10.4.1.2. Pistons

Diameter of pistons of various classes measured perpendicularly to an axis of a piston finger at distance of 51,5 mm from the piston bottom makes, mm:
class A...... 75,965–75,975
class B...... 75,975–75,985
class C...... 75,985–75,995
class D...... 75,995–76,005
class E...... 76,005–76,015

Fig. 10-29. Marking of the piston and rod: 1 — group of the piston on weight; 2 — an arrow for orientation of the piston in the cylinder; 3 — the repair size; 4 — a piston class; 5 — an opening class for a piston finger; 6 — an opening for an oil exit; 7 — number of the cylinder

Piston class (letter) and category of an opening for a piston finger (figure) are marked on the bottom of the piston (fig. 10-29). In the same place also the arrow 2 for orientation of the piston in the cylinder is marked. At assembly of the engine pistons should be installed so that the arrow was directed to a forward part of the engine (towards the camshaft drive).
Increase in diameter for the repair sizes of pistons makes 0,4 and 0,8 mm. On the bottoms of repair pistons marking in the form of a square 3 or a triangle is put. The square corresponds to increase in outer diameter by 0,8 mm, and a triangle — on 0,4 mm.
The settlement gap between the piston and the cylinder for new details makes 0,025–0,045 mm. The most admissible gap at wear — 0,15 mm.

Fig. 10-30. Installation of a piston finger on the A.60325 device for a press fitting:
1 — adaptation roller; 2 — a piston finger; 3 — a guide; 4 — the persistent screw; 5 — a remote ring

The piston finger is applied from the engine 2108. It is 6 mm shorter than a finger of engines 2103 and therefore at a press fitting of a piston finger in a rod it is necessary to use A.60325 adaptation with a remote ring of 5 (fig. 10-30). The ring has to have thickness of 4 mm, the outer diameter of 22 mm and internal — 15 mm.
Pistons are sorted by weight on three groups: normal, increased by 5 g and reduced by 5 g. To these groups there corresponds marking 1 (see fig. 10-29) on the piston bottom: "", "+" and "–". On the engine all pistons have to be one group on weight.