Although the one and two piece plugs are good solutions for low-pressure vessels, they cannot exceed pressures of 150 PSI. Looking for a way to accommodate higher pressure requirements, the mechanical tube plug was developed. Modeled off of the earlier ring and pin design, the mechanical plug took the hat off of the bushing, rotated it, and added serrations to the exterior. The tapered pin was installed the opposite way with a drill and tap location on the end of the plug. In order for this plug to be installed correctly, extra hardware was required. Operators would insert the plug and use something called a plug positioner to hold it in place. Next, a threaded rod would be inserted into the plug end and pulled back using a hydraulic ram. This action would pull the pin through the ring, causing the ring to swell until it set the seal of the plug.
While the mechanical plug was highly regarded upon its invention, some issues quickly became apparent. First, it was very difficult to install it properly. Manufacturing tolerances, materials, and operator knowledge can all impact the quality of the seal produced. If not installed properly, the plug can become dislodged from the tube, which can pose safety concerns. This is because the plug creates two seals; one between the ring and pin, the other between the pin and the tube. If both of those seals are not set correctly, then issues will occur. Additionally, the initial mechanical plug design experienced some issues with galling between the ring and pin. Galling is a form of wear that is caused when two surfaces slide together under a large amount of force, removing material from the two pieces. As a result, later versions of the plug have additional grooves on the pin to help control the force when installed inside the tube.