I have started the dimpling. Sasha and Peter (slingalongblog.sashapeter.com) were INCREDIBLY generous and lent me their DRDT-2 machine. This is both good news and bad news. The good news is that it is SO easy to do the dimpling. The bad news is that it is so easy to do the dimpling that if I am not lined up correctly, the machine will both dimple AND punch a hole. Lesson learned. I sent an e-mail to Sling to ask if this is repairable or if it needs to be replaced. They wrote back to say that it is just holding the skin to the rib, so it isn't as critical as some other ones. I can just re-dimple the rib (and eventually the skin) for a 4mm countersunk rivet and drill the hole bigger for it. Now I need to find some 4mm countersunk rivets... I got all of the ribs dimpled, though!