Oil pump alignment is a crapshoot, at best. Crankshaft is centered in the bearing journal when running, due to oil pressure, we all know that. But….turn the key off and the engine comes to a stop. At its last couple revolutions, there’s no oil pressure to keep the crank centered in the journal, and the cylinder pressure up top is pushing downward on the crank. It’s not firing, but there’s still compression pushing downward It’s not staying centered in the journal at this point and will be be low a half thou at least, maybe even a full thou, depending on where the bearing clearances are setup. Gravity plays a role here also, as it’s a 50+ pound piece of rotating mass. Installing (replacing) an oil pump in this situation is an absolute guess. Where’s the crank at? How low is it at the moment? There’s no way of knowing. Now, with a new build you’ll have a far better chance of having the crank centered in the journal, as there’s no cylinder pressure pushing downward, but I still don’t think it’s going to be right. Engine must be running to have oil pressure, and oil pressure alone is going to keep crank centered. The robots at GM that build 99.8% of LS/LT engines certainly don’t align anything. The few handbuilt engines that GM does, don’t get aligned with feeler gauges either. In my builds, I leave the pump bolts loose, spin the engine a rev or two to ensure nothing binds, and torque the bolts. That’s as close to being “aligned” that we can do, in my opinion. Send it.
I had this conversation with an engineer at Melling a few years ago or so, and he was in total agreement with me. I don’t recall his name…

 

From what I can tell, that center part of the oil pump free floats anyway, inside the pump housing, and the fitment between the crank gear and inner part of the pump don't seem to be all that tight anyway. I'd say, as long as its not sooooo far off that its rubbing the inside of the sides when spun, than its going to be fine, and I don't think that there is enough play in the alignment that it could rub anyway. One thing I saw done on youtube, was dimples drilled into the flat surfaces of the inner part of the pump, to help hold oil and reduce wear. If anything, I'd say thats more important than anything, if running just a stock pump.
However, If you're upping the Oil Pressure with a shim, or new springs, or new pump (like mine runs at 60psi idle when hot) than even the dimples at that point probably dont help. In order to push that much oil pressure thru the system, the oil pressure at the pump has to be at min, 60psi as well, and 60psi is going to push oil into that space between the inner part and outpart of the housing.
So I'm not saying NOT to shim it, if it makes you feel better..
However, I would agree, if you already have things together, I wouldn't pull it apart (risking leaks) just to shim the pump at that point. Just IMO...

 

I fully agree with your statement. I really really doubt GM takes the time to shim out the pump. At the most, the factory does what you said, and spins the motor over to help align things, but even then, I doubt they do that. Even the hand built ones from GM.
And with that said, I personally own a 1999 GMC Sierra 1500 with 5.3l and it has 447k miles on it, and the 4th 4l60e trans finally failed... It was getting low oil pressure at idle, to the point where the lifters would start clicking, but if revv'd to 2000rpm, it would get about 15psi, which was enough to shut it up.
But with that motor hitting 447k miles and most of the Gen3 LS engines I've owned hitting well over 275k before the 2nd or 3rd 4l60e trans failed, I'm going to say that GM got the design right, and pump alignment doesn't really matter.
I asked the guy who's done all my machine work for the past 20years what he does when building an LS for someone and he said "I just put it on and torque the bolts"... He's the son (and current owner) of the original owner who started the business back in the 1950s... They've built 1000's of eninges and have taught me a ton of tricks about building older SBC engines that I would've never thought mattered, but after doing some of the stuff they showed, I can actually see slight improvements...
Simple stuff from making stock parts work better, to hand grinding the block with a burr bit in an angle grinder, to eleminate some of the casting edges, and adding radius' to areas for 3 reasons. 1) Smoothing out sharp edges so you don't get cut, 2) Adding radius to edges for oil flow back to the bottom end quicker and 3) Eleminating edges helps to relieve stress areas, to help prevent stress fractures or cracks under high load.
I'm not sure if #3 really does much, but its sure nice not cutting your hands up on the block, and I can def tell more oil gets back to the pan faster under high RPM, as I used to suck the sump dry on some older small blocks still running stock oil pan setup.
There are a bunch of other tricks, but this is the big one that follows over to the LS stuff. I was able to clean about 1lb worth of excess casting metal out of the return ports in the "lifter galley" of my 2 LS 6.0L blocks. The larger hole in the front, that allows oil to run down over the timing chain, and the smaller ports that have oil drip down onto the cam then back to the bottom end... I don't need oil sitting in the head, or galley when it could be in the pan ready to be pumped around again.

 

I've never shimmed one and not saying that you shouldn't but the gear in the oil pump only goes onto the pump sprocket one way. Once you get it on the crankshaft the bolt holes line right up so start the bolts by hand and then hand turn the crank to check for any binding and if all is good then torque it down.

 

You make an excellent point on the crank "sagging" .001"-.0015" to the main cap side. I suppose if one really wanted to split angel hair, you could deliberately shim the pump lower to match the dropped crank. Now we're talking, lol!!