Excessive flexibility in the fiber strength testing mechanism of the Spinlab 900B HVI system has created controversy for several years. Initially, the HVI specifications required that the strength measurement employ a constant rate of displacement. A tensile testing mechanism which deflects during strength testing will produce a fiber test similar to a constant rate of loading device. To identify major sources of flexing, we removed the mechanism from the instrument and mounted it on a rigid work bench. The mechanism was loaded in tension with dead weights. Deflections were measured with a precision dial indicator. We found that 63% of the deflection under load, was due to flexing of the breaker arms while 9.6% was due to flexing in each of two connecting yokes. The load cell, used to measure tensile force was a source of 17.8% deflection. Ball bearings used to pivot the assembly were found to be a considerable source of free play which will cause randomness in the strength signal.