4-30 ENGINE TOP END
When boring and honing a cylinder, note the following:
There are two sizes of oversize pistons available. Oversize pistons
require oversize rings.
Oversize Pistons and Rings
Before boring a cylinder, ﬁrst measure the exact diameter of the
service data section, oversize piston, and then, according to the
standard clearance in the determine the rebore diameter. However,
if the amount of boring necessary would make the inside diameter
greater than 1.0 mm oversize, the cylinder block must be replaced.
Cylinder inside diameter must not vary more than 0.01 mm at any
Be wary of measurements taken immediately after boring since the
heat affects cylinder diameter.
In the case of a rebored cylinder and oversize piston, the service limit
for the cylinder is the diameter that the cylinder was bored to plus 0.1
mm and the service limit for the piston is the oversize piston original
diameter minus 0.20 mm. If the exact ﬁgure for the rebored diameter
is unknown, it can be roughly determined by measuring the diameter
at the base of the cylinder.
Piston Ring End Gap
Place the piston ring [A] inside the cylinder, using the piston to locate
the ring squarely in place. Set it close to the bottom of the cylinder,
where cylinder wear is low.
Measure the gap [B] between the ends of the ring with a thickness
Piston Ring End Gap
Piston Ring, Piston Ring Groove Inspection
Visually inspects the piston rings and the piston ring grooves.
If the rings are worn unevenly or damaged, they must be replaced.
If the piston ring grooves are worn unevenly or damaged, the piston
must be replaced and ﬁtted with new rings.
Check for uneven groove wear by inspecting the ring seating.
The rings should ﬁt perfectly parallel to the groove surfaces. If not,
the piston must be replaced.
With the piston rings in their grooves, make several measurements
with a thickness gauge [A] to determine piston ring groove clearance.
Piston Ring/Groove Clearance
Piston Ring Thickness
Piston Ring Groove Width