[camaroman7d]

I am trying to settle a debate/discussion about this 1/2" plug. If it is left out, what will it cause?

I was taught and have read that it will cause an internal oil leak and lower oil pressure by 20PSI.

Some people say it won't lower oil pressure at all it will just cause the oil not to be filtered.

I have not personally tried to confirm this nor have I ever forgotten to install this plug.


Here is a quote from John Lingenfelters book (page 145 in the engine blueprinting section) "Don't forget this little 1/2-inch oil galley plug found under the rear main cap. If left out, it will create an internal oil leak that can reduce oil pressure by 20 psi."

So what's the deal here? Has anyone ever verified this? What are your thoughts/beliefs?

Thanks,
Royce

[Jmark]

Well, if its the hole I"m thinking about, i've never seen it plugged before. It won't cause the oil to be unfiltered either.

When you put the main bearing in place, the hole is covered anyway. It may seep a bit of oil, but I've never had a problem with it.

Mark

[lluciano77]

I have a Hot Rod magazine article called "50 Worst Engine Building Mistakes". It says that if that plug is left out, your oil will not pass through the filter.

[Jmark]

Hmm. Maybe we need a clairafication on the hole that is being talked about. Can someone post a pic of the hole in question?

There are 2 holes IN the rear main cap itself, other than the 2 bolt holes. One for the oil drive shaft and the other is drilled on an angle for the passage to that cap.

If its the one under the rear main bearing that is IN the block, i've never seen a plug in any of the engines I've built. My current 350 has about 148,000 miles and still has 45+ pounds of pressure at hot cruise.

[Rogers Performance]

If you look at the top of the block in the rear you have a hole were you take your oil pressure from.If you stick a rod down the hole it will stop because it hits a core plug but not all these core plugs are 1/2 you need to measure the plug when you take it out if it is still in there.With the plug removed oil pressure drops and will not get filtered.Now if it is a bow tie block and is dry sump you leave this plug out because you block the hole off to the filter and use an in line filter or remote.



mike

[BanditT/A]

I am not a real chevy guy but I hope this helps!

http://www.carcraft.com/techarticles...me/index9.html

[camaroman7d]

Rogers Performance,
You are correct all small blocks don't use a 1/2" plug. The newer engines have a different size all the old engines (60's and 70's) that I am aware of have 1/2" plugs.

My argument is that you will lose oil pressure as well as have un-filtered oil.

This plug is in the block itself but under the rear main cap (I guess it is above if the engine is right side up). I can take a picture if that will help I have a bare block on the stand. The reason you may not be aware of it is, some/most shops don't remove it. so you have never had to replace one. When I tear down my engines I remove this plug.


Edit: I see someone already posted a picture (or a link to a picture).

Royce

[machine shop tom]

The plug in that gallery simply directs the oil to the filer. If the plug is not there, the oil goes directly to the mail galleries. The plug has NO effect on oil pressure.

[camaroman7d]

Thanks for the reply Tom, after a bit of research I think you are correct. It's confusing when so called "experts" say something different. One thing is for sure if you leave it out it is a BAD thing, there is no doubt about that.

Royce

[John Vetter]

The plug under the rear cap will not change the oil psi but if left out the oil will NOT get filtered.

[Rogers Performance]

yes tom is wright the oil pressure will not be effected.




mike

[kellyem]

I just put together a 283 and I cant remember for sure seeing a plug in the hole or not. Anyway, what does the hole get plugged with? Now I'm worried.

[machine shop tom]

It's usually plugged with a small cup plug. In your case a 1/2" plug. These plugs are usually left in if the block is wet-cleaned (hot-tank, etc.) but they are removed if the block is thermally cleaned (bake-oven, shot-blasted). The plug is recessed about 1.25" deep. It's in the same gallery that goes up to the sending unit hole on top, next to the distributor hole.

[GSXMEN]

Ahhh......the dreaded 'secret plug'!!

[lluciano77]

Originally posted by lluciano77
I have a Hot Rod magazine article called "50 Worst Engine Building Mistakes". It says that if that plug is left out, your oil will not pass through the filter.

Maybe it was Car Craft and it was the 10 worst. Whatever, you guys got the point.