What's the size of the oil pump pickup stock in a 1977 RB440? Milodon had me buy a 1/2", part # 18335, but I'm recalling the stock thread hole is smaller than that? Please advise.

3/8”

Thanks for the response! A big effort and any issues with increasing it to a 1/2"? Worthwhile or not?

Unless you are going to twist its tail real hard, or run large bearing clearances, or use a HV pump, I'd spend my money elsewhere.

[s][/s]I bought a Melling HV pump. I thought I've read in the past people choosing to up size it to improve oiling. And was wondering how difficult a process to do it, for a machine shop. Otherwise I'll just spend another $60 and call it a day for the smaller 3/8" pickup.

3/8" vs 1/2" is one of the classic debates...
The issue with enlarging the block inlet for a 1/2" pickup is core shift: whether there's enough material around the full circumference of the inlet.
Lots of guys run 3/8" without issue; I've done both sizes and really didn't see a difference.
But those engines had large pans & windage trays and were generally fussed over to avoid oiling problems.
For a street motor, I wouldn't bother.

I appreciate the full answer. It's a street/strip build. 508" stroker with hydraulic roller, SCAT, Diamond lower end. I've got close to $20K in parts already (which is a lot for me) and want this motor to last as it'll be my last.

I'd just stick w/ 3/8". If it was a build where I was worried that much about oiling, I'd go straight to an external pickup system.

Absolutely not worth weakening the stock boss for the pickup by removing more material and taking a chance on it cracking when you're using a tap to make the new 1/2 inch NPT threads
I've done that in the past back in the late 1990s and early 2000s, not any more after using a stock 3/8 pick up in my last 517 C.I. low deck pump gas stroker in my old Duster. I ended up shifting it at or a tiny bit above 7000 RPM (after multiple tests on shift point to get the best R/T, 60 Ft. and best ET,and) the high volume, high pressure Melling oil pump kept oil in all the right places, very good service with Valvoline VR 10/30Wt racing oil: up:

I forgot to mention that I run the full narrow groove Seal Power truck main bearings with.0035+ clearances and similar (.0030+) rod bearing clearances. In my opinion it is way better to have some extra bearing clearances instead of not QUITE having enough
Thin(5W20, 10W30) multiweight oil is your friend

Great feedback. I'll stick with the 3/8. I used Kendall 20/50 oil for 20 years in the same 440 that saw occasional nitrous and regular 6500 shifts and never a problem. But I will go with lighter oil this next engine which will have to last me into old age. :-)