Was looking for a borescope and was wondering what size to fit inside a glow plug hole. Found one that's 5.8mm and will fit inside a 1/4 opening. Is that small enough?

I don't think so but it would be close just going off memory. If no one answers you for sure PM Fingers I bet you he would know the size of them.

It will get in the hole, but not all the way to the cylinder. The hole going into the cylinder is only about 5mm

Dam, thanks.