In a general workshop, a portable boring bar might be used. This is mounted on top of the cylinder block. The method of setting up and boring cylinders with this type of boring bar will be briefly described.
It can be set up on the top of the cylinder block to bore one cylinder at a time (Figure 5.11).
When accurately set up on the cylinder block, the boring bar and its cutter feed slowly down the cylinder boring it oversize. It stops automatically when the cutter reaches the bottom of the cylinder.
Check all the bores and decide which one has the most wear. Use the size of this cylinder to determine the oversize to which all the cylinders will be bored.
The bores are machined to a definite size, for example,
74.25 mm. This would be for a 74 mm bore being bored 0.25 mm oversize. If the worst cylinder will clean up to this size, then so will the others. Work on the worst cylinder first.
Oversize pistons are used and these are slightly smaller than the bore because they have an allowance for clearance.
Typical specifications for a cylinder and oversize pistons are shown in the following Table.
Table Cylinder and piston specifications
Cylinder bore (standard)
Cylinder-bore taper (service limit)
Piston diameter (standard)
Piston diameter (0.25 mm oversize)
Piston diameter (0.50 mm oversize)
The micrometer for setting the cutter has a scale that is twice that of a standard micrometer. This compensates for the fact that metal is removed from each side of the cylinder by the cutter.