an original conrod (XC32/XC38 steel) is enough for a strictly original rebuild, provided it is checked seriously and the bolts are replaced. As soon as there is engine tuning — an increase in compression ratio, displacement, revs or boost pressure — the 4340-steel forged conrod becomes essential: it offers about twice the yield strength of an XC38, fresh metrology and a pre-stressing treatment (shot peening). The real risk is not breakage, but

The question keeps coming up during a rebuild or engine tuning: original conrod, forged conrod or custom conrod? What are the real advantages? How do you know what you're buying and which points to validate? Should price guide the choice? And can you have forged conrods made to measure?

Original conrod vs forged conrod: the quick comparison

Original conrod: design, inspection and limits

The design of a conrod, whether original or forged, is always the same: the conrod body, the cap and the bolts.

A point of vocabulary first, because the terms are often reversed: the big end of the conrod is located at the bottom (crankshaft side) and the small end at the top (piston side). You could say the engine runs on its head.

Its role: to transmit to the crankshaft the forces received by the piston. As standard, conrods are generally made from an XC32 or XC38 type steel. There are two main families of gudgeon-pin mounting: pins tightened in the conrod, often fitted by thermal expansion, and pins free (full floating), held by two circlips.

In its design, the conrod must mainly resist two deformations: compression and buckling. Above all, it must keep its rigidity, which guarantees the proper operation of the engine on three points:

1. Hold the bearing in place. Contrary to the urban myth, the location tabs have no retaining power: it is the big-end diameter, slightly smaller than that of the bearings, that creates the pressure. The compression stress at tightening is about 100 to 150 N/mm² (source: Techniques de l'ingénieur). By keeping its geometry, the back of the bearing transmits heat to the conrod to dissipate it; a defect can create a hot spot and destroy the bearing.
2. Maintain the operating clearance bearing → crankpin (about 5 to 6/100). Otherwise: material tear-out, overheating, seizing.
3. Absorb the load. In extreme cases, the conrod breaks and makes a nice window in the block.

The bolts work in tension when the piston reaches TDC. Every two crankshaft revolutions, the conrod undergoes a compression stress; on each revolution, the bolts take a tensile load. At 4000 rpm, the crankshaft turns nearly 67 times per second… hence the importance of a quality conrod.

How to inspect an original conrod?

1. The big-end bore and its ovality (tolerance ≈ 1/100), bolts torqued to spec. It is this dimension that holds the bearing.
2. The small-end bore (tolerance ≈ 0.5/100).
3. Centre distance (tolerance 5/100).
4. The squareness and the correct centering of the cap (centering bush).
5. Always replace the original bolts (stretch, cracks: impossible to be sure of their condition).
6. The condition of the big-end housing : possible presence of micro-welds (material transfer from the back of the old bearing). In this case, the conrod must be replaced.
7. The weight of the assembly, to be compared with the others (original tolerance ≈ 3 g).

These checks take a fair amount of time, to which is added the purchase of conrod bolts (when they are available). The question of moving to a forged conrod then arises — let's first look at the limits of the original.

The limits of original conrods

Original conrods are generally made of XC32 or XC38 (source: Combustion Engine Technology, Étai). Two characteristics matter: the yield strength (Re) and the ultimate tensile stress (Rm).

• XC32 : Re 430 N/mm² — Rm 570 N/mm²
• 4340 (AISI standard, forged conrod): Re 950 N/mm² — Rm 1100 N/mm²

The gap in the strength of the pure material is already clear. In tuning, the question no longer arises: a greater pressure is exerted on the piston (higher compression ratio, larger displacement, better filling, more fuel).

Add the increase in revs: the piston speed rises and works the conrod and its bolts harder, in tension as in compression — and the load increases with the square square of the speed (kinetic energy = ½ M·V²). On tuned turbo engines, the increased air and fuel supply generates a much greater stress.

What is shot peening?

Shot peening (pre-stress shot blasting) is a projection of calibrated shot that finely 'hammers' the conrod and modifies its mechanical properties. Result: an extended fatigue life and better overall strength.

The advantages of the forged conrod

1. A new conrod: no risk of micro-welds or previous deposits.
2. Fresh metrology, with no deformation due to use.
3. Conrods balanced to the gram.
4. A forged steel with far superior mechanical properties.
5. A shot peening treatment.

Our 4340-steel forged conrods, by model:

Your model is not listed? Custom manufacturing is possible (see below).

Forged conrods: how to choose them?

Buyers' number-one fear is breakage. But the real risk is not there. It is very rare to see a broken forged conrod: the only two cases encountered were due to an incompressible liquid entering the chamber (a seized high-flow injector). The valves close at TDC, the liquid conforms to the piston; with no more room, the conrod bends then breaks, leaving a hole in the block. (Photos below: these are not Mespiecesauto conrods, but faced with a chamber full of liquid the result would be identical.)

The right reflexes before buying

• Who has these conrods manufactured? What tolerances? A simple reseller does not control production: they sell what they buy. Don't trust a 'racing' brand or a nice logo.
• Metrology, the major risk. Check that the dimensions match the original in every respect. Caution: if your big-end is offset as standard, confirm that this is also the case on the forged one (otherwise poor guidance: e.g. R8 Gordini, Alfa Romeo V6).
• Avoid the quick purchase without knowing the seller. Price does not determine quality — nor does a 'racing' logo.

Having forged conrods made to measure: is it possible?

Mespiecesauto has been making custom conrods for nearly 10 years, on a very wide range of engines. A few points to know:

• Lead times: drawing, validation, tooling, die, machining and inspection. Allow about 1 month to 1 and a half months.
• Grouping: we prefer to bring together several owners of the same model. From 5 to 6 sets, the unit price can drop to €400 incl. VAT.

FAQ — Original or forged conrod