The world's best penetrating oil is Kroil, made by Kano Laboratories in Nashville, TN.

www.kanolabs.com

Get a can or two of this stuff, drain the oil out of the crankcase, and pour 3 or 4 ounces into each cylinder. Replace the spark plugs, and wait about a week. Pull the oil pan drain, and you can get almost all the Kroil out of the pan. It penetrtates the rust, and drains right past the pistons. Pour the Kroil back into the cylinders, and let it drain thru again. After a couple of these sequences, the engine should turn freely, or at least be able to be rocked free with a breaker bar or by putting the transmission in high gear and rolling the truck back and forth.
Jerry

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