UPDATE: I wound up using the slightly oversized core plugs when I had to replace the bearings. The solution was to simply grind the sides of the plug down very slightly by turning it on a bench grinder. This removed just enough material to allow the plug to perfectly fit in the hole in the head, eliminating the need for the temporary rubber expanding plug with a permanent solution.
My 3.0L engine had a bad core plug on the back of the right hand head that needed to be replaced. I found out about it from the label attached to the engine when it arrived, and the blue circle marking the plug in question. This wasn't mentioned when I bought it, but some searches revealed that these are simple enough to replace that I expect the seller didn't think it was worth mentioning.
What are Core Plugs?
The first thing I had to figure out is what a core plug was.
The casting process involves filling the metal part with sand for reasons I'm not clear on, since I don't know much about casting. Afterward, the sand is drained through holes in the part. These holes are sealed with press-fit steel plugs known as core plugs.
These are sometimes also called freeze plugs, as coolant runs through this part of the cylinder head. Legend has it that if the coolant freezes, the plugs will pop out to keep the head from cracking. In reality, the plugs are so tightly seated that usually they stay right where they are and the head cracks instead.
Core plugs are usually made of steel, but you can get aftermarket brass plugs that won't rust. The trick is getting plugs that are just the right size; too small and they'll be loose in the head and will fall out, and too large and they won't fit in the hole. This means that the plugs are made with sub-millimeter levels of precision. There are also variable-size rubber expansion plugs in case you can't find an exact match.
I first tried to get some pliers in there to pull it out, grabbing the gash with the tips. That didn't work; there wasn't enough material exposed to get a grip on anything.
I did some quick tooling and decided to first try to break the plug free by hitting its edge at an angle with a punch and a hammer, with the intention of turning the plug in its hole so that it could be twisted out like a screw or a bottle cap. This had no effect; the plug refused to move.
Next I tried using a screwdriver and (and later, a small pry bar) to lever it off, slipping the end through the rusted-through hole already present in the plug. Again, no effect; all I was doing was bending the screwdriver, and the plug remained firm.
Finally, I followed instructions from another video and tried to tilt the plug out. This technique involves placing a punch on the face of the plug near one edge and hitting it with a hammer, pushing that point inward while tilting the opposite point outward. I had to switch from the simpler claw hammer I was using to a heavier four pound hammer to get enough force to break it free, but it worked -- the plug slowly started to tip out. Once it was at about a 60 degree angle I was able to grab it with pliers and pull it out.
This plug was very near to where a sensor is mounted on the head. I should have removed the sensor first; there's a slightly ding in the end of it, but I don't expect it affects its operation in any way.
I also noticed that there was a metal ridge that I believe the plug sat against. Removing the plug in the manner I did broke the top of that ridge. I don't think this is a huge deal, but I was careful to remove all of the bits of aluminum that had fallen into the head.
The visible mix of oil and coolant in here was due to flipping the engine over. There was still some oil and coolant in the block, and it mixed together and apparently ran into the coolant passages. I'll have to flush those out at some point, once I figure out how exactly I'm going to do that.
There's also a bit of visible corrosion on the insides of the coolant passages. I'm hoping the planned flush will clear out some of that as well.
Finding a Replacement
The next trick was finding a plug to replace this one. None of the three auto parts stores near me had one that was the same size. Advanced Auto came closest, with a 1 3/8" plug (35mm); at a glance it looked the same, but it was too small when I placed it inside the hole -- It dropped right in against the remains of the ridge, and I could easily move it around with my finger.
I finally got out my trusty calipers and measured it. The old plug was 35.75mm; that three quarters of a millimeter was enough to keep the new plug from fitting.
I went back to Advanced Auto and tried the next size up, 36mm, but it actually measured 36.5mm. This was too large to fit in the hole.
NAPA had an expanding core plug available. This uses a rubber seal that is expended by cranking down on the nut on the end. While it works, some googling implied that it's not really an ideal long-term solution, but others reported great success with it over many years. Still, I wanted to try to find a steel or brass plugs if possible.
RockAuto has parts specifically for the Eagle Premier 3.0L engine, so I tried ordering a ten pack of brass core plugs from them. Unfortunately, these were half a millimeter too small to fit in the hole.
I also considered grinding the edges of the slightly-too-large core plug until it fit, but I was concerned that if I didn't do it uniformly around the entire circumference that I'd have leaks from the now-oblong-shaped plug. I'd really need a lathe to do it right, and I don't have one of those.
Installing the Rubber Expansion Core Plug
After failing to find a viable metal core plug of the correct size, I resorted to a rubber expansion plug. The plug fits loosely in the hole at first, but after tightening the nut on the end compresses the rubber piece, causing it to bulge outward and firmly locking it into place int he head.
I applied some anti-seize to the threads per some suggestions I fond online, just in case I needed to remove it in the future. I tightened it as far as I thought I could get away with, and then tried to pull it out with my fingers -- it wouldn't budge. I have yet to see how this will last once the engine is warmed up and pressurized, but I'm expecting it will hold pretty well.
I'll still be on the lookout for properly-sized metal plugs until the engine goes back in the car. I also may try to find a slightly smaller rubber plug that can slip further into the block; this one stops at the ridge inside the head that the metal plug would normally rest against, which is why it sticks out of the head so far. I figure that a rubber plug that hugs the ridge would probably provide a better seal and less likely to pop out under pressure, but in all likelihood this larger plug will be fine.