FUSO 4M4 – Engine – CYLINDER HEAD AND VALVE MECHANISM – Inspection procedure

Inspection: Connecting rod bearing span when free

•If the span is less than the specified limit, replace both the upper and lower bearings as a matched set.

•Inspection: Connecting rod bearing-to-crankshaft oil clearance
– Fit the lower bearing to the connecting rod cap and the upper bearing to the connecting rod, then tighten the nut to a torque of 49 N·m {5.0 kgf·m}.
– Measure the inside diameter of the bearing and the diameter of the crankshaft pin.
– If the clearance exceeds the limit, replace the defective part(s).
– If a bearing has to be replaced with an undersized one, machine the crankshaft pin to the specified undersize diameter. (See the CRANKSHAFT AND CRANKCASE section.)

Inspection: Piston-to-crankcase cylinder clearance
• If the measurement is not within the standard value range, perform one of the steps below according to the condition.
A: Measure on cylinder bore in crankcase (in direction of crankshaft axis)
B: Measure on cylinder bore in crankcase (in direction perpendicular to crankshaft axis)
C: Measure on piston (in direction perpendicular to piston pin hole)
• If the crankcase cylinder bore diameter exceeds the specified limit, rebore all the cylinders to an oversize diameter and replace the pistons and piston rings with the corresponding oversized ones.

•If the cylinder bore diameter is within the standard value range, replace the piston and piston rings.

Selection of oversized piston
• Available oversizes: 0.50 mm and 1.00 mm (two sizes)
• Measure the bore diameter of each cylinder. Select the appropriate oversized piston based on the largest of the measurements.
• Measure the outside diameter of the chosen oversized piston.
• Rebore each cylinder to a diameter at which the piston-to-cylinder clearance will be within the standard value range.

Reboring diameter (tolerance ± 0.005) =
Oversized piston diameter (measured value) + Piston-to-cylinder clearance (median of service standard value range) – 0.02 mm (honing margin)

• After boring the cylinders, hone them to the finish dimension (tolerance ± 0.005).
Finish dimension (tolerance ± 0.005) =
Oversized piston diameter (measured value) + Piston-to-cylinder clearance (median of service standard value range)
• Honing finish surface roughness: 1.2 to 3.5 μm
• Honing cross hatch angle: 20° ± 5° (semi angle)
• Cylinder bore out-of-squareness: 0.05 mm
• Measure the piston-to-cylinder clearance.

 

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