98 Yz400f - Noise

I have a 98 yz400f- possibly looking to rebuild me top end. Last summer it had a weird knocking noise the sounded like it came from the top end. Almost like the piston was knocking on the cylinder head. When the bike idled it would get loader then decrease in sound. I did go away after the bike is warmed up but not totally. Its something i would like to take care of before it may cost me more money. Any pointers on rebuilding top end would be appreciated. Thanks

You could be looking at a piston slap caused by wear, but any 400 with such a noise should be checked for a loose balancer drive gear and key. This is a common problem, but not a very big one. The balancer drive gear sits behind the primary drive gear on the right end of the crank.

how does piston slap exactly happen and how do you fix it?..just a new piston and rings?

The cause is usually a loose fitting or incorrectly shaped piston skirt. In the case where it didn't do it originally, but does now, it's usually a wear condition, but it could also be the result of certain kinds of damage, or even a broken ring. The cure is a top end rebuild.

Some forged high compression pistons made from higher expansion alloys than the OEM piston will have a cold slap from the beginning. This is more or less normal for them, and as long as they are quiet while warmed up, there's no problem.

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