Freeze Plug


Old core and oil gallery plugs are normally removed and replaced during cylinder block reconditioning. When installing new core plugs, make sure they are the correct size and type. The plugs' bore should be inspected for any damage that would interfere with the proper sealing. If the bore is damaged, it should be bored out for an oversized plug. Oversize (OS) plugs are identified by the OS stamped on the plug. Coat the plug or bore lightly with a nonhardening oil-resistant (oil gallery) or water-resistant (cooling jacket) sealer.

The three basic types of core plugs are installed as follows:

Disc or Dish Type - This type fits in a recess in the engine casting with the dished side facing out (Figure A above). With a hammer, hit the disc in the center of the crown and drive the plug into the bore until just the crown becomes flat. In this way the plug will expand properly and give a good tight fit.

Cup-Type - This type of core plugs is the most common type. This type of plug is installed with the flanged edge outward (Figure B above).

The flange on cup-type plugs flares outward with the largest diameter at the outer (sealing) edge. The flanged (trailing) edge must be below the chamfered edge of the bore to effectively seal the plugged bore.

To remove a core plug, use a blunt punch to knock the core plug sideways.

To replace a core plug, pound it sideways with a blunt drift punch.

Remove the plug with pliers.

A sharp punch will penetrate the core plug. The objective is to turn the plug sideways inside of its bore so that it can be easily removed. When it is sideways, it can be pulled out with pliers. Try not to let it go into the water jacket.

Do not leave an old core plug inside of the block. This is an unprofessional practice that can result in further cooling system damage.

Sometimes a slide hammer with a hook can also be used to remove a core plug. Sometimes the plug is so rusty that removal is difficult.

Expansion Type - This type of plug is installed with the flanged edge inward (Figure C above). The maximum diameter of this plug is located at the base of the flange with the flange flaring inward. When installed, the trailing (maximum) diameter must be below the chamfered edge of the bore to effectively seal.

Core Plug Installation:

  • Before installing a core plug, clean the opening in the block with emery cloth.
  • Put some sealer on the sides of a new plug and also on the side that will be exposed to the coolant.
  • Pound the core plug in with a driver or a socket that fits loosely into the inside diameter of the plug.
  • Check to see that the driver contacts the core plug on its inside surface (not on its outer sealing edge).
  • Be sure the core plug goes straight into the hole. A core plug seals on its outer lip.
  • It is correctly installed when the lip is against the bore. The picture below shows one manufacturer's recommendation.
  • Pound it in until the outside sealing edge is just below the chamfer that is on the outside of the core plug bore.
Installing a core plug. Install the core plug until its all of the way into the hole. Courtesy of Prestone Products Corporation.
Some people prefer to replace steel core plugs with brass ones that will not corrode.