How to Inspect and Replace a Boat Engine Freeze Plug

Freeze plugs — also called core plugs or expansion plugs — are designed to seal the casting holes left during engine manufacturing. In theory, they can pop out if water inside the block freezes, giving pressure somewhere to go. But in practice, especially on boat engines, they’re more likely to rust through from the inside out. That’s when problems start.

Where Freeze Plugs Are Located

Most freeze plugs are located on the sides of the engine block, under the exhaust manifolds. Some engines also have them on the rear or under the flywheel housing. On V-style engines, you’ll find at least two per side. You may also find them at the front of the heads or behind timing covers — depending on the make and model (Mercruiser, Volvo Penta, Crusader, etc.).

Signs a Freeze Plug Is Leaking or Failing

• Coolant or raw water dripping into the bilge with no obvious hose leak
• Rust trails or stains on the side of the block below the manifolds
• Salt crust or corrosion buildup around a freeze plug
• Water accumulating only during engine operation
• Steam, hot smell, or pressure loss in closed cooling systems

How to Inspect Freeze Plugs

1. Let the engine cool completely before inspecting.
2. Use a flashlight to check below the exhaust manifolds for any signs of rust or corrosion.
3. If access is blocked, use a mirror or remove manifolds for a better look.
4. Tap each plug lightly with a screwdriver handle — a dull sound can indicate rust underneath.
5. If the plug looks swollen, crusty, or pitted, it’s time to replace it — even if it hasn’t started leaking yet.

How to Remove a Freeze Plug

1. Drain the cooling system (raw or closed) to avoid spillage.
2. Use a hammer and punch to tap one side of the plug inward — it will usually rotate in the hole.
3. Grab the edge with pliers and twist or pry it out carefully.
4. Clean the hole thoroughly with sandpaper or a wire brush — the surface must be clean and smooth.

Installing a New Freeze Plug

• Use brass plugs — they last far longer than steel in marine environments.
• Coat the edge with marine-grade sealant or Permatex for extra insurance.
• Drive the new plug in evenly using a socket or freeze plug installation tool that matches the plug’s diameter.
• It should seat flush with the casting — not sunken, not protruding.
• Allow sealant to cure if specified, then pressure test the system before running under load.

Pro Tip: Consider Freeze Plug Block Heaters

If you run in cold climates, some freeze plugs can be replaced with electric block heaters. These help prevent freezing, improve cold starts, and reduce condensation buildup. But make sure to install them on properly prepared surfaces and confirm they’re rated for marine use.

Conclusion

Freeze plugs don’t fail often — but when they do, they can quietly flood your bilge, dump your coolant, or leave you overheating miles from shore. Inspect them once a season, especially if your engine is raw water cooled or stored outside in freezing temps. Replace any plug that looks crusty, corroded, or suspicious. It’s cheap insurance compared to the cost of an overheated or frozen block.