WG750-E Question about O-Ring

[maizeblue28]

Hi, I have a Grasshopper 725 with a Kubota WG750-E-MORIDGE engine. When I purchased the mower used it was running great but smoking a ton. I replaced the rings and head gasket and everything was fine for about 4-5yrs. This spring I changed the oil, made about 2 passes on the lawn and felt a pressure release out of the engine and a cloud of white smoke. There was oil leaking badly in between the head and the block. I figured I had a blown head gasket so I tore it down and since it's metal I don't really see a major fail in it. The head and block seem straight. While reading some of the comments on here and other places they talk about an o-ring that is supposed to be on the nipple sticking up out of the block. Mine doesn't have one and nor did it when I took it apart last time. I plan to buy a gasket kit but the picture doesn't really show an o-ring that would go there so I don't think it comes with it. I have tried several dealers around me and none of them are very much help. Where is a good place to go online to see parts breakdowns with numbers on the internet? I can't seem to find a good place. At least none for my gas model. I'm having a hard time finding this o-ring and would love to be able to look at some parts diagrams and see it.