Change an Engine Core Plug


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An easy but important job : credit: © Alisdair Cusick
A leaky or corroded core plug risks sudden engine failure. Alisdair explains techniques for extracting and refitting these simple, but critical, parts

Need to know

Time: 1 hour
Difficulty: 1 out of 5 stars
Models: all Land Rovers.
Tools needed: Punch, hacksaw blade, hammer, pry bar, screwdriver.
Parts used: Core plug 587628, £1.09.
Work safely:
• Wear eye protection when working under a vehicle.
• Never hit an engine block, or risk damaging edges of the block.
• Always support a vehicle on stands, never a jack.
• Use the right tool for the right job.

Of the many complex mechanical components in every type of Land Rover engine, there’s one simple, cheap part that needs to be 100 per cent – the humble core plug. It’s also called a welch plug or freeze plug, and they are simply a plug that fits tightly into an aperture in the engine block.

Why does an engine block have holes? The apertures are made when the engine block is cast. The core plug seals those casting holes, but also serves the purpose of helping prevent engine damage if the coolant freezes. In principle, if the coolant froze, the expansion should force the plug out, saving the block cracking.

In practice, core plugs get ignored for years, slowly corroding internally. Many aged core plugs would therefore fail before the force was great enough to move them in the block. If you spot even the smallest weep from a core plug, then don’t mess about, change it. Likewise, if you ever remove the engine from the vehicle, consider changing core plugs then. You’ll never have better access for the job.

Changing them is simple, and the plugs themselves are very cheap. In theory, with just a few targeted taps to the one edge of the plug, it should rotate in the block, allowing access with grips to pull it out. That is fine on recently built engines. In practice, when a plug needs attention it will have been in there many, many years, and will be structurally weakened from corrosion. Removal then becomes a task of getting the middle out, then gaining access to the outer ring to weaken it enough to allow it to be removed.

We look at replacing two plugs on this Series One petrol engine, but the process applies to every internal combustion block. Each method demonstrates the issues that usually come up with plugs that are too corroded to simply tap around in the block. We start with the easiest scenario, with engine out of the vehicle. However, when the engine is in the vehicle, this relatively simple operation can take several hours if parts such as manifolds need to removed for access, which it’s why it’s worth replacing all of them when a very old, or otherwise suspect engine is removed.

Different approaches

There’s more than one way to remove a core plug, and the many locations on engine blocks to hide them mean one method isn’t always suitable for the job in hand. Here’s LRM’s grab bag of methods to help get that awkward plug out.
Punch and pull – Tap the edge of the core plug, causing it to rotate in the block aperture. As the opposite edge moves out, pull the plug out using grips.
Screw technique – Insert a screw in the plug centre and lever it out. Be careful not to damage a cylinder liner or the engine block behind the plug.
Fold over – Using a punch, fold the plug outer edge in, eventually loosening its grip on the engine block aperture. Take care not to damage the block.
Saw – For heavily corroded plugs where the centre tears out leaving just the rim, carefully saw into the plug rim until you can get to lever it inwards. Take great care not to damage the block.
Levering –If the centre tears out, work a pry bar inside the plug edge and lever it outwards. Again, take care not to damage the block.


Typical cases on a Series I

Pinhole leak: A typical failed plug showing a slow weep puddling down the engine block. This engine had been filled with water for an initial test run, before flushing later.

Anchor point: If the plug is solid, after draining the coolant, fit a self tapping screw in the plug then use a slide hammer or pry bar lever it out.

Use the force: The screw now provides a point on the core plug to lever against. Be careful not to damage the block if you use a pry bar, like here.

That’s torn it: Even with care, an aged plug is weakened by corrosion, and the screw can pull out when force is applied. Worth a try anyway, in some cases.

Lever it out: If the centre is corroded, you have little choice but lever it out. Carefully tap a pry bar in. Strive to minimise any flakes into the coolant chamber.

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Work it: Lever on the plug edge, against the block. Turn the pry bar, and repeat the action, in a different place. Don’t lever against the edge of the block itself.

Ta-Da: It will suddenly loosen, allowing it to be pulled out with grips. There’s little strength in this 60 year old plug, other than around the very outer edge.

Even better: Remove other dubious plugs. Best to flush out the block while they’re out. The engine will definitely thank you for this. Repeat until the water flows clear.

Be square: Clean the block aperture lightly (don’t enlarge it!). Fit the new core plug by tapping it in, ensuring it stays square to the block. Sealant is optional.

Final result: This is what you want, the edge of the plug just level with the block. Some prefer to leave it a touch proud to facilitate changing it in the future.

In the car: Another failure, with the telltale trickle of blue antifreeze. With a little care, you can change a core plug with the engine in the car. Here’s how...

What should happen: Using a punch and hammer, I give a few taps to the plug rim. The plug rotates in the block. A tug with grips now removes it easily.

Replaced: A gentle tickle with some emery paper on the block aperture, and a new plug taps straight in. I leave the plug barely just proud of the block.


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