It'll obviously have a 1mm gap either side of the pole but easily padded out with a thin cloth or sponge if you want it to be perfectly snug. Otherwise it'll just move a tiny bit (barely noticeable) and spin more easily as less friction.
By Shamus G on 30 March 2017
It is about 18 cm (7") I have had mine about 3 year now and had no trouble with it. This is the depth the my Rotary washing line post drops inside the spike I think that's what you mean. I don't know where they got 20mm from that's is only 3/4 of an inch.
By Vinylnuts on 30 September 2015