Cylinders re-boring

In engine-reconditioning workshops, large boring machines are used to re-bore engine cylinders. The block is set up on the bed of’ the machine with the boring bar mounted above.
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.

Fig 5.11

Portable boring bar

A portable boring bar consists of a base that is clamped to the top of the cylinder block, a spindle which can move up and down the cylinder, a rotating boring bar with a cutter, and an electric motor that drives the boring bar.
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.

5.11 Portable boring bar parts labeled

1 diamond Lap for toot sharpening, 2 adjuster for cats paws.

3 spindle, 4 handle for raising and Lowering the spindle,

5 adjustable stop for spindle travel, 6 driving shaft from

motor, 7 clamp to cylinder block

Setting up a boring bar

The following points provide an appreciation of the method of using a portable boring bar and the need for accuracy.

Checking bore size
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.

Oversize of cylinders
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
ITEM	SPECIFICATION
Cylinder bore (standard) Cylinder-bore out-of-round (service limit) Cylinder-bore taper (service limit) Cylinder-to-piston clearance Piston diameter (standard) Piston diameter (0.25 mm oversize) Piston diameter (0.50 mm oversize)	74.00—74.02 mm 0.10 mm 0.10 mm 0.02—0.05 mm 73.97—73.99 mm 74.22—74.23 mm 74.47—74.48 mm

  

Accurate measurements

An inside micrometer is used to measure the cylinder bore and an outside micrometer is used to measure the piston. Check the outside micrometer with its setting bar and adjust it if necessary. Then use the outside micrometer to check the inside micrometer.

·           The inside and outside micrometers must read the same.

Fig 5.12

Mounting the boring bar

The surface of the block and the base of the boring bar must be clean before the boring bar is mounted on the block.

The boring bar is correctly positioned so that it bores concentric with the original bore. This is done with expanding lingers, called cats paws, on the lower end of the spindle (Figure 5.12). When the cats paws are expanded against the cylinder, they centre the spindle in the bore (Figure 5.12).

Once accurately centered, the base of the machine is clamped to the top of the cylinder block. It remains clamped in position until boring of the cylinder is completed.

Fig 5.13

The cutter, that does the boring, rotates in a carrier at the bottom of the spindle. The cutter is adjusted for depth of cut in a special micrometer (Figure 5.13). This has to be an accurate setting because it determines the finished size of the bore. The cutter is set slightly undersize to allow for a final finish by honing.
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.

Continued

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