By Andy Kennedy
When it comes to core plugs, there are various types and different opinions on how to properly install them. While different techniques for installing them can be successful, I would like to share the recommended ways to install the different types and preparation before doing so.
Cup Type Expansion Plugs
- When the recommended guidelines are followed, plugs can be installed without sealant and still retain the maximum sealing quality.
- If the holes are properly prepared, a sealant is not required.
- A lot of different shops will apply a small amount of lubricant to prevent galling during installation.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2026EPQ2/#p=52
Preparation of Holes
- The finish of the hole should be round and free of pitting.
- The hole should not be more than .002″ out of round for under 1 inch diameter plugs or more than .003″ for over 1 inch diameter plugs.
- The top of the bore for the plug should have a minimum radius of 1/4″ and a 30-degree chamfer is often used. As long as it doesn’t have a sharp edge, it will normally install just fine.
Installing the Plug
- The plugs should be driven in from the bottom and not the outer edge or rim of the cup.
- The installation driving tool should be .020-.025″ smaller than the inside diameter of the plug.
- The bottom edge of the installation driver should have a minimum 1/16 radius to prevent deforming the inside radius of the plug. It should also be flat to prevent the installation tool from contacting the crown of the plug.
- Plugs should be driven in square to the hole. The shoulder of the installation driver should be bottomed out on the casting to help with alignment and depth of the plug.
- The open end of the plug should be approximately .030″ below the point of radius of the hole.
- Plugs should not touch the bottom of a counterbored hole. This can deform the plug and cause leaks.
A good seal can be obtained by changing to these procedures when special circumstances require them. It is best however, to follow them as closely as you can.
Welch Type Expansion Plugs
- The plug should drop in the counter bore without force. The counter bore needs to be around .002″ larger than the outside diameter of the plug. If the plug is forced into the hole prior to expansion, leaks are likely to occur.
- The counter bore should be round and free of pitting and tooling marks if it has been cut for an oversized plug.
- The counter bore should be finished without a radius at the bottom. The radius can prevent proper seating of the plug. If the hole is counter bored for an oversize plug, the tool should dwell at the bottom long enough to remove chatter marks. The sharper the edge at the bottom of the bore the better the seal.
- The counter bore should be deeper than the thickness of the edge of the plug. The best condition is if the counter bore and plug thickness are as close to equal as possible.
- The angle between the sides and the bottom of the counter bore should be 95 to 97 degrees, allowing the plug to turn inside out slightly and spring back to an almost flat position providing maximum sealing.
Installing the Plug
- The best tool for setting them is a round piece of cold rolled steel 6 to 8″ long and a diameter slightly smaller than the diameter of the plug that is being installed. Place it over the plug. Strike it a couple of times with a hammer. It is best not to be completely flat after installation.
- If the plug is going to be installed without sealant, it is a good idea to lubricate the plug with oil or grease to reduce galling or scoring that could result in leaks. Sealant is used in more severe duty applications.
Pipe Plugs
When installing a pipe plug, the first thing you want to do is inspect the plug and the threaded hole for damage or cuts to the threads. Sometimes it is necessary to chase the threads in the block or head with the proper size tap. The tap should be run in until there are roughly six threads exposed on the top side of threads on the tap.
When installing the plug, ideally you want the plug to be as near flush with the top of the casting. Some applications this is not possible because if the plug is installed too deep it can block off a cross drill for an oil passage.
For the sealant on a pipe plug, there are several different options that will work. You can use Teflon tape, Teflon paste or RTV silicone, I have in cases, heard of using thread locker Loctite or equivalent.
A lot of engine builders and machinists have tapped smooth holes to accept a pipe plug rather than a cup style plug. This has always been a way to definitely make sure that the plug doesn’t come out and causes a loss of oil pressure. (Such as the front three plugs behind the cam gear on older small block Chevy engines.)
A drill/tap NPT size chart (left) and pipe plug torque chart (below) are shown for reference. If any shops out there have any tips or tricks for core plug installation, as always, we would like to hear them.
Read this article with all images in the digital issue of Engine Professional magazine https://engineprofessional.com/2026EPQ2/#p=52