A specially-shaped jig or V-block is used — the V of the jig is shaped to sit on the piston pin. Small dowels that extend from the jig are checked against the faceplate to determine rod alignment (Figure 7.25).
Slide the rod along the mandrel until the dowels of the jig just touch the surface of the faceplate. If the dowels do not touch the plate, then the rod is misaligned.
The rod should be checked for bend and then for twist. To do this, the jig is mounted in two different positions on the piston pin:
1. with the dowels vertical. This will check for bend in the rod. If the rod is straight, both vertical dowels will be in contact with the faceplate (Figure 7.25(a)). If only one dowel contacts the faceplate, then the rod is bent.
2. with the dowels horizontal. This will check for twist in the rod. If the rod is true, and not twisted, both horizontal dowels will be in contact with the faceplate (Figure 7.25(b)). If only one dowel contacts the faceplate, then the rod is twisted.
Connecting rods that are slightly misaligned can be straightened in a special jig designed for this purpose. A press or bending bar can also be used.
When straightening a connecting rod, it should be bent a little beyond the point where it is straight and then bent back to obtain correct alignment. This relieves the stress in the rod and it is less likely to tend to take up a permanent set and, for this reason, doubtful or badly misaligned rods should be replaced.
The cause of a fault must be located and corrected so that the problem does not occur again. For damaged pistons and cylinders, do not just blame the part that failed, but consider the following as possible causes:
When diagnosing a problem, consider whether it is related to the whole engine, or whether it is confined to only one cylinder. Also, consider whether the problem is related to the piston, the piston rings, or the cylinder, although one usually affects the others.
When scuffing starts, the damage can spread, being aggravated by the rough surfaces until an area of the piston is affected as shown.
The following should be checked for possible causes of scuffing or other piston failure:
2. The lubrication system for correct pressure and type of oil.
4. The connecting rod for alignment.