Removal of Cranks with Damaged Threads (square type only)
If the removal threads for the crank puller in the crank are damaged, there is a possible repair. Begin by inspecting the threads. If only the outer threads are bad, you may still be able to remove it using a normal tool. Make sure the threads of the remover are clean. Start the tool straight to the threads and carefully thread it in place. Tighten the tool into the crank threads with a wrench before attempting to pull the arm. Even if there are only a few threads left, it is worth trying to remove the arm with a crank remover.
Another repair option involves a tool from the Bicycle Research Company, the TC-8. This is basically a fluted bolt that cleans the threads. It comes with a pivoting stud to align the tool centered to the threads. This tool is not a tap, it only aligns damaged and cross-threaded threads. It will not cut new threads. Typically, it will not damage threads, so there is nothing to lose in trying it. The TC-8 will work only with square drive cranks with an 8mm threaded bolt.
If the threads are completely stripped, the arm is basically ruined. A simple removal method uses the concept of how arms are kept on the bike. Pressure from the crank bolt keeps it on, so you will need to remove that pressure. Loosen the bolt or nut three or four turns. Ride the bike hard, uphill. The crank hole is a tapered fit onto a tapered spindle. The flexing under stress will cause the arm to loosen, but it will also cause the crank hole to become enlarged, ruining the arm. Use care when riding; you don't want the arm to fall completely off when pedaling.
It is also possible to use a hacksaw to cut into the arm at the spindle joint. A cold chisel can then be used to split the arm. Again, the arm is basically ruined when the threads are completely stripped, so destructive removal should be considered as an option.