If you're going to use an impact, don't put a 1/2 inch drive heavy anvil gun on the plugs. I use a 3/8 impact on the mid setting. Alternating clockwise then counter clockwise, CW, CCW, CW, CCW. Very short bursts. Three days ago I was pulling the original 98k plugs out of the Shelby. First 4 came out perfect. My impact gave up on the 5th. So I got the breaker bar on it. Babied the thing. And broke it. Got out my battery opp impact for the sixth and seventh. Came out perfect. Battery was too drained for the last one. Instead of waiting for the battery to charge, I got the breaker bar back out and broke the last one. What a pain in los juevos.