Cylinders measurements

Fig 5.3

An inside micrometer can be used to measure the size of a cylinder, but the most accurate way of determining wear is with a cylinder-bore gauge (Figure 5.3). This consists of a dial gauge with a guide to locate it in the cylinder. As the gauge is moved up and down the cylinder, the pointer of the gauge will show any variations in the size of the bore.
Cylinders are measured in the following way:
I. Before measuring cylinder wear, wipe the cylinder clean and examine it for scores and visible wear.
2. Place the cylinder-bore gauge across the cylinder, just below the unworn ring ridge.
3. Turn the bezel to set the dial gauge to zero.
4. Move the gauge slowly up and down the cylinder to measure taper. The pointer will move on the scale.
5. Turn the cylinder-bore gauge 900 in the cylinder to measure out-of-round.

Fig 5.4

Measurements

Figure 5.4 shows where the cylinder is checked. Readings should be taken at A—A and also at B—B. They should be taken at both the top and bottom of the cylinder. These measurements will allow the wear of the cylinder to be determined as follows:
Out-of-round = difference between A—A and B—B
Taper = difference between A—A measured at the top of the cylinder and A—A measured at the bottom of the cylinder

·       The taper is checked at A—A because this is the thrust part of the cylinder. It will have more wear than B—B.

Condition of cylinders

Fig 5.5

The parts of a cylinder that normally wear are shown in Figure 5.5. Wear is caused by the piston rings rubbing on the cylinder walls. Very little wear is actually caused by the piston. Ring wear leaves ridges in the cylinder at the top and bottom limits of ring travel.

·       The piston rings naturally expand against the cylinder wall, but they are also forced outward by gas pressure.

The types of wear that can be found in the cylinders when an engine is dismantled are bore distortion, waviness, scuffing and scoring (Figure 5.6). These conditions are described in the following paragraphs.
The top of the cylinder suffers the greatest wear, and this leaves a definite ring ridge in a worn cylinder. The lower ring ridge is not as noticeable because less wear occurs at the bottom of a cylinder. The bottom of the cylinder below the ring travel is almost unworn.

·       Cylinder wear is also illustrated in Figure 4. 9 in the previous posts.

Bore distortion

This can be caused by uneven distribution of coolant around the cylinders, or by incorrect tightening of the cylinder-head studs. Bore distortion can sometimes be identified by obvious discoloration of the cylinders, but this is not always the case.

Fig 5.6

Waviness
This is a series of parallel lines or low ridges running around the bore. Waviness can be at any part of the bore but is mainly found in the top regions of ring travel. Waviness is a condition that is hard to measure, but it can be seen and can be felt by lightly running the finger vertically in the bore.

Scuffing
This is evident from small areas of the cylinder wall that have discoloration and minute raised areas in which particles of the piston ring have cold-welded themselves to the cylinder walls.

Scuffing can be the result of localised hot spots in the cylinder water-jackets. This could be caused by deposits that allow small areas of the cylinder wall to reach high temperatures. It could also be the result of continued operation of the engine with loss of coolant.
Incorrect type or grade of oil could create problems. High temperatures will break down the lubricating-oil film and allow metal-to-metal contact between the piston rings and the cylinder wall.

Scoring

This can be an advanced stage of scuffing, in which piston ring particles have scratched the cylinder surface. Broken rings or abrasives can also cause bore scoring. Provided scoring is not deep, honing the cylinder can be used to correct it.

·              With scuffing and scoring, the piston will also be similarly affected.

Continued 

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