I have a smithy drill mill and it has an oil sight glass on the head that leaks, I was wondering has anyone ever replaced one and are they hard to replace ? It is a round sight glass that you keep the oil up halfway on the glass, it isn't broken just leaking might just need an o'ring. Any information would be appreciated, like do they screw in or are they just pressed in. The mill is a stand alone mill not the combo mill lathe.