As a test model the Class 14 doesn't benefit from all the parts being finalised. Some are simply test shots that will do to prove the model but wouldn't be good enough for production.
Which is why the drive cranks are made from whitemetal. This might have been considered good enough in the old days but now we expect a bit more quality. Not that there is anything wrong with whitemetal per se but it isn't ideal for this sort of application. For a start when it comes out of the mould it shrinks. The shrinkage varies depending on the thickness of material. Real experts can allow for this but they cost a lot of money and so in the modelling world things can be a bit more hit and miss. In this case the crank throw isn't awfully accurate.
Now I could get away with this by reaming out the hole in the coupling rod a lot. The crank just needs to go around - drive can be supplied by one of the wheels instead. This works in other scales, not pretty but effective. In the larger scales though people might fancy driving the crank just like the real thing, which is one of the reasons milled steel is being investigated for this part.
Anyway, I still had to do something and decided that rather than try painting the crank in steel coloured paint which never looks convincing, I'd polish the whitmetal - it looks very different to the dull alloy you normally see with a little elbow grease. A few minutes with jewellers paper sticks and then some Brasso brought the metal to a shine. A coat of sating varnish and once lightly weathered it should look nice.