In any sealing application, the tolerances of all the parts in contact with the o-ring must be considered in order to create an effective seal. The combination of these tolerances is the tolerance stack-up.
Illustration 5.5 below shows a situation where the o-ring cross section tolerance is ± 0.003″, the groove diameter tolerance is ± 0.002″, and the bore diameter tolerance is ± 0.001″. In this example the metal and o-ring dimensions can vary up to 0.012″. If the nominal o-ring size is 0.030″, it is easy to see that the tolerance stack-up is nearly half the size of the o-ring.
This can result in too much or too little compression which can cause the o-ring to fail.