Original ISX flywheel bolts (Part number 3681326) were specified at a straight torque of 210 N·m (155 lb-ft) using a standard hex bolt. In high-horsepower applications (especially 550-600 HP with heavy clutch engagement or PTO drives), mechanics began reporting two catastrophic failures: bolt loosening leading to sheared heads, and the more dangerous "block fretting" (wear of the crankshaft flange due to micro-motion).
Ensure the flywheel housing is within the maximum allowable total indicator reading (typically 0.13 mm or 0.005 in) to prevent premature clutch and transmission wear. torque spec for flywheel bolts on isx cummins 251 updated