... ", 6-5/8" REG. GHJ series tri-cone rock bits with metal-sealing bearing system for oil well drilling.Main structure features ... , 300~60 r/m.2, Soft with low comprssive strength, such as shale, clay, sadstne, soft limestone, etc. Recommended Bit type: GHJ517 ... ~60 r/m.3, Medium soft with hard abrasive treak, such as hard shale, soft limestone, sandstone, etc. Recommended Bit type: GHJ537 ...