How do you remove a broken aluminum oil bath cap on boat trailer?

To remove a broken aluminum oil bath cap on a boat trailer, first, ensure the area is safe and drain any remaining oil. Use penetrating oil to loosen any corrosion around the cap. If the cap is partially intact, try using pliers or a wrench to grip and twist it off. If it's completely broken, you may need to drill a small hole into the center and use a screw extractor to remove the broken piece.