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How to Replace Freeze Plugs in an OMC Cobra 4.3 Liter Engine

by Lisa Wampler

An OMC Cobra 4.3-liter marine engine has six freeze plugs -- three on each sides of the lower engine block. The freeze plugs originally used in the engine were steel. If the coolant in the engine ever freezes, the plugs pop out to relieve pressure on the engine block, keeping the block from cracking. However, the original steel plugs can also rust through and start to leak. Eventually you may need to replace them with brass plugs. These plugs will not rust and still function properly in the event of freezing.

Place the boat in an area where the engine will thaw out and the coolant will unfreeze. This may require removing the boat from the water and placing it in a heated warehouse or garage. If the freeze plug blew out because of rust, you can skip this step.

Drain any coolant left in the radiator. If the freeze plug completely blew out and all of the fluid drained, you can skip this step. However, take time to clean up the spilled coolant, as it is toxic. Clean it up with old rags and then properly dispose of the rags and coolant per your local disposal laws.

Locate the defective freeze plug. Depending on the year of your engine, the plugs are 1 5/8-inch diameter or 1 ¾-inch in diameter. They are located in a row on the bottom edge of the block on either side. Each plug has a raised lip. Inspect the lips. If you see any of the lip separated from the block, remove the plug. If the plugs have rust on them, remove them and replace them. To remove them, pull on the raised lip with a pair of pliers. The plugs are relatively thin metal and will pull out rather easily.

Wipe the surface of the block clean and then clean out the hole that you removed the plugs from. You will have a moderate amount of sludge left in the engine block. Clean as much out as you can. Make sure the hole and the area around the hole is clean before reinstalling new plugs.

Place the plug into the hole and tape the plug in place with a rubber mallet. Make sure the head of the rubber mallet is larger than the diameter of the plug. This will prevent damaging the plug when installing it. Continue to pound the plug in place until the raised lip seats onto the engine block. Repeat this with each plug you replace.

Fill the radiator with 50/50 mix of radiator fluid and distilled water.

Items you will need

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