I have a substantial leak that appears to come from a plug on the rear of the left head surface partially covered by the head. The plug is present. Frustrating since everything was clean and dry. There is definitely no oil around the valve cover gasket and no oil is coming from the pressure gauge fitting. Does this plug see full oil pressure? I assume it's an oil gallery plug.

Yes, replacement is in the future. Sorry to read of the bad luck.

Dan,
Is it a threaded plug, or not. Some of the early blocks use an unthreaded 1/2" plug and they can be hard to get to seal. We usually just tap them and use a threded plug. The only other possibility, and I have seen some guys fooled by this, is the rear of the intake leaking and the clean oil runs down the back of the head to the same place. Kind of hard to see it without putting some die in the oil.
By the way, I know of one customer that fixed one of those plugs without pulling the head. He actually took a die grinder and ground away enough of the head to get an allen wrench in there and tighten/reseal the plug. Dont know how bad it looked when it was done though :(

I've also heard of members fixing this by cleaning it real good, then packing the area with j-b weld. Not a great method, but it sealed the hole without pulling the head, and the stuff will crack when it comes time to pull the head again.

I've done 3 repairs on what you have there. The fix itself is fairly easy, the hardest part is removing that original plug. If you can get it out without any drilling you'll be "home-free". I'd have to assume someone recently replaced it. If it was replaced with cup type core plug, simply drive a sharp punch right through it and work it out with screwdriver. If it's the original style, steel slug, then it becomes a little more involved. It can be done without drilling through that plug, but very carefully. I drill a small (.250") hole in the center
and try not to "break through". Get a good 5/16 x 18 tap (you may need a "bottoming" tap to get enough threads to put a 5/16 bolt in the hole. Use a long bolt and hook a set of vise grips on the bolt and carefully knock the plug out. Don't forget, when I do this I don't have to worry about chips going into the oil passage, I do the repair while the block is empty. You're in a tough spot. Now as for the fix, have any one with decent lathe make a new plug about .002" larger than the original hole (it'll take an accurate measurment)and equal to the original length (you can make it slightly longer). I don't have any dimensions recorded, but I seem to remember that plug being slightly larger than a 1/2". When you "bang" the new one in place make sure you use "Loctite". As I said, I've done 3 and no leaks. Trying to drill that to the next size (pipe) plug, 3/8", is not recommended. Thanks, Gary in N.Y.
P.S. I've got to add this here, PRIME the short block with the lifters in place BEFORE you put the heads on, and re-prime the long block with all valves set, definitely before you install the engine in the car!

Thank you for help. The original plug was replaced with a 3/8" pipe plug with an allen socket. The top of the plug is about 1/8 below the deck surface. I don't think it's an intake leak since I lost 1/3 to 1/2 a quart during the 20 minute break in from this spot. The leak increased with rpm. Wouldn't a leak in the manifold see vacuum?

When I primed the oil pump it didn't leak from this area, at least not enough for me to notice.

The machine shop is talking about removing the plug and drilling deeper. I suppose so the plug can engage more threads. Wouldn't the block have to be fully disassembled to prevent chips and junk from getting the oil passages?

I'm hoping to try and just pull the head, remove the plug, reseal and tighten. Then prime it again to see if there is a leak. How does this sound?

Way to go. Try an aluminum plug with some Permatex #1 (hard-setting). The aluminum plug will conform better to the threads they tapped in there. It sounds like they didn't get a good base for the pipe threads. Moroso has an assortment of alloy plugs. If it's going to leak it'll show up when you prime it again. Thanks, Gary in N.Y.

After you prep the hole to replug, prime the engine again and it will push out anything that falls down inside the passage.

Got the head off. Looks like a 1/4 NPT plug instead of the 3/8 I thought (.54" od). The plug was easy to remove with a 1/4" allen wrench; not especially tight. Should this thing have been a lot harder to take out? Plug threads had dark gray thick paste-like stuff in them.

The bore is about 1/2" until it steps down to about 7/16" approx 1/2" down. There were about 3 to 4 threads tapped into the 7/16 section.

What do you recommend cleaning the oil from the threads with? Brake cleaner?

Plug should have been harder to remove.
BrakLeen would be good to use to clean threads, it will evaporate later.

You might try to tap another thread into the hole with a greased up tap to catch the chips. Lots of Teflon tape on the plug would help seal the plug.

Darn, after reassembling still leaks, but not as much.

Tapped threads deeper, used new plug, coated threads with Permatex #1, and tightened well. Waited overnite to cure. Didn't leak during priming. Guess I didn't make enough pressure to leak during priming. :confused:

Hey Dan, don't give up, try that alloy plug I mentioned, it'll stop it along with the permatex. The steel plug may just be too hard. Thanks, Gary in N.Y.

Not giving up, just back giving out...from leaning over the engine. At least I'm getting fast at disassembling.

I've got the same problem. I'll be pulling the motor of the winter to take care of it so keep us posted on how you make out please

The leak last time was 1/10 as bad, so at least there was an improvement. This weekend pulled the head again. This time I tapped even deeper; cleaned hole well; put new plug in with permatex thread sealer as tight as possible. Top of plug now below surface about 1/2". Filled remaining space with JB weld. Will start up soon to see the results.

Dan,
Are you concerned with the shavings from tapping the hole making their way into the oil? I'll be doing the same if it works out for you.
-Greg

Greg,

Yes. But I did what I could. Packed hole with grease and put grease on the tap to catch chips. Afterward primed oil pump to pump out whatever else had fallen down in the hole (hopefully). I'll change to oil too. Unless anyone has a better idea. Only alternative would be to tear down motor I would think.

I'm not exactly sure where that hole goes to, but it must be just above the oil pump (rear cam bearing?)

Filled remaining space with JB weld.
:thumbsup:

OK,

That seems to have done it.

Now if I can get rid of that missing that's hard to track down...
Maybe it's because I last filled the tank in April.

Old cars are fun ;)