2004 Fat Boy has a strange cap on the oil tank. Looks like a little window in it. Is it a sight gauge? thanks, gregjo1948

 

Could it be a temp gauge....?

There are two types:

One has a small window with a button. You push the button and the oil temp shows in the window.

https://www.harley-davidson.com/us/e...ut/p/62955-09A


The other is just a analog gauge with a watch type face, and a dial with temps along the outside edge. A pointer (like a minute hand on a clock) that points at the temp.

https://www.harley-davidson.com/us/e...e/p/62667-87TA

 

Picture says a 1000 words....