New member here.... interesting read as I just spent the last couple of days pulling my B6000E / ZL600 apart for the same reason.I will look at the relief valve today, it would be a easy fix! But back to the plug, would it come out and fall into the pan, or be swallowed and ground up by the engine? I could pull the pan to get it out if I had to.
On the second oil change since I've owned it, only a few weeks, the plug fell out in the oil drain pan. The oil light had been flickering at low rpm.
I'm a retired diesel engine mechanic so I took it apart pretty much as a retirement project, just to have a look inside. Sure enough, there was the open hole in the center of the cam shaft.
After trying several things for a plug, I settled on a 1/4" NC bolt with the threads ground down to a slight taper. It tapped in nicely and got firm as the head reached the end of the shaft. Used the arc welder to secure it but after reading, will have to check to see if indeed there is enough clearance under the cover for that bolt head.
While there I checked the timing marks on the gears. Each was out exactly 4 teeth on the idler, but the timing would have still been correct. Can only assume the cover had been off in the past and someone lined the idler teeth up straight on, but being helical cut, they ended up 4 teeth out at each location when the gear was pushed all the way in.
Needless to say, it is now corrected.
Any hints or help you can offer while I have it apart would be greatly appreciated. Will attempt pics later, Thanks.