Yamaha YZ250FM 2021 Owner's Service Manual

CYLINDER HEAD5-23EAM30086REMOVING THE CYLINDER HEAD1. Remove:• Cylinder head bolt and nutTIP• Loosen the bolts in the proper sequence asshown.• Loosen each bolt 1/2 of a turn at a time. Afterfully loosening all the bolts, remove them.• M9  135 mm (5.31 in): [4], [5], [6]• M9  145 mm (5.71 in): [3]EAM30087CHECKING THE TIMING CHAIN GUIDE(INTAKE SIDE)1. Check:• Timing chain guide (intake side)Damage/wear  Replace.EAM30088CHECKING THE CYLINDER HEAD1. Eliminate:• Combustion chamber carbon depositsTIPUse a rounded scraper, not a sharp instrument,in order not to damage or scratch the spark plugbore threads.2. Check:• Cylinder headDamage/scratches  Replace.TIPWhen replacing the cylinder head, replace alsothe valve.Refer to “CHECKING THE VALVE SEATS” onpage 5-29.• Cylinder head coolant passagesMineral deposits/rust  Eliminate.3. Measure:• Cylinder head warpageOut of specification  Resurface the cylinderhead.a. Place a straightedge and a thicknessgauge across the cylinder head.b. Measure the warpage.c. If the limit is exceeded, resurface the cylin-der head as follows.d. Place a 400–600 grit wet sandpaper on asurface plate, and resurface the cylinderhead using a figure-eight sanding pattern.TIPTo ensure an even surface, turn the cylinderhead several times.EAM30089INSTALLING THE CYLINDER HEAD1. Install:• Timing chain guide (intake side) “1”• Dowel pin “2”• Cylinder head gasket “3”• Cylinder head “4”TIPWhile pulling up the timing chain, install the tim-ing chain guide (intake side) and the cylinderhead.x2 x3 x1FWD612 354612 354Warpage limit0.05 mm (0.0020 in)New