my XT350 has blown its bottom gasket which is strange as normally the head gasket goes.

I wanted to know what could have caused this (i did over fill the oil but only ran it for an hour till i realised)but did notice the leaks soon afterwards however they could have been there before.

when opening it up to replace the gaskets i don't know whether to replace the piston rings or not. One school of thought says you should but another is that the rings are hard and therefore the barrel will wear rather than the rings so no need to replace them. I'll probably replace em just in case but any thoughts on this?

If they are ok and there's not too much clearance in the gap when the rings are in the cylinder, and the piston and cylinder are ok, don't replace only the rings. They will not run in, only with a ground cylinder and new piston. So it's all or nothing.
If the head gasket is not leaking you might even consider not replacing the bottom gasket. A bit of sweating doesn't matter. I would only replace it if it really leaks.

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Fred, XTZ660, Holland.