Operational Information

The Medium Speed 4 Stroke Trunk Piston Engine

The Connecting Rod



The connecting rod in a medium speed 4 stroke engine is subject to an inertia whip loading due to the mass of the con rod swinging about the piston pin. (Because of the lower speed of a 2 stroke engine, the whip loading is not large enough to influence the design of the con rod) Added to this, the inertia loads due to the mass of the reciprocating parts cause a stress reversal from high compressive stress (during power and compression stroke) to a low tensile stress between the exhaust and inlet strokes.

This loading of the rod influences its design, and to withstand the loading described above,  connecting rods are often forged from a manganese molybdenum steel in an I or H section which reduces its mass from one made of round section steel (and thus reduces the whip loading) while maintaining strength. This is not always the case, as can be seen from the pictures shown, and often a round section rod is of sufficient strength.


Because of the large diameter of the crankpin to increase bearing area and decrease bearing load, the width of the bottom end of the con rod is greater than the diameter of the cylinder liner.

So that the piston can be withdrawn from the liner, 3 different designs are used:

  • The con rod can be fitted with a marine palm as in the photo above.

  • The con rod can be split in two parts as shown opposite.

  • The bottom end  can be split obliquely. Serrations are used to locate the two halves relative to one another.


The advantage of using a vee engine is that the overall length of the engine is reduced for a given power output.

If a normal bottom end arrangement is used then the con rods must be placed side by side which means the opposite cylinders are offset. The crankpins must be long enough to accommodate two bottom ends side by side, and of large enough diameter to resist bending. The increased length of the crankshaft means a longer engine.

Two alternative arrangements are pictured (right). Both allow the cylinders to be opposite one another. However, both arrangements restrict access to the crankcase, and because the design of the bottom ends are different, more spares have to be carried.


Because of the stress reversal mentioned above, bottom end bolts have a limited life. This varies from engine to engine, but is generally around 12-15000 hours. If a bottom end bolt was to fail in operation, then the results would be disastrous.

Bottom end bolts should be treated with care when removed from the engine during overhauls. They should be inspected for any damage to the surface from which a crack could start. This damage could be due to corrosion (water in LO) or because of incorrect handling.

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