Theory, rod measuring, specifications.

Connecting rod is a part of the ICE crank mechanism that connects piston that performs reciprocating motion to a crankshaft (or crank) that performs rotational motion.

Depending on the engine type and cylinder arrangement, connecting rods are divided into 3 groups:

• rods in-line engine;
• rods double-row engine (V, W, VR types);
• rods of engines with a cross-head crank mechanism (as a rule, diesel ICEs where rods and piston are connected by a cross-head (slide bar)).

 A connecting rod consists of a top small end, a body and a bottom big end. A bronze bush that is used as a piston pin bearing pressed into the small end. Oil is fed into the bearing under pressure through the channel in the rod body or by splashing through holes in the rod top end.

Typical faults and defects of connecting rods:

• bottom end holes wear and deformation when the rod bearing is damaged (wear, overheating, breaking, twisting);
• rod body deformation in case of water hammer, valve, valve seat breaking.
• rod top end holes wear and deformation or rod top end bronze bush wear and deformation (piston pin bearing).

Rod repair is a complex process that requires accurate measuring and machining equipment.

Before starting rod repair its geometry shall be measured. Rod inspection starts from washing and drying.

Then rod bottom end covers are tightened to an appropriate torque recommended by the manufacturer. When no data is available, the following recommendations can be used as reference:

• 35 Nm for М8х1 thread;
• 50 Nm for М9х1 thread;
• 60 Nm for all thread types with the diameter of 10 mm.

Bottom end hole is measured with a hole gauge to the accuracy of 0.004 mm. Hole ovality shall not exceed 0.02 mm.

If the rod bottom end hole is worn out, wear degree shall be evaluated by comparison of the worn hole size with that for the unworn hole or with the reference data. When measuring the hole it should be kept in mind that in general wear is greater at the hole edges. Wear degree is the reference value during repair.

The easiest way to evaluate connecting rod deformation is to use a surface plate or a line gauge.

Devices used for rod deformation evaluation measure axis misalignment of the bottom and top end holes. Measuring accuracy is 0.005–0.010 mm at the length of 100 mm.

Connection rod axis straightening

Under the effect of forces during engine operation defects like rod axis deformation occur. During repair of bent connection rods the alignment of top and bottom end holes shall be maintained. Straightening is the primary method for deformation elimination.

Slightly deformed rods are straightened cold on a special apparatus. For excessive axis bending, localized heating of the rod body with an open fire up to 450 °C is used followed by straightening. Twisting is eliminated using levers.

Rod straightening is divided into the following types:

• Straightening of axis parallel to holes.

• Straightening of axis perpendicular to holes.

• Elimination of deformations formed during rod twisting.

After straightening, heat treatment in a kiln at 180–200 °C for 3–4 hours is performed to reduce residual stress.

Rod machining

When a defective connection rod is submitted to repair, measuring shall be performed. Rod axis bending, centre-to-centre distance between the top and bottom end, as well as inside diameter of the rod top and bottom end shall be measured. Measuring is performed as shown in the figure

Rod bottom end is machined when assembled with a clip. Rod clip is tightened with bolts (nuts) to a torque according to the manufacturer data. During disassembly, inspection and assembly, the rod and the clip shall be kept as a set. The clip is aligned on finished surfaces of rod bolts.

After bottom end measuring, clip slot surface is tightened within 0.15–0.2 mm, and then a slot is bored out with allowance for honing. If 70 % of the surface area of bush installation diameter is repaired, such surface is considered to be serviceable.

Top rod end bush that is used as a piston pin friction bearing is replaced. An old bronze bush is pressed out on a pressing machine using a drift pin and rings with an inside diameter exceeding the bush outside diameter.

A new bush with the outside diameter interference of 0.06–0.08 mm is installed into the rod top end hole using liquid nitrogen. The bush is then smooth broached and bored out on a boring machine with allowance for honing of 0.03–0.05 mm.

Finish rod machining involves rod top and bottom end honing on a special hone machine Sunnen LBB 1660. This machine is used for machining inside surfaces with an accuracy specified by the manufacturer.

Machining is complete. Inspection of acquired sizes is performed. Data is recorded into the operating process chart.