06 450 top end

I took my bike to a trusted mechanic to have the valves adjusted ( just couldn't get the time to do it myself) and he found one intake valve a couple thousandths loose. No biggie. But what he did say is that the engine is noisier than normal. So for his own curiosity he pulled the filter and found some aluminum in it. He said turning the engine by hand he could hear/feel something isn't right. He says maybe piston slap because it's aluminum. So I'm gonna order a oem piston kit. The kit comes with piston, rings, wrist pin, head gasket and base gasket. I know ill need a tensioner gasket too, but is there any other gaskets ill need to complete the job? Of course ill check the bottom end out too and I have the manual and a buddy with tons of knowledge if I need him. Thanks

I don't know if you mentioned it but a new timing chain while you're in there is a good idea. Aluminum in your air boot caused by piston slap? I'm not sure I believe that one. Also I've owned 2 450s of that same generation and they were both fairly noisy from the dealer, however I'm sure the mechanic knows what he's hearing. But to answer the gasket question, I believe you should be ok with base and head gasket.

Not in the airboot. In the oil filter. Yeah I already had planned on a timing chain. Sorry I didn't mention it in the op.

The piston could well be worn out by now, but finding aluminum in the oil filter is not the least unusual for a YZ450.  Most of it comes from the clutch.  Even if the piston had suffered some fairly major damage over a short span of time, I wouldn't expect it to shed very much metal into the oil.  Don't know where he came up with that.


Check the amount of "rock" in the rod bearing.  See the manual.

He did say possibly a bearing in the lower end but that usually you find some brass or copper colored particles in the filter. I should be able to start tearing it down in the next couple days. And thanks grey I'm sure I'll have more questions as I progress.

Stator cover gasket if you plan on changing timing chain. Inspect chain guides for wear. You will also need a flywheel puller.

Finally got a chance to take the top end off. The head looks great except for carbon build up. The cylinder looks good too, except I was told the if the cross hatching was gone it's probably out of spec. No cross hatching. Don't really feel any ridge at the top of the cylinder though. But the top of the piston has some nicks in it at the exhaust side. This concerns me a little but nothing was in there and the cylinder isn't scratched. I'm ordering a oem yamaha top end kit anyways. But what would cause that? Also pulling up and down on the rod I think I feel a little movement. Hard to tell though. I'll post a pic of the piston and cylinder when I get in a better area.

Don't try to judge the rod clearance by pulling up and down.  Measure that according to the manual by measuring the amount the rod can be rocked back and forth.


Ok. So that would be the .04 to 1 mm? And the pics. Im going to take the cylinder to a machine shop to have the bore checked too.




Ok. So that would be the .04 to 1 mm?


Yes.  Don't allow the rod to slide along the crank pin while you check, or the rod's normal side clearance will be added to the reading you get.

Thank you gray

I checked the side clearance and its at .33 so thats good. And then checking small end appears to be about a mm and a half. The way I checked was by rotating the crank back some so the small end is against the front of the case and holding the crank and big end in position with my left fingers all while setting my calipers into position so I could rock the rod side to side. Sheese need an extra two hands. If you know a better trick of the trade let me know. Also I know you said not to go by weather the rod can move up or down but I'm curious and asking stupid questions is the only way to learn. Would it be normal for it to do so? Mine seems to move ever so slightly. It's almost imperceptible.

You should be using a dial gauge for that measurement.

Just to put an end to this thread, I ended up putting a new crank in her. The old one had been rebuilt and the pin was welded. She's back together. I broke her in the weekend before last and rode the piss out of her sunday at a buddy's track. Sooo good to have her back.

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