Characteristics of the crank mechanism (BMW 3 Series E21)

Pistons

The pistons are cast from aluminum alloy. Depending on the engine model, the piston bottoms can be flat or convex.

Piston brand: Mahle, Karl Schmidt (KS) or Nueral-Alcan.

Piston diameter

Piston diameters are measured at a distance "A" from the skirt.



Clearance between piston and cylinder, mm: 0.045.

Piston markings

On the bottom of the pistons there is a mark of the piston group by weight: "+" or "-".

Piston pins

The piston pin is made of steel and is ground. The pin rotates freely in the upper head of the connecting rod and in the piston bosses. It is held from axial displacement by retaining rings.

Offset of the axis of the hole for the piston pin in the piston bosses, mm: 1.0.

Piston pin outer diameter, mm:

• with white mark: 22^-0_-0,003;
• with black mark: 22^-0,003_-0,006.

Piston pin hole diameter, mm: 22^+0,004₀.

Clearance between piston pin and piston, nm:

• for pistons of Mahle and Nueral-Akan brands: 0.001-0.005;
• for KS brand pistons: 0.002-0.006.

Clearance between piston pin and connecting rod upper head bushing, mm:

• for fingers with white mark: 0.005-0.013
• for fingers with black mark: 0.008-0.016.

Piston rings

Characteristics of piston rings

* The numerator shows the values for Mahle and Nueral-Akan pistons, the denominator shows the values for KS pistons.

Spare parts include piston rings with the following repair dimensions: +0.10; +0,25; +0,50.

Connecting rods

The connecting rod is an I-section one, forged steel, with replaceable trimetallic liners. The lower connecting rod head cover is of straight section. The characteristics of the connecting rods of all engine models are the same.



Connecting rod length, mm: 135±0.1.
Hole diameter, mm:

• lower connecting rod head: 52+0.15;
• upper connecting rod head: 24+0.21.

Connecting rod upper head bushing diameter, mm:

• external: 24,060-24,100;
• internal: 22^+0,010_+0,005.

Misalignment of connecting rod head holes when measured at a distance of 150 mm from the connecting rod rod, mm, no more than: 0.04.

Maximum permissible longitudinal bending of the connecting rod: 0°30.

Difference in weight between connecting rods, g, no more than: 4.

Clearance between bearing shells and connecting rod journals, mm: 0.023-0.078.

Connecting rod bearing shells

Nominal thickness of liners, mm: 1.983-1.993.

Repair dimensions 1st: 2.108-2.118; 2nd: 2.233-2.243; 3rd: 2.358-2.368.

Crankshaft

Crankshaft - steel, forged, five-bearing (the middle main bearing is wider than the other main bearings). Design of crankshafts of engines of models "315", "316", "318", "318i" and "320", "320i" different.

Main bearing housing diameter, mm:

• for crankshaft with red index: 60^+0,010₀;
• for crankshaft with blue index: 60^+0,010_+0,019.

Characteristics of crankshafts

* The numerator shows the values for the crankshaft with the red index, and the denominator shows the values for the crankshaft with the blue index.



Permissible imbalance of a dynamically balanced crankshaft without a flywheel, gf cm, no more than: 50.

Crankshaft axial clearance, mm: 0.085-0.174.

Permissible runout of the middle main journal when supported by the outer main journals, mm, no more than: 0.1.

Flywheel

The flywheel is mounted on the crankshaft on a flange and is secured with eight bolts, and due to the presence of a centering pin, the flywheel can only be installed in one position.

Before pressing on the flywheel toothed rim, it is necessary to heat it up to a temperature of 200-230°C, then install it, directing the ends of the teeth from the engagement side to the engine, and press it until it stops. It is recommended to check the rim temperature with a thermochromic pencil.

The flywheel is statically balanced, the permissible imbalance should be no more than 15 g cm.
Flywheel runout, mm, no more than:

• when measured at a point on a diameter of 92 mm: 0.03;
• when measured at a point on a diameter of 100 mm: 0.1.

Maximum thickness of the metal layer removed when grinding the flywheel surface under the driven clutch disc, mm: 0.4+0.1.

Thickness of flywheel surface under clutch driven disk, mm, not less than: 14.5; 13.5 (for "320i").