A well drilling head comprises a housing having a sidewall structure defining a central bore and a bearing assembly removably seated within the central bore of the housing. The bearing assembly includes an outer barrel having a central bore, an inner barrel at least partially disposed within the central bore of the outer barrel and bearing units coupled between the barrels for providing concentric alignment of the barrels and allowing rotation therebetween. The inner barrel includes a central bore and a plurality of cooling structures protruding from a surface thereof within the central bore.