While generally reliable, trouble at first was encountered with the long-travel Walschaerts valve gear, and with the plate frames. While the valve gear problems were largely solved by reducing travel from the original 8 inches to inches, the plate frames continued to crack especially in the region of the firebox. While many repairs were undertaken to fix the frames, this problem was only solved by replacing the frame with the new design constructed for the KA and KB classes. This was only done as the replacement was required; as a result not all of the class received the new frames.