Thursday, June 9, 2011

Hones and cylinders honing

Cylinders with limited wear, or with any of the conditions previously outlined, can usually be restored by honing. This does not apply to badly worn or damaged cylinders which will have to be reconditioned by re-boring or sleeving.
Honing is an abrasive process which leaves small scratches on the surface of the cylinder bore. The depth of the scratches is determined by the grit size of the abrasive method being used. Oil is trapped in the scratches and provides lubrication during running-in and normal engine operation.
The cylinders of aluminium cylinder blocks can be restored by honing but, for some engines, the manufacturers do not recommend re-boring.
Fig 5.7
Cylinders in good condition are not usually honed before fitting new piston rings. Generally, the surface of the cylinders is suitable for replacement rings without any treatment. However, if the cylinders are glazed, they are usually given a light hone.
There are two types of cylinder hones. One has rigid stones and the other has a large number of small beads of abrasive arranged in the form of a brush. Hone stones are made of aluminium oxide or silicon carbide, both are very hard abrasives.
Before using a hone, all exposed parts, such as the crankshaft, should be covered with oil-soaked rags that will hold dust and particles of abrasive. The cylinders should he cleaned carefully after honing, as any abrasive material left behind will damage the engine.

Hones are driven by a portable electric drill. This rotates the hone while the operator moves it up and down in the cylinder bore to produce a cross-hatch pattern on the cylinder wall (Figure 5.7).

·              When using a hone, the vertical movement must suit the speed of the drill to produce the correct angle in the pattern.

Rigid hone
Fig 5.8
A rigid cylinder hone has four stones, or blades, each mounted in a carrier (Figure 5.8). The carriers can be expanded by a knurled nut at the top of the hone to bring the stones against the walls of the cylinder. 
The hone is used as follows:
1. Place the hone in the cylinder and expand the stones against the unworn part of the cylinder.
2. Shake the hone to make sure that all the stones contact the cylinder wall.
3. If the hone shakes, remove it and pack behind the low stones to get rid of the shake.
4. Operate the hone for about two strokes, and again try the hone for shake. Check it in another bore and. if satisfactory, proceed to hone. Lubricate the stones during use.
5. Use a slow drill speed and adjust the up-and-down movement so that the desired cross-hatch pattern is obtained. If the vertical movement is too slow or too short, the angle of the pattern will be too flat.

Figure 5.9
Brush-type hone
This type of hone consists of a central shaft with a cylindrical brush head. Small abrasive beads are attached to the ends of nylon filaments to form the head of the brush (Figure 5.9). The hones are made in a number of sizes with different abrasives. Each hone size is suitable for a range of cylinder sizes.
The hone is simple in design and use, requiring no adjustment. Through centrifugal action, the hone is self-centering and self-aligning in the cylinder bore.
The hone is driven by an electric drill and operated up and down the cylinder to produce the desired crosshatch pattern.

Plateau honing
Fig 5.10 (a & b)
Both rigid and brush-type honing methods described above produce sharp peaks when viewed under close magnification (Figure 5. 10(a)). These can create problems with some low-ring wall tension rings. Plateau honing takes the sharp tops off the peaks to provide a plateau-like finish (Figure 5.10(b)). A rigid hone with nylon filaments is used.

Cleaning cylinders after honing
When cylinders are honed or deglazed, the bores must be thoroughly cleaned before the engine is reassembled. Unless this is done, abrasive will remain in the engine and will cause rapid wear of rings, cylinder walls and bearings.
A recommended cleaning method is:
I. Clean the bores with detergent and water and a stiff brush.
2. Rinse with clean water and dry with a clean cloth.
3. Wipe the bores with a clean cloth that has been dipped in engine oil.

·   If the engine has not been dismantled, the crankshaft must be suitably covered before honing and bore cleaning.

See cylinders re-boring>>>>>>>>>>>>>>

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