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The Hi-Po main cap is a stout piece and easy to spot. The main cap is a full 15/16-inch thick and does not taper at the top like the standard 289/302 main cap. Hi-Po caps will generally have a C3 casting on the number 5 cap. (Look-alike Mexican 302 caps will have casting date codes.) If you can find the caps, all you need to do to add these to your 289 build is to have the mains line-bored.