Mystery Oil Port at Base of Cylinder - 2006 YZ 450F

I am hoping someone can help me answer a question I have about an oil port on the 2006 YZ450F. In the attached photo, there is an oil port that leads to the base of the cylinder (upper right part of the picture), but then the port is blocked by the base of the cylinder and dead ends. This port is fed by the mating surface (lower right part of the picture), where the flow has two circuits - one to feed the transmission shafts via a cross drilled hole, and the second appears to dead end at the cylinder base. Does anyone know what the purpose of this second port is?




The one at the cylinder base pumps oil up around the head bolt to the camshafts.

Thanks grayracer513. That makes perfect sense now. I wasn't thinking of the head bolt cavity doubling as an oil passage.

Since you seem to be knowledgable of the inner workings of these engines, let me try two more questions:

1. In tha attached photo, what is the purpose of the machined feature pointed out? Nothing mates with it and it seems to be a carry over from a previous engine design.

2. The service manual calls for molybdenum disulfide oil in a number places (e.g. valve stems, cylinder bolt threads) and molybdenum grease in other places. I have not been able to find molybdenum disulfide oil anywhere, and the local yamaha dealer shop says they just use motor oil instead of what the manual calls out. Has anyone else tried to find the oil that the factory manual calls for? Is it really that important to be used where called for? If overused, do you need to be concerned about fouling the clutch friction plates?

Thanks again,



1) That socket accepts the electric starter idler gear shaft. If you change the crank axle and buy enough parts, the e-start can be bolted to your crankcases.

2) Use a product called assembly lube, usually sold as a grease in a smallish tube, but only for extreme pressure points like cam lobes, and then don't over use it. For all other assembly purposes, use your ordinary motor oil.

Thanks again grayracer513. That is good information.

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