Changing the mixing elbow on 3GM30f turned into a big thing. Could not unscrew it in place. Had to pull the exhaust from the engine, clamp the riser in a BIG vise and use a BIG pipe wrench with a cheater bar to turn it off. Most of the time involved was finding a large enough vise that could clamp the riser in place so we could get the leverage to unscrew it. Took a couple of hours to remove the exhaust, unscrew the mixer and reassemble.