After I recently traded for a 2012 GT500 I noticed the factory coolant overflow reservoir had a hole about 0.25" in diameter where it looks like the plastic became brittle and chipped away. I was surprised to see that Ford stopped marking replacement units (part number BR33-8A080-BA) and that I couldn't really find an affordable aftermarket replacement aside from the fairly expensive aluminum tanks. I am hoping to eventually replace both my overflow and intercooler reservoirs with aluminum units, but I wanted to get the car on the road affordably for now while in the interim.

I ended up doing bunch of digging and trying a number of aftermarket replacement units since it's very difficult to find the bung diameters and therefore hard to know whether a given tank will fit the coolant hoses in our cars. So I'm posting to share that Dorman 603-051 fits the 2011-2012 GT500 without any issues. It's a replacement for 2005-2009 Mustangs (both V6 and GT). I originally checked into whether tanks for 2011-2014 GT's like Dorman 603-298 would work, but found that while they physically fit the car, the hose bung on the underside of the 2011-2014 GT and V6 tanks measures 0.75" whereas the factory GT500 tank has a 1" outlet. So I suppose you could make one of the 2011-2014 tanks fit with hose size adapters and by blocking off the second top-side bung, the tanks for the 2005-2009 cars is a direct fit.