Purpose of horizontal hole on right side of head?

Anybody know the purpose of the horizontal hole on right side of head on the 426? Is it to be blinded? I have bought a bike in pieces so I don't know what to do with it. On the Yamaha service manual there seams to be a bolt with a washer or gasket, but I can't find it on any part diagrams....



Center = spark plug hole drain

Forward = the Decompression arm retaining bolt

What he said ^^

Or is it the oil pressure check bolt? A pic would be nice.

Thanks, guys. Spark plug drain hole sounds likely. The hole is in the center. Have seen from pictures of other bikes that there aren't any bolts in the hole.


That's the one. Thanks!

You're welcome. How is the rebuilding process going? People around here love to see pictures of projects...

It is going quite well. The engine is all back together, but still on the counter topp. I am stripping the carb at the moment. Going to clean it. The subframe is tilted back to access the airfilter box. It is full of piston debris. Needs a new handle bar. Will start the assembly this Sunday, I think.

I have a 1999 WR 400 and when it runs for a while I have water that comes out this hole and it starts to overheat. I changed my water pump and seals and new head and base gaskets plus didn't see any cracks in the combustion chamber (between valve seats, etc.) of the head but didn't take out the valves and check in the valve pockets anybody have any ideas?

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