterclock-wise, until the dist. will slide back down into the intake/engine.(As you install the dist., the rotor will turn clockwise a bit, so that's why you need to go a little farther c-clockwise to start.)

If the dist. will not go all the way down, then the oil pump drive-shaft will need to be lined up with the drive tang on the bottom of the dist. gear. This means you need to pull the dist. all the way out, and note the position of the drive tang,when the rotor is pointed to the mark you made on the base. Then, look down into the hole where the dist. goes, and see where the slot on the pump-shaft is. You'll need to use a long, flat bladed screwdriver to turn the pump shaft to match the position of the dive tang on the dist, remembering to turn it just a bit farther c-clockwise, same as the rotor. Then, the dist should slide all the way down.
Whew...

The really EASIEST way to get the ign. wires timed, would be to locate the rotor position at TDC as it is, and then just move the wires on the cap to line up with the rotor point. If you have the engine at TDC now, just remove the cap, and note where the rotor is pointing. That should be #1. Then, find the #1 plug wire, and move IT to that terminal on the cap that the rotor is lined up with. Then just finish routing the rest of the wires from that point.
Once you do this, if the engine still won't start, or it pops and backfires, then you know you didn't have it on the #1 compression stroke, and the wires will be 180 degrees out. If this happens, just remove the #1 wire from the cap, and move it to the terminal exactly across the cap from it.(180deg) Then move the rest of the wires the same way, just swaping positions one at a time.

I'm afraid none of this makes any sense... I'm MUCH better with visuals!

thanks Joel, friday we will try to make that vette run again.

If you want to save just a bit more time.   Remove the number 1 spark plug and stick your thumb over the hole and have someone bump the engine over just a bit at a time.   WHen you feel compression pushing pressure on you thumb stop bumping the engine.   Then turn the engine by hand until the timing marks line up.    Not zero, but the timing you want.  Now the above procedure to install the dist, and it will be very close.

tonite been trying to get that vette ( an 81) running but no go.