How do you check if the Top End is still good

1. Buy a compression tester, and test the bikes compression after every two or three rides. When the compression starts to fall off do a top end. I actually do my compression tests even more often than that, but I am anal. :laugh:
2. Tear the top end down at least once a year and do a vissual inspection and measure all of the clearances.

By doing these two things and having the bike properly jetted, your cylinder will last for many years.

You could do a compression test which should tell you how well your rings are sealing, that may be an indicator as to how things are looking. Personally, if I've done a decent amount of riding, I will replace rings one year and rings and piston the next with a good forged wisco piston. I usually won't go much over 40-45 hours of trail ridding on rings an 80-100 on the wisco piston. That's on a trail ridden 250 and I make sure that the filter is clean and oiled every ride. I don't see much MX anymore.

With a 125 that you have to keep in the revs in order to stay in the power band, I may lower the hours down to 30-35 hrs of trail riding before I ripped it apart. Opinions vary on this, and if your racing then I would rip it down a lot more frequently. Keep the premix ratio correct and the engine clean and it will live longer.

Or, you could just wait till it blows up to replace the top end. Thats somehow what I usually end up doing. But you can also feel if you need a top end... its easier to kick over, and it doesnt hit as hard. If youre not racing, the top end should last for a few years.