How to Install a Transmission Drain Plug

What You'll Need
Thread Lock
Bench Vice
1/2 Inch NPT Bit
1/2 Inch NPT Tap
Tap Handle
Cutting Oil
1/2 Inch NPT Drain Plug

Installing a transmission drain plug is one of the projects necessary when building a specialty vehicle, or using rebuilt parts for vehicle restorations. Any rebuilding job can be very challenging, because you must install, or fabricate, many parts yourself. This goes for a simple thing like the transmission drain plug.

Creating a transmission drain plug is something that requires a lot of special attention to detail, tools and precision. While this can be done in a backyard garage shop, you will want to make sure you know exactly what you are doing first.

Step 1: Position Transmission in Vice

In order to give yourself the best opportunity to install the transmission drain plug, with the least amount of problems, set the transmission onto a stable workbench. You should use a vice to hold the part in place, but if one is not available, make sure it is held in position by clamps or bench dogs.

Turn the transmission, so that the area where the drain plug is going to be located is facing you. This is generally in the rear of the transmission housing.

Step 2: Drill with 1/2 NPT Bit

Set a 1/2 inch NPT drill bit into your drill chuck. Tighten it down, and place the tip onto the area where the transmission drain plug will be installed. Begin drilling very slowly, for the NPT drill bit to make a mark into the metal of the transmission skin. Once the mark has been made, and the drill bit is moving into the metal, you can drill a little faster. Make sure to use some cutting oil occasionally, to cool down the bit. Clean away shavings as you drill.

Step 3: Drill Carefully

Once the drill bit pushes through the metal casing, stop the drilling. As the hole to the transmission is opened up, you must drill very slowly and with sharp drill bits. This will make sure you cut large metal ribbons rather than small splinters. Keep track of every piece of metal shard, so that it does not fall into the transmission housing.

Step 4: Tap Hole for Threads

Once the hole is completed, use a 1/2 inch NPT tap to create the threads for the drain plug. Lubricate the tap with the cutting oil, and insert it onto the hole. Keep the tap straight as you cut into the metal. Once you turn the tap a few time clockwise, back it off, to remove the metal debris. Repeat this process, until the hole is completed.

Step 5: Install Transmission Drain Plug

Clean away any of the metal shavings from the drilling or the tapping process. Make sure that everything is absolutely clean before applying the drain plug. Apply some thread lock to the threads of the NPT drain plug, and start it into the tapped hole. Continue to thread the plug into the hole, until the loosening adapter is flush with the outside of the transmission.