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What is a core plug

A core plug is a thin piece of metal either in a cup or dish shape which is used to block off a hole or some form of opening on an engine block or cylinder head that was made in the manufacturing process. Most common types of openings can be coolant castings on the engine block or drilled oil galleries within the engine block or cylinder head.

Other common names for core plug are welch plug, freeze plug and expansion plug.

Core plugs are fitted to nearly all diesel and petrol automotive, marine and industrial engines.

Materials vary including mild and stainless steel, brass and aluminium.

Most common are mild steel core plugs which are usually finished with a zinc-nickel electroplating finish to form a corrosion resistance against fluids and high temperatures once fitted to the engine or cylinder head.

It is common practice when rebuilding or overhauling an engine block or cylinder head to remove all core plugs to clean the all the coolant and oil gallery areas thoroughly especially if the engine block or cylinder head have been soda or shot blasted. All the core plugs are replaced when it is being built back up.

Common Issues Failures Core & Freeze Plugs

The most common issue for core plug failure is an incorrect ratio or lack of antifreeze in the engine coolant. Either due to the vehicle being topped up with just water when it is showing low on the coolant tank or because the antifreeze is just old and needs replacing. It is vital the coolant is tested with an ethylene glycol tester at last once a year or even when the vehicle has regular service intervals.

Lack of antifreeze in the coolant will cause the core plugs to rust and rot out from the inside of the engine.

Another related issue to lack of antifreeze in the coolant more so in winter time is the coolant completely freezing, expanding and popping the core plugs. Most often than not causing water pump failure as well. In severe cases, it will ever crack the block or cylinder head.

How To Choose The Correct Core Plug

Here at Engine Core Plugs All core plugs are manufactured slightly oversize by 0.25mm – 0.40mm (0.010” -0.015”) which is the interference fit required to seal them snug in whatever opening they are being installed.

It is always best to have the original core plug to measure and compare when replacing. Although sometimes that is not always possible if the original has rusted away or been damaged beyond recognition. Digital micrometre is always preferred when measuring over a tape measure or rule where possible to get an accurate measurement.

If you cannot find the correct size for your application it is worth converting metric to imperial (or vice versa) to try and find the correct size. Please see the conversion chart on the website.

Generally vintage and classic automotive vehicles will use imperial size core plugs whereas more modern automotive vehicles will use metric core plugs.

Same goes for core plug type, vintage and classic automotive vehicles will use dish type core plugs most often than not on the engine block. Whereas more modern automotive vehicles will use cup core plugs throughout the engine block and cylinder head.

Should you require any help choosing your core plug please contact us.

 How To Remove a Core Plug

Tools required:

Safety goggles

Ball pein hammer

Punch or old flat slotted screwdriver

Power drill and drill bit

Small pry bar

Water pump or vice grip pliers

How to remove a core plug video


***In an ideal world the engine or cylinder head would be removed and on the bench ready for rebuild, with good access to the core plugs. If fitting with the engine or cylinder head in situ access might be blocked by any number of different items in the engine bay which could require removing to gain access to the core plug openings.***

The engine block or cylinder head should be drained of any remaining coolant before removing any core plugs

The easiest way to remove the original cup core plug is to use a punch or flat slotted screwdriver on one side so that the plug spins in the opening making it easier to get a small pry bar or screwdriver behind it.

Dish type core plugs can sometimes have a solid lip that they butt up against, in which case drilling or piercing a hole in the plug and then using a bar or screwdriver is the easiest way to pry the plug out.

How To Fit a Core Plug

Tools required:

Safety goggles

Ball pein hammer

Flat round Punch


Power drill with wire attachments or small wire brush

Old sockets – slightly smaller than cup core plug you are trying to fit

And of course correct size core plug or core plug kit you are installing

What is a core plug ?

***Again because of various items in the engine bay. It may not be easy to access the openings for the core plug to be installed***

As with most automotive repairs, the area you are working on (in this case the core plug opening).  this must be thoroughly dry of any coolant and cleaned of any rust, old paint or localised debris before installing. It is also worthwhile using some sort of sealant to provide a guaranteed seal once the new core plug is installed. High-quality sealants like Elring Dirko, Victor Reinz Reinzosil or Hylomar Blue are recommended but use only a small smear around the edge of the core plug.

A cup type core plug can be installed with an automotive socket slightly smaller than the plug its self. The core plug should be a snug fit, making sure it is square in the opening and not crooked. Once positioned the socket can be gently tapped moving the cup core plug into its final position.

Dish type core plugs should be installed with the concave side facing inwards and the convex (bump) facing outwards. A dished type core plug will need to be positioned correctly in the opening. Then given a sharp hit to the centre of the convex (bump) with a ball pein hammer or flat ended punch. The plug will then expands slightly sealing tight to the end of the opening. Sometimes the opening will have a lip. Which the core plug will sit against which makes it a lot easier to install.

It is also worth noting that some sealants take longer to cure than others. Check the curing times before refilling the coolant. Be mindful of any leaks before and during running the engine up to temperature.

Unfortunately, there is no off the shelf catalogue available for any engine core plugs sets/kits. The core plugs sets and kits listed on the web store are created from scratch. Using engine data or engines that have come through our workshop for repair.

Should you require a kit that is not listed. We would be more than happy to create a bespoke kit for your application.

Shop for Metric cup type plugs here.

Shop for Metric dish type plugs here.

Shop for Imperial cup type plugs here.

Shop for Imperial dish type plugs here.

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