how easy is it for the oil pump drive shaft to be inadvertently rotated while heads are off and distributor is out

While heads were off did you rotate the engine for any reason? This sounds baffling for sure...that would move TDC from original and is now on TDC exhaust instead of compression. I can not see how the oil pump drive could get moved ..especially if you were never in the bottom end. So it's off one rotor contact....and you did everything right marking and taking pictures. Geez....thinking I am missing something...

Yes the engine was rotated

Also ... I have installed the shaft in both positions to make sure that I am on TDC at the proper time

if the engine was rototed while the distributor was out then the shaft isn't aligned anymore and has to be set back by hand

Hey Andrew So you're saying that if the gear portion of the shaft isn't in place when the engine is turned over ...that the oil pump drive is able to rotate ? If that's the case ... Then best thing to do would be put a screwdriver in and bring the slot back inline ... Rather than move all the plug wires ?

If possible rotate the engine back where it was until the rotor points at #1 TDC, take out the spark plugs to rotate easier

I've turned this engine over many. ... Many times to try and achieve this ... TDC for # 1 is a piece of cake but at no time does that mean that it aligns properly with the keyway in the oil pump. The only logical answer would be the oil pump key has rotated . Before I make matters worse ... I wanted to be sure this was the case and hoped someone might know

Set the engine to top dead center, then use a long flat screwdriver to align the slot on the pump. Keep in mind, when you drop the distributor back in, the distributor drive gear is "helical". The shaft will turn, as you drop it in. So,... you will have to estimate how much further you will need to turn the oil pump shaft, with the screwdriver, to get it aligned. You might need to take the distributor out a couple of times, and re-adjust with the screwdriver, to get the proper position.

Thanks I appreciate the input ... Again this is necessary because the oil pump key can rotate without the distributor shaft being in place if the motor has been turned over ?

No, the oil pump shaft will not rotate without the distributor installed. But, if you crank the engine with the distributor off/out, the oil pump shaft may not align correctly (even if you brought the engine back to "top-dead-center), you could easily go one tooth off on the distributor. The distributor is driven by the camshaft gear. Lets put it this way, ... if you take the distributor out quickly (and nothing else), and then try to replace it, you can easily be off on the oil pump shaft. Why?... because the oil pump shaft turns very easily, and the engaging slot is chamfered, and if you place the distributor down where you might think it will align, it will miss-align the oil pump shaft.

Take the distributor out. Look in the hole on the engine, with a flashlight (you will see the oil pump shaft), look at the bottom of the the distributor inside the gear (You will see the spring pin). These 2 parts must must align, when you drop the distributor in.

I've always made sure the slot aligns with the pin but I understand what you're saying. Thanks ... Appreciate the input .

If your distributor keeps landing in the wrong spot, use the screwdriver to move the oil pump shaft a bit further, then try dropping the the distributor in.

Don't try to drop the distributor into the position it will rest at. It doesn't work that way. You need to drop it in, one tooth/position off, and the oil pump shaft has to line up to it. Adjust the oil pump shaft first with a screwdriver, and a good eyeball on the distributor. If the distributor seats in the wrong spot, you need to turn/adjust the oil pump shaft. Don't crank the engine, don't change the plug wire position, but "do" take out the distributor and change the oil pump shaft position. When start to drop the distributor in, it should be about one firing position off, until it seats all the way down,... Got it?

Sounds good thanks

Answering your question to me: turning the engine over without the distributor in is EXACTLY was casues it to be out of alignment. Its not that the oil pump shaft moves but the opposite, it doesn't move and the engine does. The gear on the dist is turned by a gear on the camshaft. Then the oil pump drive is turned by the keyway on the dist gear end. Get the engine to TDC, look at the dist pointed to cylinder 1 on the cap, then look at the keyway on the botton of the dist gear an make not of the way it is positioned. Now the oil pump shaft needs to line up with how the dist gear is, you can turn it inside the engine with a long flathead screwdriver. Align it and plop it in, now you may be spot on or 180 degree off depending on which TDC your at but should be an easy trial and error to figure that out.

Worked like a charm ... Thanks guys

See Youtube video, Chevy distributor installation timing problem. https://www.youtube.com/watch?v=wQYX8uNU8aM

Hey quick side question to this process. Let's say I had replaced the distributor but when installing I put it on the wrong stroke for #1tdc and didnt get a start upon cranking but instead got a crank and a loud banging sound. Learned about the oil pump, got it all lined up and running, but once started it died out after a minute, ending with another bang that made me cut the key. Went through and checked things a week or two later, started it with the oil cap off and could see no oil flowing from the lifters or any splashing in general. Am I on the right path to think the pump and drive shaft are toasted and get them ordered and replaced? Or is it possible I'm not driving the oil pump because it's still not lining up to the distributor properly? Sorry for the lengthy tag on to this thread but you all seem to know the right info s