I started riding again at the age of 47. During a trip to CA to visit some friends who own and race a vintage car, my wife knocked my sock off when she told me “Men need a hobby, all work and no play is no good so, if you want to get a dirt bike, that would be OK”. The next week was opening day of Dove Hunting Season and I saw some friends I only see that day every year. They were leaving after the morning hunt and when I asked why, they explained that they raced Cross Country and had a race to get to. The light bulb went off and within a couple weeks I had a new bike. Several years later now, at 51 I not only ride Cross Country races (in between injuries), but I’ve found I enjoy working on my bike.In talking maintenance to some buddies recently on practice day, the subject of pistons came up and I was surprised to learn that replacing the piston was a regular maintenance item, least on a modern day 4 stroke. Hmm, my bike is 4 yrs old now, been ridden hard but is well maintained. However, I’ve never replaced the piston…
Now, back in the day, you didn’t replace a piston unless it was clapped out, and at that point you would go to the next bore size. These guys are using stock size replacements. So my questions are:
*Is replacing the stock piston a common & valid maintenance practice today?
If so, don’t you risk losing the mating of cylinder, piston & rings that previously existed?*Is there a suggested break-in routine?*When do you suggest going to the next bore?*Finally, what is the advantage/dis-advantage of going to a higher compression piston?The magazine is a great read and you guys do a fine job of reviewing both the bikes, after-market products, maintenance & riding techniques. Keep up the good work!Crash43
With the sort of rpm that modern four-strokes are turning, and the super-short pistons the bikes have to run in the short-stroke motors, you don’t want to wait until the piston is “clapped out.” Before it gets to the point of piston slap like an old engine, the piston will hang in the bore and pull the rod in half. You didn’t say what four-stroke you are riding or how hard you are spinning it, but 50 hours on a 250F and 80 to 200 hours on a 450 piston depending on the brand are good starting points. For an engine that is four years old I would change the cam chain as well. Modern bikes don’t have cylinders that can be “bored out.” At least they can’t be easily, since the cylinder is aluminum and is coated with a super-hard, slick coating like Nikasil. That is a metallic ceramic, and if the engine is only digesting clean air and oil, the cylinder bore will last a long time.It is routine to still see the hone marks on cylinders with 200 hours on them. In fact, if the hone marks are not visible it is likely the cylinder needs to be replaced or replated. So it is common to use a stock size replacement. There are big-bore kits that are sleeved or have the cylinder bored and replated. The coating used it so hard that it takes a pressurized hone to establish the stock surface; so most mechanics break the glaze with a quick hone or just knock down any deposits with a Scotchbrite pad. Usually two or three heat cycles is plenty of break-in for a piston. That means to run the engine without a hard load or extreme rpm until it is hat, then letting it cool three time.A higher compression piston will generally give the bike more response on the low-to mid rpm ranges. Some of the modern pistons will still allow you to run pump gas, but they generally like at least a mixture of pump gas and race gas. The engine is dealing with more pressure, so in theory the crank and other parts are dealers with added loads, but in reality it isn’t usually a problem. Higher compression will generate more heat, though, and often the cooling system is already taxed in off-road conditions. If you are riding a 250F, then a big-bore kit can be a very nice addition. -Karel Kramer