The plug that is there from the factory (as your's looks to be), is a press in plug. No threads, just pushed into place. Poor choice on Toyota's part. That's why your's is leaking and why they are known to fail. As mentioned in the other thread that is linked to above, when they do it is a major problem. There is pressurinzed oil behind the plug. When the plug comes out or simple starts leaking badly enough (and it will eventually) you pump your oil right out the side of the engine. The "cold" side as it happens, so you don't get a lot of smoke and stink like you would if it was hitting the hot exhaust. (Of course you don't get an oil fed fire on the exhaust manifold either, so there is *some* good there...) The first thing most folks know about the failure is when the engine seizes from lack of lubrication.
FIX IT. FIX IT NOW.
IF Toyota sells a replacement press in plug, don't get it.
You can drill out the existing plug, tap the hole and use a threaded plug (there are no existing threads there... )
You can braze the hole up. Awkward to do with the head installed, but very doable if you are good with a torch and brazing rod. You colkd likewise solder it. Still awkward at best with the head mounted.
You can plug the hole with epoxy. You will want it very very clean. I would probably use a putty, thicker than the regular JB weld, so that it doesn't ooze out before curing. Although if you can keep it in place, JG weld works very well.
I have actually fixed this problem in the field by packing a press in zerk full of JB weld, pushing some of the JB in ahead of the zerk and then pressing the zerk into the hole. Drove it away after just a few minutes and it held for at least a year. Probably many years, I've just lost track of what ever happened to that particular head.
Mark...
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