Renewing a core plug
TIME: 30 minutes to change plug (up to 10 hours to reach it) TOOLS: Small blunt chisel or drift, hammer, emery tape SAFETY: Wear eye protection DIFFICULTY: (changing the plug only) COST: £1.20
BEFORE A new engine block is cast by pouring molten metal into a mould, the internal spaces, such as coolant passages, are formed by core material which is removed through holes after the casting has cooled and solidified. These holes are then sealed off with steel disc core plugs, which we see on our engine blocks and cylinder heads and are typically approximately 25 or 50 mm diameter. They are simply pushed into the machined hole, held by interference (or press) fit. Because they are sealing the coolant space, if the coolant freezes through lack of antifreeze they are likely to pop out or at least be dislodged, though this won’t necessarily save the engine from cracking. They also corrode from inside, especially if the rust-inhibiting antifreeze is not replaced when required by the service schedule, or if poor quality antifreeze is used. Either way, movement caused by freezing, or internal corrosion, results in coolant leakage which will worsen and the only answer is to fit a new core plug. Bodging the leak with chemical repair products invites unexpected and sudden large leaks in the future.
Replacing a core plug is easy and they cost only pence, but reaching it can need the engine out of the vehicle. The last one I replaced was on the side of my Series III
diesel and I was lucky to be able to access it by removing only the dipstick tube. Whenever an aged engine is removed, or parts such as the wing removed, it’s worth replacing all accessible core plugs with new. The pictures show the simple method of replacing a core plug; that is, after you’ve done the work of reaching it.
1 A few hammer blows using a blunt chisel on the flanged edge of the core plug loosens one side of it, taking care not to mark the machined hole in the engine block.
2 Eventually, the plug rotates in the hole sufficiently to grip it with pliers and pull the plug out as is shown here.
3 Here’s the old rusted plug (picture left) and the much thicker new replacement (on the right). These are readily available from suppliers for most engine positions.
4 To fit the new plug, it needs to be driven in squarely using a drift of smaller diameter, this socket extension is perfect for this one-inch plug.
5 Before tapping the plug in, clean the hole with emery. Slight marks only, made during removal, can be filled with chemical metal and smoothed without losing the circularity.
6 Put a thin smear of sealant around the edge of the plug before fitting. To avoid sealant entering the coolant space, don’t apply sealant to machined hole.