'06 YZ450F - diagnosing top end "rattle" that only when engine is warm

I am diagnosing an '06 YZ450F this week (not my own), and when the bike is warmed up it has a nasty loud rattle coming from the top end (seems to be more on the right side of the motor when I listen with a screwdriver). Can this just be the valves out of adjustment? Loose chain? I thought about super loose crank bearings, but I've never heard any THAT loud, plus I could hear that those were rolling smoothly.

I'll be pulling the top end apart this week most likely - suggestions on what to look for?

Some possibilities:

  • Loose primary gear on the crank
  • Loose big end bearing allowing piston to contact head
  • Damaged piston and/or rings

Got it. The last two seem like more likely possibilities given that the noise is only present once the bike is warmed up.

Splitting the cases on these isn't any more of a big deal than it is on a 2-stroke motor, right?

Same thing, once the top is off.

So life got in the way of working on bikes lately. Getting to this one this weekend I hope.

I can qualify the connecting rod bearings by using a dial indicator through the spark plug hole, correct? Same way I set the timing on my 2T, but doubles as a rod bearing check.

Any other way to check the piston/cylinder health of these besides pulling the head?

Going to start with checking valves, and removing right side cover to check the primary gear nut. Bike runs real healthy other than this stupid noise, fires up real easy and it seems like a really low hour bike (<60-70, just owned by idiots).

I can qualify the connecting rod bearings by using a dial indicator through the spark plug hole, correct?

Any other way to check the piston/cylinder health of these besides pulling the head?

That method won't tell you much of anything. The manual shows how to measure the rod bearing clearance by rocking the rod sideways and measuring the movement. An excess indicates wear. Measuring through the plug hole will show extraneous things like the piston rocking in the bore, small end clearance, etc.

Are you sure the noise is abnormal? These engines are not known for how quietly they run.

It sounded abnormal to me, but I don't know 4T single motors that much. Listening to some videos of these bikes idling they seem to make the same noise. I'm hoping I can somehow verify that's ACTUALLY it without tearing the whole motor apart. If something is about to go, it seems prudent to find out before it goes to catastrophic failure.

As gray stated, they are quite loud even in good shape for whatever reason.

I'd try to find one close by you can listen to to see what it should sound like, if that's an option.

Well, I have the '06 YZ450F of my own in my garage which sounds nothing like it, but listening to videos of others running in previous threads, it's way the hell quieter than any of those.

This widespread YZF rattling has to be coming from somewhere. Noises like that aren't completely inexplicable.

Most of the noise is explicable. But listening to an audio stream from someone's GoPro and trying to diagnose any of what comes out of the speakers on your flat screen monitor isn't very effective, usually. It's entirely different from being there.

A lot of the noise is caused by the way the engine runs at speeds below about 2500 RPM. The YZ450 engine has an extremely light rotating assembly for its bore size, even with an add-on weighted flywheel. At low speeds, it fires and accelerates everything it's connected to it, the main drive gears, the balancer, two trans shafts, ten transmission gears, etc. It makes noise as all that slack comes out. Then the engine and all that stuff coasts until it hits compression, when it looses speed in a hurry, and all the slack in all those parts comes out again. Then it fires. You get the idea.

Another noise related to the RPM pattern at idle is in the cams and valve train. The same drive forces that the clutch gets from the jerky idle are transmitted to the cams, too. This is one reason that keeping the chain and tensioner free is so important. If the chain gets "kinked", it can trick the tensioner and cause the engine to jump time.

Then, there's what I call "cam flip". If you ever turned a camshaft by hand, you know that it's hard to turn the lobe up onto the lifter, but it flips forward under its own power on the way past that. At a low enough speed, higher in engines with fewer cylinders, the "flip" energy is higher than the rest of the mechanical drag, and a rattle results.

So, the trouble lies in trying to explain that to someone who hasn't been around them much when you really can't hear it AND see it, and help him figure out if there's something to worry about or not.

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