How to plug the big block oil holes?

[davepl]

After a couple of years, I'd like to start my engine today, or at least soon. First, however, I need to re-prime it, which means oil, which leads me to:

Above the filter there are three holes. Unlike most pictures I can find, on mine the larger is toward the rear and the smallest toward the front.

The center is the factory oil pressure sender, I know that much.

One of them must be the clutch pivot ball. I'm guessing the larger, and it seems to go nowhere (I stuck an awl in it, not much depth).

So my question questions are:

1) What's the front little hole? I've tried screwing an oil pressure sending unit in there, which looks to be the same size, but no go. Won't start threading.

2) Did they reverse these two holes somewhere between Mark IV and Mark VI engines?

Thanks!
Dave

[imjeff]

They are different between two bolt and four bolt mains.

[imjeff]

Sorry, I should have known from the description that it's a two bolt main. The 2-bolt main block has the 5/16" front hole that is not drilled and tapped into the oil galley, the center hole is drilled and tapped into the oil galley with a 3/8" pipe thread hole and the rear hole is 1/2" USS coarse that is not drilled and tapped into the oil galley. This rear hole has a machined surface and is used to mount the pivot ball for a manual transmission clutch release cross-shaft. Hope that helps.

Jeff

[davepl]

It's a 2-bolt block, 1970. The 5/16 (front) is drilled and tapped on mine, which is why I'm worried. I was able to start a 5/16 bolt into it, whereas 5/16 pipe thread I couldn't start. I didn't run anything in past finger tight.

So what to do when the little front one has threads and seems to go somewhere?

[imjeff]

Originally Posted by davepl

It's a 2-bolt block, 1970. The 5/16 (front) is drilled and tapped on mine, which is why I'm worried. I was able to start a 5/16 bolt into it, whereas 5/16 pipe thread I couldn't start. I didn't run anything in past finger tight.

So what to do when the little front one has threads and seems to go somewhere?

The front one should just be a 5/16" hole that isn't threaded or into the oil galley. You might just be getting bite from the end of the bolt like any hole without threads. You can always check it when you spool the oil pump up to pre-lube. It shouldn't go into the oil galley and I can't imagine why anyone would thread it. The clutch ball always goes on the hole with the machined surface around it.

[davepl]

I threaded, by hand, a 3/8 bolt into the front one and a 1/2 bolt into the back one. Don't know what to tell you, other than maybe someone had to drill and retap it, but so far as I'm aware this engine has never been out of the truck. Original owner's records show nothing more invasive than a burned valve. Numbers matching original 402 truck. Correct casting number and date codes.

Both have flat machined surfaces too (or at least flat, the rear one is clearly machined). Especially because it's tapped with bolt threads rather than tapered pipe threads I can envision a ton of things (4-speed conversion and they guy used the wrong hole for the clutch stud, had to fix it) but just not with this motor in this truck.

Sorry about the ugly teflon tape :-)

[imjeff]

Sorry, bad communications on my part. The forward hole on your block shouldn't be threaded into the oil galley. It is threaded from the factory as I recall. The four bolt blocks had a larger NPT hole in that forward location. The only one that should strike oil is the center one.

[midniteblues]

hey dave did your front hole(where you have the bolt) have a threaded plug in it?

mine had a plug in it. why would it be plugged if it dont strike oil?

[davepl]

Originally Posted by midniteblues

hey dave did your front hole(where you have the bolt) have a threaded plug in it?

mine had a plug in it. why would it be plugged if it dont strike oil?

Sadly I don't know because I didn't note it before it went off to the machine shop. I talked to the builder today, should have asked him if he had to plug it on the dyno.

I taped up the bolt and added it because I could "go around a corner" with pick tool if I went in deep enough. No idea if it's been drilled into an oil gallery at some point or not. Home made oil cooler? Who knows.

I've always been of the opinion that there was no sure-fire foolproof way to ID a 2-bolt from a 4-bolt block. And yet these two holes do seem reversed on most 4-bolt blocks.

I know it can't be a "rule" however as GM used the same engine casting sometimes for both 2 and 4 bolt big blocks. Just machined them to 2 or 4 based on the application.

[midniteblues]

I've seen the holes reversed but only on tall deck engines.

boy i'm hoping it works as thats where i ran my gauge line. but reading between your lines if there wasnt a plug in there when you got your block back and it was dynoed i'm kinda thinkin it dont need a plug.
why would the dyno man pull the plug back out is what i'm getting at.

awe man something else for me too redo.