How to know when to replace the top end

Compression test. Go buy a tester at an automotive store. Have them steer you in the right direction if you have no clue what I'm talking about. Follow the directions for installation. Before doing so, warm the bike up a little Once installed, hold the throttle wide open and kick the bike a few times. Record the reading and do it 2 more times. See that your 3 readings are all close to be sure you are doing it properly and they are accurate. I always kick until the needle stops moving. If you are well into the 100's for p.s.i. you are probably ok. How long since the last time you did rings or a complete top-end?

X2 on what scooter5002 said!

Time. Start a log and write down every time you ride and for how long. When you get to so many hours it is time for a rebuild. High strung race engines have a tendancy to blow up when not maintained. 125 say 15 hrs for a top end. and rebuild the crank when it goes it wont hurt anything just a bunch of rattling.

Once rebuilt, either log information, in the manual , or better yet invest in a Hour meter!

It really depends on if it the engine is stock or modified to the length of time between rebuilds along with how it is being ridden/raced. A trail rider or a fun racer is a whole different ball game also. Like what is said earlier check your compression and go from there.

When you lose compression you can tell when u go to kick it there is no reistance until the bottom of the stroke

Compression testing is good for determining the ring seal condition to the bore. However, pulling the cylinder, and measuring the piston diameter / cylinder bore / ring end gap will tell you more precisely where you are at.

I have seen many engines that make good compression numbers, only to pull them down and see a crack developing in the skirt areas of the piston.

I invested in an hour meter last winter during my rebuild. Its a very good investment and cheap insurance. If you keep up on maintenence, 30hrs is a good time for a complete topend rebuild. With an hour meter you can also track oil changes. I change mine every 3hrs.

I also have a compression test kit. $20 at auto zone.

As noted above a compression tester is a great tool for understanding the sealing condition of your cylinder.

What it does not do though is tell you if anything is structural fatigued.

A good maintenance schedule and timely top end replacements after a pre determined amount of hours based on your riding level is a good route to go to prevent any failures.

As Project Racer said measuring the parts is also a great habit. Understanding your bike is a big advantage and saves $$$

raise gas octane level and it will make your top end last longer. i run 104 octane on my 125.