Rooting through the toolbox junk drawer for something else I came up on one of these the other day too..
It's similar to the lower pic of Joe's w/ a diamond shape on the head but in a deeper recessed circle..
I looked to this thread to compare but didn't repond when seeing Barry had noted he wanted a 'correct' one and the head on his pic shown is smooth also taller in the square part of the plug than this one has..
Aside from the diamond/circle part, this one's a 1" plug and looking into the bottom it's recessed smoothly and looks kinda' like the inside of a trim tag rivet..
It's about 1-1/16" total height, the thread part is about 5/8" high and the square part above the threads about 7/16" high...the square itself is 5/8"..
Happy to pop it in the mail to Barry if it's suitable...or if it's no good to him, someone else can have it after he responds?.
Thinking further, are waterpump bypass plugs the same size and an outie head too?.
~ Pete
