Following experience of occasional cracks appearing near the spring brackets of the Britannias and Clans, a substantial rearrangement took place in this area that resulted in the locomotive riding on three cast steel " sub-frames " carrying the ten front-most spring brackets and lengthened spring brackets behind the rear driven axle.

Following experience of occasional cracks appearing in the frame plates near the spring brackets, had the second batch of Class 6 Standard Pacifics been built, the chassis ' would have been rearranged to be similar to that used on the solitary Class 8 Pacific.

Following experience of occasional cracks appearing near the spring brackets had one or more further batches of Class 7 Standard Pacifics been built to complete the intended 91 engines the chassis ' would have been rearranged to be similar to that used on the solitary Class 8 Standard Pacific and that drawn up by Derby Drawing Office for the 2nd batch of Class 6 Standard Pacifics.

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