No substitutes: Polyurethane just can't be replaced.
Tags: Can I use butyl nitrate o rings o-rings orings instead of polyurethane automag AGD
Anyways, I had a terrible leak down the barrel, turned off when the trigger was pulled, changed the power tube spacer (o rings .010) multiple times (.220 .225 .230), and nothign worked. I was using butyl.
I switched it out with some 5 year old polyurethane orings i had laying around. BAM leak gone.
I also noticed that polyurethane was important for the regulator valve pin assembly (.006). I got it to work with butyl - eventually, but I feel poly would have solved all the problems.
So, I think the point of this thread is people should mention what parts of the valve simply can't be replaced with butyl - which is becoming more prevelant and polyurethane being more rare.
Also, people should mention what sorts of local stores carry polyurethane!