Figure 84. Assembly drawing of the piston with connecting rod, as they are installed in a standard engine with 8 or 12 valves. On DOHC and diesel engines, the piston pin is held in place by one retaining ring on each side. 1. Piston ring; 2. Piston; 3 Piston pin; 4. Connecting rod bolt; 5. Connecting rod; 6. Connecting rod nut; 7. Connecting rod bearing shell
Check all details thoroughly. If there are cutter marks, scratches or wear on the parts, they must be replaced.
Measure the clearance at the height of the piston rings in the piston grooves by inserting the piston rings one by one into the corresponding grooves (figure 85). Using a template, determine the gap between the ring surface and the plane of the groove in the piston. The nominal gap size is 0.03-0.07 mm for the top piston ring and the second ring. These values apply to a gasoline engine. In a diesel engine, the gap at the top ring is 0.05-0.09 mm, and at the second ring it is 0.04-0.08 mm. The wear limit is approximately 0.15 mm (gasoline engine) or 0.20 mm (diesel engine). If the wear limit is exceeded, either the rings or the pistons need to be replaced. If new pistons are installed, the rings must also be changed.
Figure 85. Measuring the clearance along the height of the piston ring in the piston groove.
Next, you need to insert all the piston rings from the bottom of the crankcase into the cylinder holes one by one. According to the drawing 86 Insert rings into pistons with pistons upside down. This will ensure that they fall directly into the holes.
Figure 86. Measuring the piston ring thermal clearance.
Insert the template into the gap between the two edges of the rings to measure the piston ring clearance (figure 86). If the data in the tables of parameters and dimensions are exceeded, the piston rings must be replaced. Remember that the thermal clearances for all rings in gasoline and diesel engines are the same. If the gap is too small (for new rings, for example, which need to be measured) you can process the edges of the rings. To do this, you need to clamp a personal (finishing) file in a vice, as shown in the picture 87. Check the piston pins for wear or corrosion. If even just one connecting rod is not in perfect condition, the entire set must be replaced. Check the connecting rod bearing nuts in a connecting rod testing device for twisting or bending, which should be minimal. Figure 86 shows this device. If such a device is not available, then the connecting rods must be checked in a workshop.
Figure 87. Reduction of the piston ring thermal clearance for new rings.
Figure 88. Checking the connecting rod for warpage or twisting.