Thank you sirpedrosa.
Thanks for looking into this. I agree that It makes no sense for Lister to be making a metric keyway in the 1930's given the imperial system was widely used. My engine was built in 1939 based on the serial number information. The only parts on the mine that have metric components (as far as I have seen) are the fuel pumps that have a metric banjo. However, I have seen other folks mention the confusion around the crank keyway and 14mm. It appears you were able to machine your own keys and fit them accurately using the 9/16" stock you purchased. So at least I have one other person that has done this
I have come to the conclusion that the Keys that I purchased are useless paper weights given how far below 9/16" they are. I placed a new order to get a set from the UK. They are actually 9/16". It may be helpful for those CS owners who are looking to replace their Gib Keys to ask for the measurement of width before purchasing.
The specifications for tapered key design are clear and a wide gap in the keyway is not acceptable. I have cross checked this across many references including DIN, British and US standards of the past and present. A key without correct tolerances on the width will be problematic independent of how well the taper is matched. Any forces that arise due to slightest flywheel imbalances will cause the key to come loose unless it was driven in by excessive force which caused deformation to the hub.
Moe