The chocolate chamber is where all the chocolate is stored and compressed in.
For my chamber I used a Ø60mm aluminium tube with wall thickness 10mm. the length I decided on is 150mm but could be anything from 50mm up.(Couldn't be much shorter because piston is large). I bought this peice of aluminium from ebay seller forward metals (http://www.ebay.co.uk/itm/Aluminium-Tube-60mm-Diam-x-10mm-Wall-x-250mm-Long?item=150337213831&cmd=ViewItem&_trksid=p5197.m7&_trkparms=algo%3DLVI%26itu%3DUCI%26otn%3D1%26po%3DLVI%26ps%3D63%26clkid%3D8273219770097453398
As you can imagine, (not very clear in picture), this is quite a chunky piece of aluminium. It has to be so chunky so that holes can be drilled in the ends to hold the mounts and extrusion plates. Both sets of holes consist of 4 drilled and tapped M6 holes.
drilling these holes straight proved to be quite tricky for me, as the milling machine I had access too was a bit too small with the vise already on it which is why in the photo there are two vises on the bed(the old one is smaller).
Whenever doing low volume tapping operations after drilling on the milling machine, I start the thread off in the machine as to getit straight from the beggining. I think the photos show this operation clearly but do leave a comment if you have trouble understanding it.