This may be a stupid question, but I just got my block back from the machine shop and all of the block's threaded plugs were removed and returned to me in a baggie. I can only find 4 plug holes and I got back 5 plugs. The ones in the sides of the block with hex head were not removed and are not being counted, only the allen head type. I have 3 on the back side, and one in the oil filter boss area. What am I missing?

That is not a stupid Q at all. Look inside the 2 threaded bosses on the rear bell face, the one on the drivers side is larger as you need to thread in a 3rd npt plug inside of this large boss to cap the dr side horizontle main oil gallery inside.

The one at the end of the lifter galleys is one you will only miss once.
When you find out the hard way you'll never do it again

The only stupid question, is the question not asked.

check to make sure the little cup plug is in the oil passage below the oil sender also

thats a tricky one..machine shop might or might not have knocked it out

IIRC run a small wire down the oil sender unit and see if the cup is in there

if not IIRC it goes in from under the main bearing

Thanks, Of course the larger plug is the one I put in first, so I couldn't see the inner threads.
Could of been disasterous. You guys rock.

was the small cup plug that has to be tapped in like a core plug there???

I would deff check for it also

the oil passage comes up vertical from the pump/rear cap then makes a 90 deg turn out horizontle to the filter/thru the filter then back in horizontle into the block & back to the same vertical passage and a 90 deg turn back into it & the oil continues further up vertical to the horizontle oil gallery. The vertical passage between the 2 horizontle passages is drilled so the oil would go strait up without going out to the filter and it needs a mini freeze plug driven in to a depth between these 2 passages. There's a cupped depression sort of there for it to seat at the right depth between the 2 horizontle passages. under the rear main cap are 4 holes, 2 for the main cap bolts, one for the intermediate shaft that drives the pump and one for the long vertical passage and that one is where you drive the mini plug in. EDIT to drive the old plug out so you can bristle brush clean all the galleries drive a rod into the oil psi sending unit hole above at the top rear of the block to drive the plug out the bottom aforementioned hole

Most shops never remove the plug, but it re-directs the oil flow to the oil filter. I hope this picture helps?

That worries me. I don't remember seeing that plug in my 340 block. I cleaned it out with oil galley brushes when I assembled it. I took all of the plugs out but don't remember a drive in plug at all. It runs fine but oil pressure is low at idle about 20 psi.
Doug