The domed-disc plug fits against a shoulder. The cup-type plug is driven in until only a little of the lip protrudes.
Typical plug sites are in the side of the block under a manifold; at the back, inside or above the clutch housing – awkwardly near the bulkhead; and in the top of the cylinder head, often hidden under an overhead camshaft.
Plugs can blow out completely, resulting in a sudden loss of coolant. They can also leak slowly through rust holes or around the edge. A slow leak usually leaves a stain down the block. Look all over the engine for such a mark.
Some of the plugs are difficult to find – use a mirror to look in hidden spots – such as the back of the block, next to the bulkhead. An engine may also use more than one size of plug, so measure the hole when buying a replacement.
Access is the next problem. You may have to disconnect the carburettor.
Plugs facing the bulkhead may be unreachable without lifting out the engine, so you may have to have the job done at a garage.
Do not cut a hole in the bulkhead. It is load-bearing and, unless you weld a plate back over the hole, weakening it might invalidate your insurance. Riveting the plate on will not suffice.
Disconnect the battery and drain the engine block, saving the coolant if you want to reuse it (See How to flush an engine radiator).
Catch the coolant in a clean container, and filter it through muslin before pouring it back. Check the strength of the solution (See Checking and topping up car antifreeze coolant).
There are two common types of core plug, the cap type and the domed-disc type, which are inserted in different ways.
Removing a core plug