I believe its 5/8 unf certain this is the adaptor thats comes with the Tim 52mm gauge with capillary sender and it fitted straight into the 200 disco stat housing
I hope its the same in the def 200 as i have another gauge coming!
If you wait until the end of the week i will try and confirm as it should be here this week hopefully!
But if you want to look its the Tim gauges on e bay