Forged Connecting Rods

The connecting rod is the link that turns the piston's straight-line motion into the crankshaft's rotation. Its small end carries the gudgeon pin (wrist pin) inside the piston; its big end wraps around the crankpin on bearing shells; and a slender shank joins the two. On most engines and compressors the big end is split, with a separate cap held on by bolts so the rod can be assembled around the crankshaft.

A rod must do three things at once: be strong enough for peak loads, stiff enough that the big end stays round under load so the bearing survives, and light enough that its own inertia does not add unnecessary load to itself, the crankshaft and the bearings. Those requirements pull in opposite directions, which is why rod design is a careful balance rather than simply making the part heavier.

The load reverses between tension and compression every cycle. That makes connecting rods a fatigue problem first and a strength problem second. Cracks, when they occur, typically start at stress concentrations: the transitions between shank and ends, the bolt holes and bosses at the big end, and any oil hole or surface defect.

Why the shank is an I-section

Most forged rods have an I- or H-section shank. The flanges of the I resist bending in the plane of rotation, while the overall section resists buckling under compression, all with far less mass than a solid rectangular section. Forging produces that section directly, with grain flowing along the flanges and web.

Rods can be cast, machined from billet, powder forged or conventionally forged. For engines and compressors with high cylinder pressures or long service lives, conventional forging remains the dominant route.

Forging's advantage is structural. Grain flows along the shank and curves around into each end, so the material's strongest direction follows the load path. There is no porosity at the shank transitions where fatigue cracks would start. See why forged parts are stronger.

Quenched and tempered alloy steels

For diesel engines, compressors and gensets, chromium-molybdenum steel quenched and tempered gives a strong, tough rod that tolerates the high peak loads and long service lives of that equipment. The grade's hardenability ensures the bosses and shank transitions develop properties right through their section.

Micro-alloyed and fracture-split steels

High-volume automotive production often uses micro-alloyed steels that reach strength by controlled cooling straight from forging, removing the quench and temper step. C70S6, a high-carbon micro-alloyed steel, is used for fracture-split rods: the big end is forged in one piece, notched, and then cracked apart under load. The rough fracture faces interlock perfectly when reassembled, locating the cap without dowels or machined joint faces. Fracture splitting requires dedicated equipment and is a high-volume technology.

Forging

Rods are closed die forged. Because they have a thin shank between two heavy ends, metal must be redistributed before finishing: fullering thins the centre and pushes material towards the ends, and edging gathers it where the small and big ends will form. A blocker impression then shapes the rod roughly before the finisher forms the I-section and ends. Flash is trimmed hot, and the web inside the big end is pierced out.

Some designs forge the cap and rod together and separate them later; others forge the cap as a separate part. Either way, the forging sequence is designed so grain flows around the big end rather than being cut where the bolts will pass.

Heat treatment and cleaning

Alloy steel rods are quenched and tempered to a specified hardness band; micro-alloyed rods are control-cooled. Shot blasting follows, and many high-duty rods are shot peened, which leaves compressive residual stress on the shank surface and raises fatigue strength.

Machining

1. 01Grinding or milling the side faces to establish datums and width
2. 02Rough boring the big and small ends
3. 03Drilling and tapping or reaming the bolt holes
4. 04Separating the cap by sawing, or by fracture splitting on fracture-split designs
5. 05Machining the joint faces and assembling the cap with bolts to specified torque
6. 06Finish boring and honing the big end with the cap fitted
7. 07Pressing in and finish boring the small end bush, where used
8. 08Drilling oil holes and deburring

The big end is finished with the cap assembled and torqued, because the bore must be round in its assembled, loaded condition. Our precision CNC machining page describes our machining capability.

Weight matching

In a multi-cylinder engine, rods of different weights create unbalanced forces and vibration. Rods are weighed after machining and sorted into weight classes so that a set fitted to one engine falls within the designer's specified band. Many specifications control the weight of the big end and small end separately, because the reciprocating mass at the small end and the rotating mass at the big end affect balance differently.

Bolts are part of the rod

Connecting rod bolts are among the most highly stressed fasteners in an engine. They must be the specified grade, tightened by the specified method — often torque plus angle — and usually replaced rather than reused. A correct rod with incorrect bolt tightening can fail as readily as a defective rod. See why bolts fail.

• Spectro verification of every incoming heat lot
• Forged weight sampling to confirm consistent die fill
• Hardness verification after heat treatment, on every batch
• 100% magnetic particle inspection for surface cracks and laps, particularly at shank transitions and bolt bosses
• Dimensional checks of centre distance, bores, bend, twist and faces
• Weight classification into specified bands
• Macroetch examination of grain flow during die validation, to ASTM E381
• PPAP documentation for automotive customers, with IATF 16949-aligned processes

A bent rod must be replaced, not straightened: bending indicates the material has yielded, and straightening leaves residual stress and possible cracking.

A rod that is easy to forge is also a rod that is consistent, sound and economical. Most connecting rods are forged lying flat, with the parting line running through the centre plane of the shank and ends, so the I-section forms symmetrically in the upper and lower dies.

• Draft on the I-beam flanges and bosses lets the rod release from the die; the machined side faces remove it where parallel faces are needed.
• Generous radii at the shank-to-end transitions improve metal flow and reduce stress concentration at exactly the places fatigue cracks start.
• Adequate web thickness in the I-section allows the web to fill; an excessively thin web chills against the die and resists flow.
• Machining allowance on bores and faces covers scale, draft and die mismatch without leaving the part undersize.
• Bolt boss geometry needs enough material around bolt holes that drilling does not break into highly stressed regions.
• Consistent mass distribution in the forging helps weight matching, because less material has to be machined off to bring rods into class.

We review rod drawings for forgeability before tooling is cut. See forging design guidelines.

The small end carries the gudgeon pin, and it is loaded in the same reversing pattern as the rest of the rod but with a small bearing area. Two broad arrangements are used.

On bushed rods, the bush is pressed in after heat treatment and then finish bored or honed so its bore is concentric with and parallel to the big end. Oil reaches the pin through a drilling or slot in the small end, whose edges are deburred so they do not become crack starters.

Many diesel engines, compressors and gensets remain in service long after the original manufacturer stops supplying parts. A connecting rod can be reproduced from a sample, provided both the geometry and the properties are captured.

1. 01Crack test the sample with magnetic particle inspection, and examine any failure to understand the cause.
2. 02Identify the steel by spectro analysis and map hardness to establish the original heat treatment.
3. 03Measure critical geometry — centre distance, bore sizes, side widths, bolt hole positions — compensating for wear using bearing and pin data.
4. 04Weigh the sample and, where a set is being replaced, establish the weight class required.
5. 05Issue a drawing for approval before tooling or material is committed.
6. 06Manufacture, weight match and inspect against the approved drawing.

Related components include forged crankshafts, camshafts and piston rods. We supply compressor and genset manufacturers as well as automotive customers. Send a drawing or sample and request a quote.