Online parts diagrams
 

So been sitting here working out and getting the motor reassembled and ready to drop back in. Still on the trail of finding another bellhousing (long story, damn dog). The notes the boy & I took during disassembly were pretty good and we were able to get 95% of the motor reassembled and ready to drop back in.

Found a couple weird sensors and holes I don't remember though. Down by the oil filter there's the single-wire oil pressure gauge (that I don't know if it was on or off the extension, but it seems to fit pretty well on the extension in there). About at the 1:00 position above it there is another threaded hole that goes into the bell housing. I can't seem to locate anything that fits in here in my parts stash. Trying to find an online diagram thing to help direct me in the right direction.

What year Blazer? The GM service manuals will have diagrams of the routing of some of the wiring harnesses.

Could one of the sensor wires be for a knock sensor? What is this "extension" you mention?

Check out the exploded parts diagrams at the pages below. You might have to cross reference the parts numbers elsewhere to get more information. Also, look for the dates in the upper right-hand corner of the diagrams.

https://nemigaparts.com/cat_spares/epc/chevrolet/
https://nemigaparts.com/cat_spares/e...olet/54s-t/02/

I was thinking a knock sensor as well, sounds about the right location for it.

The block drains are around there as well, could that possibly be it?

Let's see how this works - I'll throw the image inline. I know there are block drains on either side of the block (about middle of the block on either side). On the passenger side is the knock sensor (white connector). Here's the hole(s) in question. Not the gold extension thing, I have the 1 wire oil pressure gauge to go here. It's the one just up and to the right of the hole. It goes straight through to the bellhousing area. Don't recall if it's threaded or not.

I'll check out these links above - thanks y'all. Someone asked the year - thought I had it in the profile somewhere. It's an '87 Blazer with a 305 and sm465. Chatting with another gearhead someone said something about clutch controls or something. I think its moot for me though as this one has hydraulics vs. cable clutch.

https://cimg5.ibsrv.net/gimg/blazerf...6fa60f493b.jpg

If memory serves me right the lowest large hole was for the clutch ball for the crossover swing arm on the old school manual type clutch and the two upper ones were to hold wiring looms, heat shields and such.
Also the oil sending unit should be up by the dist, the one by the oil filter is usually a test port and plugged off

Thanks. The manuals I have don't list too much information or diagrams, but I found a couple good reference pictures that helped out. I have two oil pressure things - a 2 wire up top by the distributor and then the dash gauge one down by the filter. I'm leaning towards blocking the lower one and just running a T-tap up on the top to run both sensors. The top one is the 2 wire for the electric choke thing from what I can gather by the wiring diagram I found.

Ok I see now.
Usually on the two wire sending units one wire runs the gauge/idiot light and or computer interface the other usually runs a low pressure emergency shut off for a elect fuel pump.
My toy in my avatar has the computers still so when I ran after market gauges I did the T tap by the dist to run the two sending units that way every thing is up high.
Working around the dist. is a pain but up there I have no worry's about it getting screwed up by something hitting it