At what point should the 3/8" pickup be enlarged to 1/2" or even an external oiling? Is it based off RPM's turned?

You will never go wrong using the biggest pick up you can get.

Which motor and oil pump and oil pan size?

Seen a couple fresh blocks broke from enlarging that pickup. Just a warning that it can happen. Then external is the next option.

The 3/8" pickup with (at most) cleaned up oil passages seems to do fine up to about 6,500 RPM (mine does for example). Above 7,000 RPM the stock type oil system seems to struggle from what I have seen.

It goes without saying the pickup needs to be submerged for it to work right so the oil pan needs to be right for the application. For road track I run the Milodon road race pan.

I would go with external before trying to drill and tap for a bigger pickup. Pretty much a mill type operation, screw up and your buying a new block. A side note, make sure you plug the pickup hole in the block when you go to external oiling.

I looked at your title again, I won't enlarge the oil pump pick up hole in a stock block ever again after having two stock blocks crack there
My last pump gas stroker motor made over 700 HP and I shifted it at or above 7000 RPM at every race with a stock 3/8 pick up and a stock 6 quart Mopar oil pan
I do use the Milidon external dual line swinging pick up on some race motors also as well as the Milidon single line external pickup with the stationary pickup in the road race pan

My good friend and partner went 4.80s in the eigth with a 3/8 pickup in his dragster. Ran it for years like that and that motor is still running to this day and sold recently.
Todd

Car is a drag race only, BB in a A body with stock K frame, I do run an 8 QT pan, but with a 3/8 pickup. Current combo gets shifted at 6000 and crosses the strip around 6300-6400. Looking at building a 511 base on a 400 block and see that some grinding is needed near the pickup. Really don't want to go to the external oiling route, but I don't plan on spinning the motor over 6500 either. Just trying to get all the little details worked out before I start the build so I know exactly what I need and the route I need to take.

For those that have experienced cracking the block, was it from the drilling and tapping operation, or from screwing in the pick-up?

I have been running an ext single line swinging p.u.for a long time with a stock k un cut 69 b body.The line just wraps around the front of the k and to the pan,no problem.

I agree w/ rowin4...I'd go w/ a single line external pickup system before I bothered enlarging the p/u hole in the block to 1/2" NPT.
Both of my street/strip motors have single line systems on them. Turning about 7500 rpm. One makes about 700 hp, the other 1100 hp. No issues.

The two blocks that I tried to enlarge the pickups cracked when I installed the pickups
On your next build my first 400 stroker motor was 4.375 bore with a 4.250 stroke crank on pump gas, it made peak HP at 5500 RPM, low deck Eddy six pack intake and stock type 440 carbs on a set of ported 906 heads, I ended up shifting at or above 7000 RPM to get the best E.T. and MPH.
I swap the crank out to a 4.300 stroke to raise the compression from 9.25 to 1 with the original iron heads to 10.4 to with the new CNC ported Eddy RPM heads I put on three years later, it had the stock 1970/71 6 quart Hemi and 440 six pack oil pan with the stock 440 six pack 3/8 pick up from day one
IHTHs

I always drill the hole, taper the top 3/8" with a die grinder, tap hole install pickup, never broke one. If you crank the pickup a full turn past tight it will break for sure, 1/2 turn with locktite is fine. Tap the hole a little more if its too tight before final positioning. I relieve the pan rail for a straight shot at the hole, use black pipe thread cutting fluid, and a new tap. It cuts like butter, just never an issue for 40 years.

Here is an observation: The Factory pick-up is not a tapered thread. Most aftermarket pick-ups are. The tapered thread puts a lot of pressure on the top threads and it is like driving a wedge into the block. I use a straight thread like the factory and make sure that I tap the block pretty deep to make sure that the hoop stress is deeper into the block. But I've only done a couple.

I am under the impression that all the Mopar oil pick ups used NPT threads
What size and thread count are the correct 3/8 and 1/2 taps

Ding Ding Ding! We Have a Winner

Crazy to screw around with a block--if you are at the level where it might matter go with single line external and FORGET about it
Dual Line Milodons are not cool- they are a 60's bandaid-the Milodon pick up does not have the capacity to handle a dual line feed so that is why Indy has a swinging pick up that will!
If a single line external will not keep your stock big block alive then you are doing something way wrong

I also know of that 3/8ths pick up doing 4.80's for years that Sasquatch refers too--our pal ...knew what it takes--still does!!
And it ain't what folks think that makes speed and durability
Do not drill and tap ( way too shallow 99% of the time ) a stock block--you are fooling yourself and taking a chance on screwing up a good block It is not the answer for todays massive Mops

Both NPT and NPS have the same thread angle, shape, and pitch (threads per inch). However, NPT threads are tapered and NPS threads are straight.

People usually argue with me on this, but if you have a factory unit and another with NPT, you can see the difference if you compare closely, and certainly can measure the difference.

People like to argue. I bought a set of NPS taps back in the 1970's for for putting drain plugs into aluminum Muncie transmission cases. They work fine for putting a tapered pipe plug into the straight hole. You don't end up with a plug that is sunk down like you will when someone taps a hole with a tapered pipe tap.

I agree the 3/8 is fine on mild street/strip builds that dont see over 6500 rpm.My brother and I have drilled out many for the 1/2 pick-up also and never had a block break. I honestly dont really feel like I needed to to it on my 63 but I did drill and tap the block for the 1/2" pick-up since I was not sure how hard I would twist it and it has worked great. Ron