How to Replace Core Plugs

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Core plugs are convex metal discs used to seal round holes that are found in many castings. They are incorporated in the castings as part of their manufacturing process and are intended to allow the sand, used as a “core”, to be removed from inside the casting.



core plugs 2Unfortunately with time they tend to corrode, usually from the inside, and then leak. They can also be forced out in frosty weather if you have not filled your cooling system with anti-freeze!

The Seven engine usually has 5 core plugs, 3 in the top face of the head and 2 larger ones one at each end of the block.



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The good news - Those in the head can be replaced in situ.

The bad news - Those in the block can’t!. To replace the one in the front you will need to remove the radiator and the dynamo and housing to gain access. To replace the rear one you really need to remove the engine since there is no clearance between the block and the bulkhead. However….. In the past when your car was less loved somebody may have cut, chewed or smashed a hole through the bulkhead to enable them to replace the plug without taking the engine out. Check to see whether your car has suffered the “treatment” !!

I would strongly advise that if you have your engine out for a rebuild replace the core plugs in the block as a matter of course whether or not they show signs of leakage.


Drain water from the radiator to below the level of the leaking plug. Remove the leaking plug.

There are a number of ways of doing this. The two most common ways are;-



core plugs 4Using a small, sharp cold chisel make a slot in the plug, insert the blade of a screwdriver in the slot 





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and lever the plug out. …….


Drill a 1/4” ish diameter hole in the plug and insert the tip of a pointed screwdriver into the hole and lever out.




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The plug sits on a locating ledge inside the hole.

The sides of the hole and the ledge need to be scraped clean (file tang/wire brush etc) to provide a clean surface for the new plug to seat on.  Make sure that corrosion has not enlarged the hole which could lead to the standard core plug being too small. If the core plug simply drops into place beware.

Whilst its not strictly necessary I usually apply a light coating of jointing compound around the inside of the hole before I place the new core plug in position.


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Place the new plug, DOME UPWARDS, into the hole and GENTLY tap the plug down so that it is seating squarely on the ledge.



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Select a suitable steel, aluminium or brass drift, of approximately the same diameter as the core plug.



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Position it squarely on top of plug and using a heavy hammer hit the top of the drift with a single firm blow. This will cause the plug to spread and grip the sides of the hole. Do not use a series of light blows or keep hitting it with a heavy hammer!  If you do your new plug will almost certainly leak or fall out!  You have been warned!







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All that remains is for you to refill your radiator, not forgetting anti-freeze, and you should now have a watertight engine.



This article, written by Andrew Jarmin, originally appeared in CA7C Seven Focus in Feb 2001 pp 20-23.