KX85: General hints on what to consider when replacing piston

[johanb]

Hi all!
I bought my son a 2003 KX85 and
+ got it for a very decent price
+ it seems it has not been much used at all, very good general condition
+ starts on first kick
- not taken care of, ie greasy chain, dirty airfilter, oil needed change etc
- no driving log: no info on the number of hours since the piston was changed.

Questions:
1. Is there any way to find out if the piston needs to be replaced without opening up the cylinder? Even that will as I understand it not give me much information. It sounds fine and runs smoothely, so I feel a little reluctant to just spending money and time if the piston is fine...
2. Sooner or later I will have to change piston anyway. I will try to do it myself and have a repairmanual (Clymer). Are there any general wisdoms that I will not find in the manual? What should I consider?

I have tried to search for help on this with very little result.

Thanks
Johan

[tayyb]

You could just do a compression test, but I would just replace the piston anyways just to be sure. When i bought my kx125 the guy had no idea either so its just a good safe thing to do. Plus, you gain the knowledge on how it works. 100 bucks should get you a piston and a top end gasket set. Im no mechanic and for me the installation was a breeze. As for the repair manual, yea i bought one, but I dident look at it once for that procedure.

[nastyhabit]

For the price of a new piston and I would just replace it. This gives you a chance to have a good look at the whole bike and replace anything that is not in good order. If you son is racing the bike, you should be replacing the piston every 30 hours and going over the bike from front to back before he rides every time. You can do the top end yourself it is pretty straight forward and will save you a few dollars also.