Pop,

The end plug on the HPOP is an 18MM threaded bolt that looks like a larger version of any drain plug you've ever encountered. As you'll likely see in the $29 o-ring kit you buy from Ford, the plug should be torqued to 25 ft-lbs upon reinstallation (along with some loctite).

I had considered trying to locate some generic o-rings that were the same size, but now that I've seen the o-ring that comes in the kit, I don't think I would advise someone to go this route. The rubber in the new O-ring kit seems to be a much harder compound than most run-of-the-mill O-rings I've seen. So spend the $29 here (assuming this is the problem and hopefully you won't have to scrub your driveway every week.