Guide To Four Stroke Motorcycle Engines, Part 9
Piston and Cylinder Service
As you know, the piston is designed to operate in the cylinder with a little clearance to allow for heat expansion and lubrication. If there's too little clearance, the engine will seize up; too much and piston slap, eventually breakage, may result. Most motorcycles call for 0.001" to 0.003" of piston-to-wall clearance. You can generally assume that most pistons use about 0.001" clearance per inch of cylinder bore. Your manual gives you the exact specification. You can measure your existing piston and barrel using a micrometer and snap gauge to determine clearance.
Before you measure these parts, however, the machine should be clean, so give it a good washing. The steps involved in removing the head on most models will vary considerably with some even require removing the engine from the frame. Follow the instructions in your manual relative to engine and cylinder removal.
Once the head is reworked and carefully stored, you can inspect the tops of the pistons for excessive carbon, burns, or damaged piston crowns. Carefully remove the cylinder assembly from the engine, following these precautions:
- Never pry cylinders with a screwdriver tap them with a soft rubber or plastic-tipped hammer.
- Stuff clean rags into the crankcase to protect the lower end as soon as you have lifted the barrel an inch or so.
- Lift the barrel straight up - don't rotate or twist it.
With the barrel off, the piston can be removed from the connecting rod. In nearly all pistons there are retaining clips to remove before the pin can be extracted. These clips can usually be removed with a good pair of needle-nose pliers. Don't scratch the piston or damage the clip if you must use a small screwdriver to pick out the clip. Be careful that the clip doesn't fly out and hit you in the eye as it springs out of the piston.
Sometimes gentle finger pressure will enable the piston pin to slide right out and free the piston from the rod. Other times, however, the piston must be heated gently with a torch, or a piston-pin extractor must be used. A C-clamp and the proper size sockets might be used to press out the pin.
Piston-to-Wall Clearance Measurement
Pistons are tapered somewhat because high temperatures cause the top of the piston to expand more than the skirt area. For this reason pistons are measured in the skirt area rather than across the crown. Also, pistons are cam ground; that is, they are oval-shaped because they expand more in the direction of the pins.
To get the proper piston measurement, measure it at its largest diameter. You must measure the piston near the bottom of the skirt perpendicular to the pin.
By using a micrometer it is possible to obtain 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 piston to cylinder wall clearance.
A cheap and easy, though less accurate, way to determine piston-wall clearance is to use long, thin feeler gauges (0.0015" to 0.005"). Slip the clean 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 piston-wall clearance you have. Carefully inspect the piston-pin bearing or bushing area of the connecting rod. If the bearing feels gritty, or the pin appears scored, replace both. If the 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 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 piston and rings with a new set of the same size. If the piston is worn considerably and replacing it with the same size new 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 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 piston. Always buy the new piston first and take the piston and cylinder to the machinist who is to do the job. Without the piston, he can only guess how far to bore and hone the cylinder. If he has the 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 piston-ring end gap before installing rings on the 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 piston.
After assembling the piston, pin, and rod, be sure that the pin clips are securely seated in the 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 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 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.