I was upgrading the 3/8-inch Rawl five-piece bolts on a route, hoping to replace them with half-inch, but one of the bolts just spins and won't unscrew.
Holy botch jobs, a spinner! A Rawl five-piece (now known as the Power-Bolt by Powers Fasteners) will spin because the expansion cone in the back of the hole is turning with the bolt shaft, instead of getting pinned in place as it's supposed to be. Dust in the hole acting as a lubricant is a common cause, and so are under- or over-sizing the bolt hole. When you drill, blow out the hole, use a fresh bit, and hold the drill straight. Note that even brand-new bits, due to manufacturing inconsistencies, can bore holes that are too small, especially in hard rock. If the hole is too small you really have to beat the bolt into the hole, possibly damaging the expansion cone. When you place the bolt, tap it straight in. If you let the bolt angle in even a little at the start, you can bugger up the cone, also causing a spinner. Placing a bolt is simple, but you do need to be a craftsman to do it right. Pretend you are tuning y our pacemaker.
Whoever placed your spinner was a bolting novice, dolt, homicidal maniac or all three. Maybe it was you. To clean up the mess, pry out on the bolt while simultaneously unscrewing it. That's right, you'll need to be as ambidextrous as Paris Hilton in a bedroom video (yes!). A big crowbar under the bolt hanger usually works, but you might succeed simply by clipping the hanger to your harness belay loop with a draw, and pushing out with your legs as if you're passing a three-pound brick of extra- sharp Wisconsin cheddar. If you are lucky, the cone will come out with the guts of the bolt. Usually, the cone stays in the back of the hole and you have to either drill it out with a sharp metal-cutting bit, or patch the hole and drill a brand new one (now is your chance to finally put the bolt in the right place!) A mix of Bondo and bits of local rock worked up into a gruel works well as a patch.
If the spinner won't succumb to prying and screwing, you are going to have to go ape shit. Get a sharp cold chisel and cut the head off of the bolt by beating on it. Count on breaking your hand and the rock. When you do finally cut the head off of the bolt, the hole will be a mess and the bolt shaft will be sheared off inside the hole. Punch the steel shard deep into the rock, and patch over the wound. Another nearly as undesirable option is to drill a hole adjacent to the bolt, widen its hole, freeing the bolt. Patch with Bondo. Now, get it right next time! Gear