"Signs" of needing new top end

[OLD-MAN]

New here and also for MX bikes. What are some of the signs or signals of a bike needing a new top end. Mine is a 2004 YZ125 the wifes is a 2001 YZ80, and the boys, kx65. All of these bikes were bought used and run pretty well, but the YZ 80 kinda has a bog until the power band hits, but when it does Wow! front end comes up in 4 of the 5 gears. Thanks for any and all replies.

[MXtras]

The smaller the engine, the more frequent the need for at least a ring.

When the compression begins to drop, the bike will become harder and harder to start. Bottom end power will go away. Lower compression leads to the bike running rich - unburned fuel and oil will lead to more of a blubbery exhaust note.

You can re-ring a piston a few times before the piston needs to be replaced - it's a cheap fix, too. The ring is what wears the most and is what has the largerst influence on compression for the most part (assuming your bore is not completely shot).

When you slowly depress the kickstarter by hand, you should feel a sharp hit when reaching the compression stroke and it should slowly sink. This is tough to put into words but if you feel the resistance after a fresh top end and compare it to before, you will know what I mean. Once you know the 'feel' of this, it can be a decent, seat-of-the-pants test to judge the general condition of the top end. A compression test is the best way, of course.

Scott

[OLD-MAN]

there is also a whine sound after you rev it. when it is revving back down it has a whining sound. Is that the piston and or rings, or a bearing?