In general, a FAST'S coupling is preferred by users because of it's long service life, whereas the Series H offers a lower cost alternative; FAST'S have larger lubrication capacity than o-ring style couplings, all-metal (nonperishable) seals, allow oil lubrication, and allow longer lubrication intervals. Series H (o-ring style) couplings have a larger bore capacity which can allow a smaller coupling in some cases.