We felt that the C-7 core bit was optimum for this leg and formation. A C-57 might have performed better than the C-7 in some lithologies (in the coarse-grained gabbros) but would have been more vulnerable to cutter damage in the finer grained, foliated rocks. The bearings performed well and never exhibited any potential for catastrophic failure. Bit life on all runs was determined by wear on the ID nose cutters. These carbide buttons wore significantly even on the bit with only 8+ rotating hours. Eventually, the buttons were completely worn back into the matrix material. Interestingly enough, the bits continued to drill ahead at reasonable penetration rates and recovered gauge core with the nose buttons completely gone. For future drilling at this site, consideration should be given to applying a diamond coating to the carbide buttons to see if cutter life could be extended. In general, bit performance was excellent.
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