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You might consider heating the plug with propane torch after you drill it out just prior to using easy out (to use expansion to break it loose). I have broken those off in the hole when the attempt was made which made thigs worse. If its tight enough to round off a large allen hole, its pretty tough!
I did heat it but not until after I broke the ez out off in the plug. Ended up drilling tiny holes around the ez out end, punching it into the oil passage, and using big chisel and hammer to unscrew the fitting. Used a shotgun cleaning rod to push the ez out end out the other end. Buggered threads in block a little. Ran a pipe tap to clean up threads. Think it will seal. Hope so anyway.
Here is a picture of my new friend. Leave on at least two hours, even overnight. Grease is gone. Paint too. Much better than the low VOC degreasers available here in CA.
So what exactly do you find when you come back to those parts? Does this lift and make a crust, or turn everything to a slime? It must be water soluble since it is used in kitchens. I used the last of my old self-emulsifying engine cleaner on Red's motor, maybe this is worth a shot.
It rinses off easily even after leaving on overnight. It has a lot of ammonia so you need to use outdoors. I had to do some of the really nasty parts twice but much better than the parts house degreasers I have tried.