Cylinder And Piston


Strip motorcycle as described in "Stripping Motorcycle for Engine Repair."

Remove cylinder head as described in "Disassembling Cylinder Head."

Remove all cylinder base stud nuts and spacers (1) except one on rear cylinder using Cylinder Base Nut Wrench, Part No. 94585-30. Raise front cylinder and piston enough to permit placing a cloth over crankcase opening. This will prevent dirt or pieces of broken ring from falling into crank-case. With piston at bottom of stroke, remove cylinder (2). Remove remaining stud nut from rear cylinder. Remove rear cylinder in same manner. Discard cylinder to crankcase gasket (3).


Cylinder base


nut and spacer (4)





Cylinder base gasket



Compression ring


(1st groove)



Compression ring

(2nd groove)

Piston pin lock ring (2) Piston pin Piston pin bushing Piston

Figure 3-8. Cylinder — Exploded View

Spring piston rings (4) outward until they clear ring grooves in piston and lift off. Use a commercial ring expander if necessary. Remove piston pin lock rings (6) using internal lock ring pliers. Part No. 96215-49. Support piston and tap out piston pin (7) with a suitable drift.


Place cylinders and pistons in "Gunk Hydro-Seal" or other carbon and gum dissolving agent until deposits are soft. Scrub piston dome and outside of cylinder to remove deposits. Where carbon deposit is thick and hard, it is advisable to scrape carbon before cleaning. Use a putty knife or ground tip on an old file. Use care to keep from scraping into aluminum of piston.

Wash all parts in solvent and blow dry with compressed air. Force air through feed and return oil passages in cylinder. Clean piston ring grooves with a piece of compression ring ground to a chisel shape.

Examine piston pin to see that it is not pitted or scored. Check the piston pin bushing to see that it is not loose in connecting rod, grooved, pitted or scored. A piston pin, properly fitted, is a light hand press fit in piston and has .001 in. clearance in connecting rod upper bearing. If piston pin to bushing free fit exceeds .002 in., replace worn parts, (see "Connecting Rod Bushings").

Check piston pin lock rings to make sure they fit tightly in lock ring grooves. Make sure lock ring grooves are clean.

Examine piston and cylinder for cracks, burrs, burned spots, grooves and gouges.

Check rods for up and down play on lower bearings. When up and down play is detected, lower bearing should be refitted. This requires removing and disassembling engine crankcase.


Gauge pistons and cylinders to see if they are worn to the point where cylinders must be rebored and oversize pistons installed. Inside and outside micrometers used for piston to cylinder fitting should be checked together to be sure they are adjusted to read exactly the same. Subtract piston measurement from bore measurement to obtain clearance. Bore measurement of a cylinder should be taken in ring path, starting about 1/2 in. from the top of cylinder, measuring front to rear then side to side. Repeat procedure at the center and at the bottom of ring travel (see figure 3-9). This process will determine if cylinder is out of round or "egged" and will also show any cylinder taper or bulge.

Pistons are measured front to rear at base of piston skirt as shown in figure 3-10. Pistons are cam ground to an egged or oval shape so only front and rear surfaces are touching cylinder wall.

If cylinders are not scuffed, scored and are worn less than .002 in., it is not necessary to rebore oversize at time of cylinder repair. It may be done at time of next complete engine overhaul. If desired, a new piston may be installed to reduce clearance for more quiet operation.

If cylinders show more than .002 in. wear, they should be rebored and/or honed to next standard oversize and refitted with corresponding pistons and rings.

Cylinders can be refinished oversize with a hone only, or with a boring bar followed by a finishing hone. In general only cylinders not scored and not badly worn are refinished entirely with a hone. Cylinders badly worn or deeply scored are first rebored to nearly the required oversize using a Sunnen Roughing Stone (70 grit) and Medium Finishing Stone Set (220 grit). Then finish hone to exact size, using Polishing set (280 grit). Exact final size of the cylinder bore is determined by size of the piston to be used in that cylinder. Measure piston diameter accurately as described previously, then add desired piston clearance in cylinder. This will equal the exact final size to which cylinder bore should be refinished, example: the 1200cc .020 in. oversize piston to be used measures 3.4575 in., adding .0015 in. (desired clearance) equals 3.4590 in. (finish-honed size). When cylinders require reboring to beyond .070 in. oversize to

Figure 3-9. Measuring Cylinder Bore

Figure 3-10. Measuring Piston

clean up, their oversize limit has been exceeded and the cylinders must be replaced.

When cylinders are worn less than the 0.002 in. maximum, and reboring is unnecessary, unless they are scuffed or grooved the same pistons may be used with the replacement of rings and the roughing of cylinder walls to facilitate ring seating. Use a 240 S.C. (Silicon Carbide) Flex Hone.


Piston rings are of two types — compression (plain face) and oil control ring. The two compression rings are positioned in the two upper piston ring grooves with the stamped word "TOP," a dot (.) or a (-) upward. Rings are regularly supplied to fit standard oversize pistons.

Figure 3-11. Measuring Ring Side Clearance

Figure 3-9. Measuring Cylinder Bore

Figure 3-11. Measuring Ring Side Clearance

Compression rings must have proper side clearance in ring grooves. Check with thickness gauge as shown in figure 3-11.

See Figure 3-11 A. Taper face type compression rings are being used in the 2nd from top ring groove of 1340cc engines beginning with crankcase no. 1479-345-165, and in 1200cc engines beginning with crankcase no. 179-023-001.

The new ring replaces the chrome ring formerly used in the 2nd ring groove, and is identifiable by the black color instead of chrome on the outer edge. When installed in the ring groove, the lower edge of the ring seals against the cylinder wall to improve compression anci oil conlrol.

The bottom oil ring is a three piece oil controf ring using a spring expander.

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