I made the back of the piston tonight. It took about 5 hours. Turned out perfectly though.
The o-ring guide I’ve been looking at recommends about 20% compression of the o-ring (.110 for my 1/8″ o-rings). That fit was a bit tight on my last part, so I decreased the compression to 10% (.124). My pipe is 3.745 ID, so I turned the bottom of the o-ring groove to a diameter of 3.497. It still felt too tight, but I applied some oil and now it slides pretty easily.
I had to take off a lot of material…
Another thing I just noticed. This 4″ PVC pipe fits perfectly over my 4″ OD aluminum tube. This might come in handy later.