When you band commutator extensions with skill and a few tricks of the trade, all seams are sealed and carbon won't infiltrate. Preventing carbon from entering behind the copper bars, or between the mica v-ring and the steel can prevent problems with arcing bar-to-bar, as well as bar-to-ground. The pressure used, technique applied, and finishing touches make a big difference. A feeler gauge should not be able to enter a seam. If it can, your comm is at risk. Contact us if we can help walk you through it the next time you have to replace a band.