Not being 100% happy with the threaded C frame on my Zero, constantly having to adjust the 510 between atomisers, and some not even holding I decided to do something about it. After a bit of research I plumped for the varitube spring loaded 510. Having completed this I can safely say, the C frame is stainless steel. 4 brand new twist drill bits reduced to molten slag. It's not pretty under there. You can't solder to stainless with a simple iron, and you will burn your hand if you use a blowtorch iron and forget that metal conducts heat as well as electricity. (Still fecking sore) However it is possible. And people with better skills than me would find it very straightforward. I would do it again, but I would get the frame drilled out at a machine shop.