... 2" ~ 6-5/8" REG. GHJ series tri-cone rock bits with metal-sealing bearing system for oil well drilling ... of GHJ series tri-cone rock bits for well drilling: 1, Very soft with low compressive strength, such as soft marl, ... speed, 240~60 r/m.2, Soft with low comprssive strength, such as shale, clay, sadstne, soft limestone, etc. ... speed, 200~40 r/m. F/FC series tricone rock bits for oil well drilling