Core plug

One of the purposes of core plugs is to serve as a cap at the end of these passages used to prevent water or a coolant mixture from leaking out of the engine.

[2] Core plugs are usually thin metal cups press fitted into the casting holes, but may be made of rubber or other materials.

[4] Although modern antifreeze chemicals do not evaporate and may be considered "permanent", anti-corrosion additives gradually deplete and must be replenished via a flush and refill.

This differs from other dish-shaped core plug designs, which form a seal when their tapered sides are pressed into the casting hole.

In order to get back on the road, one of the Welch brothers installed a press-fit quarter or half dollar coin into the hole using a hammer.