It sounds like the threads have been damaged, which my be a real head ache for you. Singer threading and screws are not something you can duplicate with any die tap I know of. I had to rethread a screw hole on a 319 once and it's not very fun. This is going to be something that you will need to remove periodically to clean so I really don't know what would be the best route. You may have to retap the screw and buy a new screw to fit the new threads you make. And it probably wouldn't match the other one in the plate, so that would be a bummer. But, do try other options before going this way.