I have an old Kaweco Dia 805 (late 1950s or earlier is my guess). When I got it (ebay), the cap was loose but after soaking the pen in water to clean it, the cap fit perfectly.
This was temporary. After a few months the cap is loose again - so loose it falls off. Curiously the blind cap does not have this problem only the section cap.
The cap of the pen looks to be plastic - as is the body - but the thread area below the ink window is faded brown.
Could this be hard rubber and water absorbant?
Is there any non-harmful substance, apart from water, that will keep the threads 'plump' and well fitting for a long period?
Edited by AidenMark, 14 April 2020 - 09:22.