2004 YZ450 Cam Seize

[yarbonwick]

I'm not a thumper rider but I received a call from someone who is. It apears, after break-down, the intake cam cap and cylinder head seat for the intake cam show oil starvation. Bike was recently handed over to a shop to have the valves checked and carb issue. Two days after getting it back...seize. Over the phone I had him check the oil filter (clean), oil level (full) and the rubber plug for the auto decompressor (still there).
Motor is out of the frame.

I know this is a rare occurrence for the YZ450 (or atleast I assume) so I'm very curious as to what caused this. Additionally, would it be acceptable to have the cylinder head and cam caps machined again? I'm thinking this will cause spec issues with the stock cams though. Any help would be appreciated. Thanks in advance.

 

[Rich Rohrich]

It's unfortunately fairly common for the cam caps to be incorrectly torqued after a valve shim change followed shortly thereafter by a seizure.

You need to use an accurate low range torque wrench (usually 1/4 drive) and tighten the caps in stages to limit the chances of this happening.

Depending on the extent of the damage I have fixed a lot of YZF heads by cleaning up the bearing surface of the head with a 22mm 320 or 240 grit ball hone and using 320 followed by 600 and finally 1000 grit paper on the cam journal.

If it's too far gone Mike at Engine Dynamics does an amazing job of repairing these failures. I've seen him fix heads I was sure were junk. http://enginedynamics.com/

 

[yarbonwick]

Thanks Rich. I did some research online and found some info that Mike suggests the torque specs in the book are too much and reducing 5lbs will help prevent this. I still haven't actually seen the damage yet so I can't speculate if it'll need to be boxed up and sent to Mike. All in all though, this is incouraging news. Last thing you want is to chase a gremlin with this things.

 

[vinceprorider724]

I am having this problem right now it is making my bike jump time on the intake side I'm going to tear the head back apart I just put $1,000 into the bottom end so any suggestions would help??????

 

[vinceprorider724]

I am having this problem right now it is making my bike jump time on the intake side I'm going to tear the head back apart I just put $1,000 into the bottom end so any suggestions would help??????

I put it back in time and it started but I don't want to trust that so I bought a new gasket kit and timing chain!