Expansion plugs — often called freeze plugs or core plugs — seal the openings left in the block and cylinder head from the casting process. They are inexpensive parts, but a poorly installed plug can leak coolant or work loose, putting a freshly rebuilt engine at risk. Many rebuilders replace all of them while the block is bare.
Common Plug Types
- Cup type: the most common style. The open side faces out and the plug is pressed in so its side wall seals against the bore.
- Dish (convex) type: a slightly domed disc that seats against a shoulder in the bore. It is expanded into place by flattening the dome.
- Threaded plugs: used in some applications and in oil galleries; sealed with a suitable thread sealant.
Plugs are available in steel and brass. Brass resists corrosion and is often preferred for engines where coolant maintenance may be poor.
Preparing the Bore
- Remove the old plug and any rust, scale or old sealant.
- Clean the bore so it is smooth and round; light corrosion can be removed with emery cloth.
- Inspect for cracks, pitting or a damaged lead-in chamfer.
- Confirm the replacement plug is the correct diameter and depth for the bore.
Installing Cup-Type Plugs
- Apply a thin coat of a sealant suitable for coolant contact to the bore or plug side wall.
- Use a driver that contacts the inside flange of the cup, not the center. Driving on the center can distort the plug.
- Drive the plug straight and square until it sits just below the chamfer, or to the depth specified.
Installing Dish-Type Plugs
- Coat the bore shoulder with sealant and place the plug with the dome facing out.
- Tap the center of the dome with a flat-faced tool until the plug flattens slightly and expands against the bore. Do not over-flatten it.
Final Checks
Let the sealant cure as directed, then pressure-test the cooling system or check for leaks at first warm-up. Plugs located behind the flywheel or under manifolds are difficult to reach later, so take extra care on those.
Evergreen Rebuild Kits
Depending on the part number, Evergreen engine rebuild kits and gasket sets may include expansion plugs. Check the kit contents in the Evergreen eCatalog. Browse Evergreen expansion plug kits.
