Hey all you Pinsiders! First of all, thank you to anyone who can help me with this, because I feel that this is a dumb question, but I'm totally lost.

My Gottlieb Hi Dolly ball count unit is not telling the machine properly what ball is being played, and often isn't displaying a ball at all. Well, I opened up the back, and would you know - I look at the swiper that rests on top of the unit and it's held in position by two set screws, just like the flippers on my Bally. But... One of the set screws must have been loose and flew right out of its place! Well.... That explains it.

However, I have absolutely no idea how to measure or tell what size set screw I need. I know it's the same size Allan wrench as my Bally flippers - 1/8. But set screws are measured by thread size, right? How do I know what set screw I need?

The correct size was 1/4-20 and they did in fact have them at the local Lowe's where the guy was able to measure it for me.