Core Plugs, Welch Plugs, Freeze Plugs, etc

By Norman Nock

Originally published in the Austin Healey Magazine, Vol. 22-2


Engine blocks are cast in sand molds and have holes designed into them to remove the sand after the casting has cooled.

Later plugs are fitted into these holes and coolant is circulated in the watertight space. Some people have the misconception that these plugs have been put there to pop out if the coolant in the engine block happens to freeze. That is why some people call them freeze plugs.

These core plugs rust from the inside and need to be checked.


To check the condition of your core plugs, use a sharp spike and try to push it into different places on the plug. If any of the plugs feel soft, you should replace them all before they leak coolant and cause the engine to overheat.

A badly overheated engine will damage pistons, crankshaft bearings, and block or the head could warp.


Welch/core plugs on Healeys are located behind the exhaust manifold with two behind the block and one behind the head (not on 100-4). Removal of the plugs behind the manifold is easy after the manifolds have been removed. But the two or three plugs behind the engine are a little more awkward. Remove the transmission tunnel and on early models a vertical cover can also be removed. This will give you access to the other plugs behind the engine. (figure 1)


When replacing the plugs at the rear of the head, rear of the block, and the one behind the manifold at the back, there is not enough room to install the correct plug. So we suggest installing an expansion type plug. If you have the 100-4 or the 100-6, you would use a neoprene expansion plug to replace the cup or expandable type that was fitted to these blocks. If you have a 3000, they use the cup type of plug so you would need to use the copper expansion plug. These items are available at your local auto parts store made by Dorman Products. (figure 2)


To identify which engine is installed in your Healey find the engine numbers plate on your engine. (Many engines have been switched and you may not have a 3000 engine just because you have a 3000 body.) The 3000 cc engine has the engine number below the first core plug from the front of the engine. The 100-6 has the engine number below and between the first two plugs. The sizes of plugs fitted to big Healeys are 1¼”, 1 5/16”, 1 5/8” and 1 ¾”.


We recommend that you find the sizes needed for your engine and carry a set at all times.

These plugs have been known to pop out. BANG! without any warning.


Not what you were looking for? Don't forget you can check our back issues using the AHCUSA Magazine Index.

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