So, I crossed the first hurdle already. The company that makes the O-Rings sold by Home Depot is Danco, and annoyingly they have their own numbering system for o-ring sizes and Home Depot doesn't list the dimensions of the o-rings on their site. Last night I contacted Danco to ask if they have a conversion chart that shows their size numbers relative to ASME DASH numbers and they already sent me a spreadsheet that lists those as well as the OD and ID for each size.
It's not sorted very well, but now I know which o-rings at home depot are usable for making plunger heads. Their "#043" size is the same as a Dash 216. I'm not currently sure if I'm going to use SCH40 pipe for the plunger tube, or just try to convert the door closer itself into one. The door closer comes with the o-ring already. The only part that cannot be trusted is the plunger head that comes with it.
Aren't superlative heads made from a fender washer that fits just inside 1-1/4" PVC?
So I'm assuming 1/2" OD PEX, but what kind of drywall anchor? That's intriguing just because drilling a centered hole in a 1/2" rod is challenging even with a drill press. Here I was thinking threaded rod would be the way to go.
Superlative heads use two sizes of rubber fender washers, a screw, and a washer. The sandwich of the rubber washers forces the 1-1/2" OD rubber washer into a conical shape to distort it down to an OD that's compatible with the ID of SCH40 pipe. You get something that looks sort of like a skirt seal, but slightly flimsier.
I wouldn't really want to use threaded rod due to the weight of it. It would be easy to construct one that way, but it might not perform all that well.
Here's the thread where makeitgo shows how he makes plunger rods out of PEX and drywall anchors.
The little critters of nature, they don't know that they're ugly. That's very funny, a fly marrying a bumble bee. I told you I'd shoot, but you didn't believe me. Why didn't you believe me?