... API best bearing system 20 years history Milled Tooth Rock Roller Bits: 7 7/8 to 14 1 ... tri-cone rock bits Rock formationBit typeBit sizeDrilling weightRotary speedmminchkN/mmr/minVery softSoft Marl, Clays, Sands, Salts etc.1187. ... /8441~735120~50softClays, Soft Shale, Marl, Sands, Salts etc.295.2~174.63 3/4-6 7/ ... 2539-83360-40medium hardHard Dolomite, Limestone, Hard Sandy, Rock etc.595.2~174.63 3/4-6 7 ...