Replace pistons when the upper piston ring groove or piston skirt is worn
In partially worn cylinders, install pistons of the same size (nominal or overhaul) as the pistons that previously worked in this engine.
It is advisable to choose a set of larger pistons to reduce the gap between the piston skirt and the cylinder bore.
In this case, check the gap between the piston skirt and the cylinder mirror in the lower, least worn part of the cylinder. Do not allow the clearance in this part of the cylinder to decrease to less than 0.02 mm.
Pistons are supplied as spare parts along with matched piston pins and circlips (see Table 2 Dimensions of UAZ-3151 engine parts according to the catalog).
For selection, pistons of nominal size are sorted by the outer diameter of the skirt.
On the bottoms of the pistons, the letter designations of the size group are stamped, which are indicated in table. 1 Dimensions of UAZ-3151 engine parts according to the catalog
On oversized pistons, their diameter is also stamped.
In addition to selecting pistons for cylinder liners by skirt diameter, they are also selected by weight.
The difference in weight between the lightest and heaviest piston for one engine must not exceed 4 g.
When assembling, install pistons in liners of the same group.
Fig. 1. Device for installing a piston with rings in a cylinder
Install the pistons to the cylinders using the Model 59-85 tool shown in fig. 1.
When installing pistons in cylinders, the “forward” mark cast on the piston should face the front of the engine, on a piston with a split skirt, the “back” mark should be towards the clutch housing.
On all pistons of repair sizes, holes in the bosses for the piston pin are made of a nominal size, broken down into groups.
If necessary, these holes are bored or reamed to the nearest repair size with a tolerance of -0.005 -0.015 mm.
The taper and ovality of the hole is no more than 0.0025 mm.
When processing, ensure that the axis of the hole is perpendicular to the axis of the piston, the permissible deviation is no more than 0.04 mm over a length of 100 mm.