The piston must have correct clearance in the cylinder to avoid piston slap, the compression rings must seal to prevent pressure loss during compression and combustion, the oil ring must control the oil on the cylinder walls, and the piston pin must be the correct fit to prevent noise.
The connecting-rod bearings must not be damaged or worn, and the crank-pin journals must be in good condition to prevent oil loss and noise.
The following are points that relate to dismantling a piston and connecting-rod assembly:
1. Remove the piston rings by winding them out of the grooves in the piston. A thick feeler gauge used under the rings will help.
2. Push the halves of the bearing from the connecting rod and cap. Check for difference between the upper and lower halves. The upper half of the bearing may have an oil hole.
3. Check the piston pin by trying to move the connecting rod in relation to the piston.
4. If the piston is to be removed from the connecting rod, check for markings that identify the front of the piston in relation to the connecting rod.
5. Separate the piston from the connecting rod by removing the piston pin. Depending on the type, the pin may have to be tapped or pressed out.
6. Temporarily install the cap on the connecting rod so that the bearing and all the other parts of the assembly are together.
7. Clean the various parts and check for wear.
Removing and installing a floating piston pin is shown in Figure 7.2. This has circlips that have to be removed from the piston before the piston pin can be removed.
The piston pin can be tapped out, using a light hammer and a punch. The piston must be suitably supported to prevent it from being damaged and the punch should have a pilot that fits into the hollow piston pin.
In some cases, a worn bush in the eye of a connecting rod can be replaced by pressing out the old bush and pressing in a new one. Figure 7.3 shows this operation for a connecting rod of a diesel engine. This is a rugged connecting rod with a relatively thick bush. The eye of the rod must be supported and a suitable pressing tool used. The bush has
Before installing the piston pin, the piston should be heated so that the pin can be installed without damage. New circlips should be used.
(a) the circlips are removed with a small
screwdriver (b) the piston pin is installed to a heated piston (c) new circlips are fitted
A pressing tool and a supporting tool are needed when removing or installing piston pins that are a press fit.
Figure 7.4 shows how the piston is being supported while the piston pin is being pressed out of the connecting rod. The pin must not be driven out. Before removing a piston pin, note its exact position in the connecting rod so that it can be replaced in the same position.
The pressing tool and support arc used to press the pin back into the connecting rod. This must be done quickly so that the pin is in its correct position before the connecting rod cools and contracts.
To heat the piston, place it in a container of water and heat the water to almost boiling point (Figure 7.5). This will expand the piston and make the piston pin easier to remove. This can be done before dismantling and also before reassembling.
A piston pin that is fitted too tightly could cause distortion, while a pin that is too loose could cause a knock.
Engine manufacturers specify clearances, such as 0.002—0.008 mm between the piston pin and the piston, with a wear limit of 0.02 mm. These are very small clearances. The small clearances are possible by having the piston pin a selective fit in the piston. The piston pin and the hole in the piston have extremely fine surface-finishes and this reduces wear.
There are also ways of describing the fits between parts — three descriptions of the fit of piston pins are illustrated in Figure 7.6. In order of tightness, these are hand-push fit, thumb-push fit and finger-push fit. There are other descriptions that can be used, such as tap fit, light-tap fit and drive fit.