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.