The last one I did I found one of those little steel rods that go on the drum brake wheel cylinders laying around. Tapered on one end and just about the right size. After checking the fit, I put some red LocTite on it and beat it into the block with a hammer. With care, you could potentially crack the cast iron this way. After letting it set up I ground the protruding end down flush. After painting the block it's invisible and I have no doubt is absolutely permanent.
I used the 1/8" NPT tap method. The hole is actually a little large for a 1/8" tap, and the Allen plug will seat a little deep, but that won't hurt anything. Sealed the plug using Permatex #2. Locktite can loosen when heated, but Permatex is in there almost forever.
'97 TBird project. AED supercharger, forged bottom end. Runs high 11's, when it runs.
"Absolutely permanent" became unpermanent. I eneded up needing that 5.0 to go back into an EFI application. To remove the plug I used I pulled the pan (had to swap it anyway), heated the plug with a propane torch and knocked it out from the backside with a hammer and punch. Sure am glad I didn't tap the hole for threads or I don't know how I would have gotten the stock EFI style dipstick to seal correctly. Big gob of silicone or something similarly crappy.
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