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Posted by: 4strokesyfz

hi guys, im going for my third season of mx with my yfz 450 2004, i broke my frame at 4 different place, broke my swing arm but i have never blew a motor. the motor of my yfz is completely stock with juste a jetting kit with a duncan fat boy pipe. my question is if i run this motor for the 2006 season what is the chances that the motor doesn't broke? what is the price of a completely rebuild motor if i changes piston and crank?

Posted by: Toadz400

Well, first I suggest you taking an English class over again.

After you're done with that, you can start working on the engine of your YFZ.

Depending on what you rebuild in your engine, the costs will vary, especially where you buy the parts from and if you do the labor yourself or not.

Since you are going on your third season of MX racing, I would suggest rebuilding the entire engine just to be on the safe side. You'll need to check the cams to see if they need to be reshimmed, I'd order a top end kit and I would have the crank checked to see if it is within clearance (bearings). If the crank is fine, I would leave it and not bother replacing it unless there is something wrong with it.

A rebuild kit shouldn't cost you much, I don't know what the four-stroke rebuild kits are going for, but I believe it's only around 120-150 and I'm not sure what the shim kits go for.

Posted by: Ride1Rob

You can get two sets of HotCam shim kits for $65 off of YFZCentral.

Posted by: 4strokesyfz

sorry for my english, im a dude french!!!

Posted by: jshtex

I just put a new piston and rings in ours and we had a little over a season of hard XC racing on it.

While you are in there you should get the cylinder checked to make sure it is still in spec. If it gets to big the piston will try to rock over in the bore and thats bad. Look at the front of it (exhaust side) for signs of scuffing, but ultimatly get it checked to see if it is still within acceptable specs. Cylinders are not cheap but they are a whole lot cheaper than what will happen if it lets go.

I also put a new timing chain in ours, they are pretty cheap. And if you havent done it already (like they said above) you are way past due for checking your valve adjustment.

The head bolts are torque to yield, and you are not supposed to reuse them, although many have done it with no problems, but they are cheap so I bought new ones.

I also pulled the valves out and lapped them to get the seats back into shape.

We change our oil every race weekend, and the filter every other. The motor was very clean inside.

By the way I went back with all OEM Yamaha stuff but for me reliability is the big thing. Good luck with yours.