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Dirt Bike Pistons are made thinner at the top than at the skirt since high temperatures from combustion cause the top of the Dirt Bike Piston to expand a larger amount than the skirt area. For this reason Dirt Bike Pistons are measured at the skirt rather than across the crown. Dirt Bike Pistons are also cam ground meaning they are oval-shaped because they expand more in the direction of the pins. To get the proper Dirt Bike Piston measurement, measure it at its largest diameter. The ideal place to meanure the Dirt Bike Piston is near the bottom of the skirt perpendicular to the pin.
A micrometer will help to obtain Dirt Bike Piston-size readings that are accurate to less than a thousandth of an inch. With either an inside micrometer or a telescoping gauge, it is then possible to get an accurate reading on the cylinder size. The difference in these two figures is the Dirt Bike Piston to cylinder wall clearance.
A cheap and easy, though less accurate, way to determine Dirt Bike Piston-wall clearance is to use long, thin feeler gauges (0.0015" to 0.005"). Slip the clean Dirt Bike Piston into the clean cylinder along with the feeler gauge placed perpendicular to the pin. Try to fit larger or smaller feeler gauge stips until you find one that will just pull free with your fingers. That gauge represents approximately the amount of Dirt Bike Piston-wall clearance you have. Carefully inspect the Dirt Bike Piston-pin bearing or bushing area of the connecting rod. If the bearing feels gritty, or the pin appears scored, replace both. If the Dirt Bike Piston pin rocks or binds severely in its rod bushing, both the pin and the bushing should be replaced.
Now that you've determined the actual Dirt Bike Piston-to-wall clearance and looked up the recommended factory specifications, you have several options:
Option 1: Reinstall the old parts. If the clearance specifications are within recommended factory tolerances, you're safe to simply reassemble the upper end after cleaning and lubricating the parts.
Option 2: Replace the rings only with a new ring set of the same size. If the clearances are toward the loose side of factory recommended tolerances, deglaze the cylinder with a hone, then install new rings.
Option 3: Replace the Dirt Bike Piston and rings with a new set of the same size. If the Dirt Bike Piston is worn considerably and replacing it with the same size new Dirt Bike Piston would decrease the clearance to factory tolerances again, you can save the cost of a bore job (up to $100.00 a hole) by this simple replacement. Again, you must deglaze the cylinder and use new rings.
Option 4: Re-bore the cylinder, then install proper oversize Dirt Bike Pistons and rings. To update an engine to peak performance, a bore job is called for. This requires re-machining the cylinder with a boring bar or special hone so it will accept a larger Dirt Bike Piston. Always buy the new Dirt Bike Piston first and take the Dirt Bike Piston and cylinder to the machinist who is to do the job. Without the Dirt Bike Piston, he can only guess how far to bore and hone the cylinder. If he has the Dirt Bike Piston he can measure exactly.
It's a good idea to clean the fins and spray them lightly with some flat black high-temperature exhaust paint. This helps the engine cool better. Check the Dirt Bike Piston-ring end gap before installing rings on the Dirt Bike Pistons. If the gap is tight, carefully make a light cut or two across the ends of the rings with a fine file. Remove just enough metal from the end of the ring so the gap will be at or slightly larger than factory specifications. Follow the manufacturer's recommendation for locating the end gaps on the Dirt Bike Piston.
After assembling the Dirt Bike Piston, pin, and rod, be sure that the pin clips are securely seated in the Dirt Bike Piston before reinstalling the barrel. Also, don't forget to put on the base gasket before you put on the barrel.
Installing the cylinder assembly over the Dirt Bike Pistons requires special care and patience. Don't force anything. The base of the cylinder is tapered to allow the rings to slide into the barrel more easily. Use some clean light oil on the Dirt Bike Piston rings and cylinder when reassembling them to guard against scoring when the bike is first started.
Follow the manufacturer's instructions for re-torquing cylinder head studs as aluminum heads distort easily if you don't. Many mechanics take several passes at this first, torquing the nuts to half, then three-quarters, and finally to full torque setting. Be sure to follow the recommended sequence when torquing the head.
If you're working with an overhead-cam engine don't forget to keep the cam chain suspended throughout the installation of both the barrel and the head. Refasten the cam chain only after the cam is properly reindexed according to the technique recommended in the factory service manual.