20 means 20mm. That is the standard size for GTV's and early spiders. I think that 22mm was the standard in later years. I swapped out my 20 for a 22 on my Spider. It firmed up the pedal a bit. Andy Kress at Peformatek sells genuine ATE MC's in a variety of sizes for about $100. He can give you good advice on rebuilding/replacing.