Last thing is, how are you measuring? Certainly not meant to offend or accuse, but tight hand with a micrometer, less than accurate bore gauge, old bad eyes, etc, all matter.
This^^
A light vs heavy touch on a micrometer easily makes for a .005 difference. And if all the gauges/micrometers aren't calibrated to each other with a verified checker, then all numbers are pretty much meaningless anyway. A 1" calibration checker, or whatever they're called, is really necessary. Then mark any differences between all gauges and micrometers so that number can be added/subtracted from any final numbers you come up with. They can easily vary by +-.005, if not more, depending on age and quality. Just a couple of things to keep in mind when trying to determine exact clearances.
That's a great point, checking your mics against a standard should be a daily occurrence for the guys that use them every day and a good habit upon usage for the DIY'ers. However, if they're off, no need to add/subtract numbers, you just use the micrometer wrench and readjust the barrel. Your micrometer for checking bearing clearances on an FE will be a 2-3" micrometer, so it should be a 2" standard.
Bore gauges will never be "off" because you set them to the micrometer or setting fixture that you're using each time.
Another good point that you made, is that a micrometer is a "touch" tool. If I gave 20 guys the same mic and told them to measure a crank journal, there would probably be a tenth or two (.0001-.0002") spread between all the measurements. Even using the barrel against the clutch can sometimes show a .0001" difference.
I also agree with Scott, there are probably specific situations where a .0015" oil clearance might live, but it will be a very specific situation: all .0015" measurements across the board, no taper or out-of-round situations, very light viscosity oil, and a life of idling around. I would also have to lump in new, rigid connecting rods in with that. Back "in the day" guys were having to do all kinds of stupid stuff to make FE's live at high-rpm/high-hp, including honing the rod journals out of round. Now we slap in a good set of Molnar rods and turn 'em way up.
In the OP's situation, it's quite possible that the tolerances just stacked up against him and that's the clearances that he got. If you hand a machinist a bunch of parts and tell them to go to work, they don't generally put everything together, measure clearances, and then adjust everything to fit. Unless given specific instructions, they just grind/machine everything to spec: main housing bore to middle of spec, crank journals to middle of spec, rod big ends to middle of spec, etc.
When I send a crank to my local grinder, I give him specific instructions based on what's worked for me in the past. I've got 2 cranks at the grinder right now getting "adjusted". I only use coated bearings and bearing selections have been straight up stupid for the past 2 years, so I have to use what's available on the shelf and "adjust".
If this were a SBC, then you could just go buy a set of .009" or .011" bearings and mix and match. However, the amount of bearings out there for FE's are few and far between. That's why your new aftermarket blocks are all cut for Cleveland bearings. You can't even get a true "X" main bearing for an FE, you have to grind the tangs off Cleveland bearings and use those.
