Replacing O Ring on Timing Gear Case Cover

[Greg19]

My Timing Gear Case Cover is leaking oil near a bolt directly below where you fill the engine with oil on my 853. From looking at the Service Manual it looks like there is one long O Ring (#20) that goes in a groove on the backside. When replacing the O Ring do I also have to replace the crankshaft front oil seal (#19) that is in the cover if it is not leaking? Should I replace it since I have the cover off? Do I also have to replace the Gaskets #21 and #18. Not sure what those are for? How hard is this to do with the engine in the machine?

Thank you, Greg

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[flyerdan]

Chances are that the paper gaskets (21) have been glued on with gasket sealer and will come off in pieces, so they will need to be replaced as a matter of course. For as little as it costs, replace the front seal; check the wear area on the crankshaft for scoring or anything that might eat the seal prematurely. When installing the new seal, fill the recess between the lips with grease so it will go on easier and won't wear immediately on a dry shaft.
Gasket 18 looks like a split main seal; if it has hardened and taken a set you probably won't get it back in the exact spot and will have seepage. If it's still pliable the odds of success are better.
It's highly likely that there is a gasket kit with all these components that would probably be around the same price as a couple of individual pieces.