Not sure if the end needs to be hemi or not. My guess is that hemi is best because it better fits the end contour of the glass tube and will more completely move the air out of the glass tube, which is what the displacer needs to do...I collected some aluminium cigar tubes for this... but have not made an engine... yet...
- Does the end need to be hemispherical? - or is it just aesthetics? Does look good anyway.
- Do you fill the displacer with wire wool or anything? A model I bought told me to do so, but didn't give any clues as to "how much" wire wool to use... Any suggestions?
Nothing inside the displacer; just hollow. Needs to be as light as possible so it doesn't create any more friction when running as possible. Also needs to be air tight.
I've made three of these engines now and, knock on wood, didn't have any problems making the displacer tubes and I made them quite thin. Finish the inside first and then take light cuts to finish the OD. I formed the inside radius with a round end end mill. I also made up a steel plug with a matching radius on the end to go inside for turning it around and forming the outside radius. Just take your time and take light cuts. Measure frequently so you know how thin you are getting. If you hurry, you might be sorry...