The 2.25" OD Velocity Stack is machined from 6061 aircraft quality aluminum. This is designed specifically for either of our plenum materials and our 2-1/4" OD 6061 Aluminum tubing. Designed like the rest of our velocity stacks in the fact that all you need is a 3" hole saw to install these. These are made to key to the bottom of the plenum material and allow for seamless joints both at the top of the stack and base where the runner begins.