They'll get pretty hot, but if you've got a lot of oil/grease up on the block, better consider something else.
I use mine mostly for my diffs if it's extremely cold. They're also great for warming up water in a pot. <img src="/forums/images/graemlins/notooth.gif" alt="" />

Consider one for putting in a frost plug, or better yet, a heat pad for your oil pan.

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