PISTON REPLACEMENT TIME? MR. KNOW-IT-ALL

WR450F PISTON REPLACEMENT?
Dear Mr. Know-It-All,
I am a slightly overweight, 50-plus-year-old slow guy who has enjoyed my 2005 Yamaha WR450F since purchasing it new in 2004. I religiously service the bike myself, including regular oil and air filter changes, valve checks (they have never needed adjustment—ever) and run 100-octane fuel. The motor runs strong, and does not smoke, use oil or make any odd sounds. People always seem surprised when I tell them it’s a 2005. I do not have an hour meter, but I estimate the bike must have several hundred hours on it by now. My question is about piston replacement. The factory service manual says 25-hour interval for piston replacement. I am sure this recommendation is geared more for racing use; however, given the cumulative hours, should I be worried about piston/top-end failure?
T. Parkinson
via [email protected]

The fact that your WR’s valves have never needed any adjustment confirms that you do indeed take good care of your bike and are careful about maintenance. Excessive engine heat, bouncing off the rev limiter and sucking dirt (or water) through the air filter are all causes of valve problems. Yamahas offer better valve life than a lot of brands; however, no matter how well you care for the engine, the piston is still going up and down, and that will cause ring and piston wear. All modern, high-rpm four-strokes have pistons without a great deal of skirt length to stabilize the piston. Again, the Yamaha WR450F is better than most, but a new piston and rings are still a good idea. It is unlikely that you will notice a performance improvement, but you want the piston to be stable in the bore, and you don’t want it to break or pull sideways. Check to make sure that the cylinder bore is straight as well. If you can no longer see cross-hatching in the cylinder wall where the rings contact the surface, have the bore checked. The cylinder may need to be replaced or re-plated. You should absolutely change the cam chain and the cam-chain slides. Check the oil pump and pump gears, and replace any that are plastic. Check the water-pump impeller for wear as well. Naturally, check for play in the connecting rod as well.

You could have the valves and head done at the same time, but that is the most expensive part of a rebuild. Have a technician you trust evaluate whether the valves are sealing, and the condition of the tappets (the buckets that cover the valve springs) and the condition of the cam lobes. That should allow you to make a good decision about leaving the valves alone.