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Since ABSOLUTELY NOTHING WHATSOEVER on these motors is in Imperial units anymore, my money is on a 16mm drill for the passage.
Try using drill bits to measure the hole dia rather than calipers or the like.
That would make sense. I was just measuring the barbell, as I don't have the motor apart yet and want to verify before doing so.
If it is actually 16mm, that would leave a gap at 63 microns at the largest. That sound pretty good to me.
Your measurement of the billet barbell's outer diameter (OD) at "just over 0.626 inches / 15.9mm" confirms that your new part is likely machined to precise specifications designed to fit snugly within that 5/8" bore.
The fit is often designed to be a very light press-fit or a snug slip-fit to seal the passage correctly and force the oil through the filter and cooler ports as intended.
On aftermarket billet versions, yes. The plastic OEM style barbells do allow a little oil by them as a bypass design….o-ring is on the rear of those and seals the oil from exiting the circuit. The front of the OEM style is merely a plastic nose without any o-ring used. I’m sure you guys know this, as I’m just clarifying info here.
Since ABSOLUTELY NOTHING WHATSOEVER on these motors is in Imperial units anymore, my money is on a 16mm drill for the passage.
Try using drill bits to measure the hole dia rather than calipers or the like.
If GM ran a drill thru there as a finished diameter, the oil would go past the barbell like an electric car passes a gas station. My guess is they're drilled and then reamed to size. Surface finish has to be good enough so the O-ring seals and lets no unfiltered oil by it.......
As Che70 said, the factory barbell doesn't have an O-ring on its business end at all. And for that matter, even if it did, the MAXIMUM pressure differential across it, would only be the oil filter bypass valve's setting; something in the 5 - 10 psi-ish kind of range. And, all it can leeeeeeek anyway, is unfiltered oil, around the filter and into the filtered oil passage. Butt since the gap is probably only in the sub-100 micron neighborhood anyway, i.e. less than .004", any leeeeeeek it commits will be TINY.
The back side, where it plugs the very end of the passage and DOES have an O-ring, might have a greater tendency to leeeeeeek a tiny amount, since there's full post-filter oil pressure (what you see on your gauge) across the O-ring there; butt it would hardly matter, since all it would do is drip back into the pan.
I don't know of the factory EVER "reamed to size" ANYTHING unless absolutely unavoidable. A place like that, as minimally critical as it is, would certainly never get such an expensive extra treatment.
If I was the betting kind, I'd bet they ran a 16mm drill in there, and shipped it.
A drill thru the low grade aluminum casting of the block would leak past any barbell and also past the O-ring. I was speaking of both ends. A drilled finish would suck. I guarantee you these aluminum blocks are what we in the trades called "gummy" aluminum. If GM didn't get a decent finish on that I.D., shame on them.
You do have me curious, though. Next time I'm around a bare aluminum block, I'm going to check the finish. On second thought, you're probably right. When it comes to quality, GMs train left the quality depot a long time ago...
Can't speak specifically or authoritatively to "your" LQ4; butt both the LM7 in my truck, and the LQ9 I have on a stand to replace it, have 10mm BH bolts.
Again, can't speak to "your" block, butt this is one of the bolts holding my LQ9 to its stand. Sorry, the whole thing is turned around backwards, had to use a mirror to be able to see it. I can assure you that this 10mm bolt threaded in by hand. It's just yerbasic hardware store bolt if memory serves; had to be like 120mm long or something, to go through the stand and thread into the block. You can see the metric marking (10.9) on it pretty plain.
So yeah, I STRONGLY urge you to go back and re-check your work, if you graunched 3/8"-16 bolts in there. You might be in for more than one nasty surprise.
C5R block is imperial. Every other GM Gen3/4/5….and soon to be 6…blocks are all metric. Your bellhousing threads are gonna need drilled, tapped, and Helicoild. Or just drill and tap to the next bigger size and go with the correct fasteners…just PLEASE be sure to inform the next owner of said engine before he/she makes a mistake down the line.
UH-Oh---- Hope I didn't destroy the threads in my LQ4 on the stand....
How did you run the bolts in? If you did it by hand, you'd know right away. I suspect you used a power tool. The 1.5 metric pitch equates to 1 turn=.059" movement. The 16 SAE thread is 1 turn=.0625" movement. I doubt you'd get 1 turn if starting them by hand. One way to check whether or not this was SAE or metric would be to use calipers to check the drilled hole size. On a 3/8-16, the hole size should be 5/16". On a M10-1.5, it would be .335". Sorry to hear of your issue. A good rule of thumb, and one I've always used when installing a bolt when I'm unsure of the female thread, is to start it by hand a turn or 2, then use the power tool. BTW, on every SAE thread that has 16 TPI, hole size is always 1/16" smaller. On every metric thread, hole size can be determined to be the size of the diameter, in this case, 10mm, minus the pitch, in this case 1.5. 10 minus 1.5=8.5, so an 8.5mm (.335") drill can be used. FWIW......
Anyone know the diameter of the oil diverter hole? 
It's just yerbasic hardware store bolt if memory serves; had to be like 120mm long or something, to go through the stand and thread into the block. You can see the metric marking (10.9) on it pretty plain.
