Repair of connecting rods comes down to replacing the upper head bushing and then processing it to fit a piston pin of nominal size or to machining the bushing in the connecting rod for a repair size pin
Spare parts are supplied with bushings of the same size, made of bronze tape ОЦС4–4–2.5, 1 mm thick.
When pressing a new bushing into the connecting rod, ensure that the hole in the bushing matches the hole in the upper head of the connecting rod.
The holes are used to supply lubricant to the piston pin.
After pressing the bushing, seal its inner surface with a smooth brooch to a diameter of 24.3+0.045 mm, and then ream or bore to the nominal or repair size with a tolerance of +0.007 -0.003 mm.
For example, expand or bore the bushing to fit a nominal size pin up to a diameter of 25 +0.007 –0.003 mm or a repair size pin up to a diameter of 25.20 +0.07 < sup>–0.003 mm.
The distance between the axes of the holes of the lower and upper connecting rod heads should be (168 ± 0.05) mm [(175 ± 0.05) mm for model 4218 engines];
The permissible misalignment of the axes in two mutually perpendicular planes over a length of 100 mm should be no more than 0.04 mm; ovality and taper should not exceed 0.005 mm.
In order to meet the specified dimensions and tolerances, turn the connecting rod end sleeve in the jig.
Fig. 1. Finishing the hole in the upper head of the connecting rod: 1 - holder; 2 - grinding head; 3 - clamp
After reaming, finish the hole on the special grinding head, holding the connecting rod in your hands (Fig. 1).
Set the grinding stones of the head with a micrometer screw to the required repair size.
Connecting rods with bearing holes in the lower head with an ovality of more than 0.05 mm must be replaced.
Replacement and repair of piston pins
Repair dimensions of piston pins and kit numbers are given in table. 2. Tolerances of the main parts of the UAZ-3151 engine
To replace piston pins without pre-machining the holes in the piston and in the upper head of the connecting rod, piston pins are used, increased in diameter by 0.08 mm.
The use of pins oversized by 0.12 mm and 0.20 mm requires pre-machining of holes in the piston bosses and in the upper head of the connecting rod as described above.
Fig. 2. Removing the piston pin circlip
Fig. 3. Device for pressing out and pressing in the piston pin: 1 - guide; 2 - finger; 3 - plunger
Before pressing out the piston pin, remove the piston pin circlips from the piston with pliers as shown in fig. 2.
Pull out and press in the pin on the fixture, as shown in fig. 3.
Before pressing out the pin, heat the piston in hot water to 70°C.
Repair of piston pins consists in grinding them from large repair sizes to smaller ones or in chrome plating, followed by processing to a nominal or repair size.
Fingers with breaks, chipping and cracks of any size and location, as well as traces of overheating (tint color) cannot be repaired.
Assembly of the connecting rod and piston group
Fig. 4. Piston pin selection
Select the piston pin to the upper head of the connecting rod with a gap of 0.0045 - 0.0095 mm.
At normal room temperature, the finger should move smoothly in the hole of the upper head of the connecting rod from the force of the thumb (Fig. 4).
The piston pin must be lightly lubricated with low-viscosity oil.
Install the pin into the piston with an interference of 0.0025–0.0075 mm.
In practice, the piston pin is selected in such a way that at normal room temperature (20°C) it would not enter the piston by hand, and when the piston is heated in hot water to a temperature of 70°C, it would enter it freely.
Therefore, before assembling, heat the piston in hot water up to 70 ° C.
Pressing in the pin without pre Excessive heating of the piston will lead to damage to the surface of the holes in the piston bosses, as well as to deformation of the piston itself.
Assemble the connecting rod and piston group using the same device as disassembly (see Fig. 3).
To ensure proper engine balance, the difference in weight between the pistons and connecting rods installed in the engine should not exceed 8 g.
The piston pin circlips should sit in their grooves with a slight amount of interference. Do not use used rings.
Install the piston rings on the piston as indicated in the article "How to replace piston rings UAZ-3151".
Given the difficulty of matching the piston pin to the piston and connecting rod (to ensure nominal fits), the pistons are delivered in spare parts complete with a piston pin, circlips and piston rings.