idk about how good it is for penetrating a frozen engine, but vinegar works great at loosening up rust
8.75 chunk I got as a giant frozen rust ball and now I have it all rust free and painted. Process took about 4 weeks total. I let it sit for a week at a time then took it out and hosed it off...repeat. By the 2nd week the rust had loosened around all the fasteners and all the components came apart without too much trouble
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