DONT GRAB ONE END AND PULL thats a way that you have a huge chance of messing up the threads, especially b/c they are aluminum. What you need to do is take a look at this
this is the official tool to uninstall them, make one yourself, you place it squarely in, and lightly tap the top, while keeping downward pressure you simply unscrew the helicoil. make sure you are 90 degrees away from where the top of the helicoil is. While pulling it out may work, the risk of screwing up the threads is definitely there. Also if the hole is too messed up for a helicoil, you can look into the keen-serts or time-serts. These are basically steel threaded bushings, where the OD is threaded, and the ID is threaded to your spark plug size. you need to remove more material than a helicoil, or you can actually double up helicoils. good luck let us know how it turns out, o yeah, if at all possible make yourself a drill/tap guide to go in squarely and do it on a machine. even if you only have a drill press you can clamp the cylinder up and put the tap in the chuck, and grab the chuck by hand back n fourth to tap it. this allows you the feel but keeps you square. don't forget lots of cutting lube
