Seats get wider with wear. By lapping valves on a used engine, you only make the seats wider. I use three different stones on the seats, but possibly the things that Hotrod cited on eBay may work. But whatever you do, don't reassemble the engine before you get the correct width on your valve seats. I am in Germany at the moment so I don't have access to all of my books, but I think if you would buy about a 70 year old Motor's Manual off of eBay it would explain a lot of things. They can be bought for about $20 including shipping. They were written by mechanics out in the field compared to service manuals that were written by engineers at the factory.