What is this hole?

I have a 03 YZ450F, and at my last trip to the track I layed the bike down in a turn and while picking it up noticed oil all over my radiatior and header (it was burning off the header). After a long search we found the hole it was comming from. Its on the front right side of the motor facing the right radiator. If you remove the valve cover you can see right through it (I'm guessing your not supposed to be able to do so). I asked another guy at the track if I could take a look at his, and it was just filled with JB-weld. So I am guessing that this is just a plug of some sort, as it has a screw perpindicular to the hole that is still in there (and tight), so I assume the plug managed to sneak its way out in a hard impact (I've been casing a very large table which was hard enough to yank my carb out of the bootie, since the dirt was horrible that day)

First, what is this hole? Second where should I look for a replacement, as I have always been able to find things on bikebandits.com (and this doesnt even seem to be in their diagram). I guess I could always order from yamaha, but they are always slow as christmas around here.

here is a diagram (stolen from bikebandits.com) with an arrow pointing to the hole.


For once I am glad I dropped the bike, because I caught it before running out of oil. That would have been really bad. By the time I made it to the pits the bike was just at the low oil level, and had been almost at the high mark before starting to ride (had ridden about 30 mins)

Thats where the decompression lever used to be on the older bikes. You can get a plug from TT that goes there and is held in place by the little bolt #8. Check and make sure your breather line is not plugged off or crimped some how. Very common thing from 03 to 05 bikes. If you get the plug from TT and when u put it in go very easy on that bolt, cause the threads will strip out quickly.

from what i understand, that is the hole where the manual decompression lever on the old 400's and 426's were. Yamaha still left the head casting with the hole in it and put a rubber plug in there factory. The tt store has a billet plug that takes care of that. I have also seen them on ebay. Grayracer told me you could put a freeze plug in it. I think he said 18mm, but i could not find one that size in my town. The bolt you talked about holds the billet plug in on mine. You might also check the vent hose to make sure it is not plugged up and popped that plug out. That is exactly where the decomp lever was on my 400 and bolt on top of it was where the cable system mounted.

man i gotta learn to type faster !!!

If it's leaking replace it with the above mentioned part. Or try to safety wire it if it's not too late.

man i gotta learn to type faster !!!

I know what you mean.The other night ,I tried to respond to a post and by the time I got thru,I was two pages behind.:thumbsdn:

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