Straightening Connecting Rods

In refitting and reassembling connecting rods, and finally fitting pistons, rods may be bent or twisted, throwing upper bearing and lower bearing out of alignment with each other.

After pistons have been installed, rods must be checked and realigned as necessary. If a rod is bent or twisted, piston has a "cocked" relation to cylinder bore and the result is excessive noise and rapid wear.

Check rod alignment with Piston Squaring Plate, Part No. 96179-18, as shown in Figure 3-17. Be sure crankcase face is clean and free from burrs so that squaring plate seats fully.

Figure 3-17. Checking Rod Alignment NOTE

Piston skirt is cut away at bottom (belowpiston pin) for flywheel clearance, therefore, it cannot be used with squaring plate for checking rod alignment. Temporarily install special piston with flat bottom skirt, Part No. 96180-76, to check rod alignment.

If a rod is in perfect alignment piston bottom will rest squarely on plate when flywheels are turned so that crank pin is in forward and rear position. This check, to be accurate, depends upon checking with crank pin alternately in both forward and rear positions. It is the change of rod angle, resulting from changing crank pin from one position to the other that influences the seat of piston on squaring plate and thus indicates whether or not rod is in alignment.

Insert narrow strips of paper of equal thickness underneath piston, one on each side, below piston pin, as shown in Figure 3-17. Press piston down lightly with finger tips resting on center of piston head and pull first one paper, than the other, partially from underneath piston. If piston is perfectly square (rod in alignment), both will have the same amount of drag.

If rod proves to be out of alignment, it can be straightened by means of a bar inserted through piston pin, as shown in Figure 3-18. Use a bar with a diameter as close to the hole diameter in the piston pin as possible. The manner in which piston seats on squaring plate indicates as follows:

1. Piston high on same side, both crank pin positions; rod is bent.

2. Piston high on opposite sides as crank pin position is changed; rod is twisted.

3. Piston square or nearly square with crank pin in one position and high on one side with crank pin in other position; rod is bent and twisted.

Correct as follows:

1. To straighten a bent rod, insert straightening bai through piston pin hole on low side of piston and apply upward force.

2. To straighten a twisted rod, insert straightening bai through piston pin hole on high side of piston, and if cranl" pin position is to front apply force to rear - if crank pin position is to the rear apply force to front.

3. To straighten a bent and twisted rod (combination of a bend and twist) remove bend first and then remove twist.

After rods have been aligned check to see that pistons center in crankcase cylinder opening, without side pressure on upper rod ends. If further realigning is necessary to center pistons, correct by dressing offend of rod bushing on interfering side with a file. This allows the piston to shift slightly on rod to find a more suitable alignment of rod, piston, and cylinder bore.

0 0

Post a comment