Yamaha WR250F(D) Owner's Service Manual

Contents

5-26CYLINDER AND PISTONREMOVING THE PISTON ANDPISTON RING1. Remove:• Piston pin clip "1"• Piston pin "2"• Piston "3"• Put identification marks on eachpiston head for reference during re-installation.• Before removing each piston pin,deburr the clip groove and pin holearea. If the piston pin groove is de-burred and the piston pin is still dif-ficult to remove, use the piston pinpuller set "4".Do not use a hammer to drive thepiston pin out.2. Remove:• Piston ring "1"Spread the end gaps apart while atthe same time lifting the piston ringover the top of the piston crown, asshown in the illustration.CHECKING THE CYLINDER ANDPISTON1. Inspect:• Cylinder and piston wallsVertical scratches → Replace cyl-inder and piston.2. Measure:• Piston-to-cylinder clearanceMeasurement steps:a. Measure the cylinder bore "C"with a cylinder bore gauge.Measure the cylinder bore "C" in par-allel to and at right angles to thecrankshaft. Then, find the average ofthe measurements.b. If out of specification, replace thecylinder, and replace the pistonand piston rings as set.c. Measure the piston skirt diameter"P" with a micrometer.a. 8 mm (0.31 in) from the pistonbottom edged. If out of specification, replace thepiston and piston rings as a set.e. Calculate the piston-to-cylinderclearance with following formula:f. If out of specification, replace thecylinder, and replace the pistonand piston rings as set.CHECKING THE PISTON RING1. Measure:• Ring side clearanceUse a feeler gauge "1".Out of specification→Replace thepiston and rings as a set.Clean carbon from the piston ringgrooves and rings before measuringthe side clearance.2. Position:• Piston ring(in cylinder)Insert a ring into the cylinder andpush it approximately 10 mm (0.39 in)into the cylinder. Push the ring withthe piston crown so that the ring willbe at a right angle to the cylinderbore.Piston pin puller set:YU-1304/90890-01304 Cylinder bore"C"77.00–77.01mm (3.0315–3.0319 in)Taper limit "T" 0.05 mm (0.002in)Out of round"R"0.05 mm (0.002in)"C" = Maximum D"T" = (Maximum D1 or D2) - (Max-imum D5 or D6)"R" = (Maximum D1, D3 or D5) -(Minimum D2, D4 or D6)Piston size "P"Standard76.955–76.970mm (3.0297–3.0303 in)Piston-to-cylinder clearance =Cylinder bore "C" - Piston skirtdiameter "P"Piston-to-cylinder clear-ance:0.030–0.055 mm(0.0012–0.0022 in):0.1 mm (0.004in)Side clearance:Standard Topring0.030–0.065mm (0.0012–0.0026 in)0.12mm(0.005in)2ndring0.020–0.055mm (0.0008–0.0022 in)0.12mm(0.005in)