Hi, further to Samn40's reply the attached photo shows the dimensions of what is for us the MF 163 Control Spring Nut Wrench. Time spent making one will be well rewarded especially if it is likely to be used on more than one tractor.
You can do as Sam suggestes or my normal way is to tack weld the rod head to the cup and use a stilson to unscrew the rod. You will need a lathe to remove the weld after. The thread is very fine so it is vital that you use a penetrating fluid such as WD40 immediately you have movement. There is a very real possibility that the parts will 'pick-up' and you will need to replacve both. When reassembling the rod MUST be adjusted so that the spring is tight to turn BY HAND. Don't forget to fit the rubber boot!
Follow remaining instructions as per the workshop manual.
DavidP, South Wales