The relationship between procedures A and B of the ASTM D 3945 diesel injector shear stability test has been examined with respect to sheardown rate constants and severity after extended shearing. Significant differences were found between Procedure A and procedure B. Tests of varying duration in the two instruments showed that the shear down rates are such that the standard 30 cycle test of Procedure A is equivalent to 40 cycles or twice the number of cycles in procedure B. The severities of the two instruments after extended duration tests were approximately equivalent for two common Viscosity Index improvers. However, for polymethacrylate (PMA) VI Improvers, the Procedure B apparatus was 20 percent more severe. On the basis of these results, it is recommended that procedures A and B in ASTM D 3945 be given separate methods.