Different attributes make
con rod bolts critical fasteners inside the engine. One of these is
double tightening, which guarantees a consistent axial load distribution.
In fact, during the engine’s production cycle, the con rod bolts must be tightened twice in the
plastic field, just like the
main bearing cap bolts.
This means that during tightening the bolt is drawn beyond the limit that it can guarantee in terms of elasticity, and thus it experiences a plastic
deformation. Once unscrewed, the bolt will have a different appearance than before, slightly longer.
Con rod bolts are critical also because they are
among the most highly stressed bolts of the engine.
First of all, they are subjected to the same stresses as the
cylinder head bolts, that is to say, the
thermal expansions due to the combustions in the engine’s combustion chamber.
The piston connected to the con rod releases each burst onto this component and, as a result, onto the
con rod bolts.
But that’s not all. Con rod bolts are subjected to three-dimensional forces. Whereas the main bearing and head (and therefore their respective bolts) are subjected to a typically axial task, the con rod carries out both a piston up and down movement and a rotary movement of the camshaft.
This gives rise to the most peculiar characteristic of the con rod bolts: moving components pulled along with every engine revolution.
These extreme stresses reveal the extreme importance of con rod bolts.
They must be small fasteners to get low inertia while maintaining an excellent performance level in racing or special power vehicles. Although they are minute, they must be strong enough to prevent the risk of breakage, upon penalty of damage to the engine.