Asymmetric field coils have been installed on Alcator C-Mod and used to establish the locked mode threshold and scaling for this compact size device. The magnitude and phase of the intrinsic field error has been identified and is found to be consistent with a comprehensive model of the sources of field errors. There proves to be no major difference between the fractional (m = 2,n = 1) field level (typically 10-4 of toroidal field) for mode locking on Alcator and that on JET (which has 4.3 times the linear dimensions) for typical densities on each machine. The results are consistent with a linear scaling of the total asymmetric field locking threshold with density, as has been observed elsewhere, are within a factor of 2 of the scaling previously deduced from the toroidal field scaling on JET or DIIID using Connor-Taylor constraints, and agree with specific dimensionless identity experiments on JET.