pops van wont start

Starters can develope a "dead spot" on the commutator...my 70 duster's did. It drove me crazy til I tried a spare and it cured the problem. It would start about half the time, the other half it wouldn't turn over at all. The funniest one was a ricer who detailed his engine compartment. He even painted the battery post and connectors. He couldn't get a thing out of it. I scraped off the neg post and terminal, car fired up. The car had sat for a week at the repair shop...go figure!