UPDATE JAN 1, 2021: Aluminum Motorcraft T-stat housing has been on my 2008 XLT Ironman for over 1.5 yrs now & 15k+ miles, not one single drop of anti-freeze has leaked or been added to my cooling system since new housing was put on w/new t-stat & fresh coolant in summer 2019
******************************************************************************
Just thought I'd chime in with my experience in the Ford 4.0 aluminum thermostat housing... for anyone contemplating the aluminum housing swap & what make/version to go with, there's more the 2 options available. I just replaced my thermostat housing assembly for the 3rd time this past weekend, used the aluminum Motorcraft RT-1220 / AL2Z-8575-B on my 2008 XLT 4.0 Ironman edition.
After much research fall 2018 & reading all the pros & cons of the Simmons/Austekk models, price, etc I found that there was a Motorcraft version, although discontinued & still available from Ford dealers online & via eBay. The cost varies ($95-$120) more then the Austekk, but is less then the Simmons, plus it comes with a
new $22 Motorcraft screw-in temperature sensor, new bolts, new gaskets, all you need is to re-use or new thermostat. I'm quite happy with it so far, obviously it's something that Ford never should have made nylon/plastic to begin with. Replaced the original Motorcraft housing @ 79k miles, made a mistake & bought a Dorman replacement that started leaking in under 2k miles, replaced that with a Motorcraft OEM and that lasted about 30k miles b4 leaks @ the temperature sensor would not stop no matter the sensor, the o-ring, the thread tape... guessing the hole was just warped enough that nothing was going to work.
So now @ 112k miles hopefully this aluminum housing is the last one I need to replace. Reading the QC issues & comments about the other 2 brands being marketed lead me to the Motorcraft RT-1220. Couldn't find anything negative about it online anywhere, so figured had to be the better of the 3 options. Fit & finish was top-notch, no worries about gasket falling out of the bottom when installing, just clean all surfaces good. Only minor issue I had was getting system to "burp" air out, ran heat full blast for 10 mins, not quite hot. Started cold next morning, ran fan high & max heat defrost for 10 mins, felt the hot-hot heat about 3 mins into that cycle. Added a few more ounces of coolant, lightly rinsed off top of engine block & intake. So far no more smell of hot radiator fluid on my intake. Will post an update if the Motorcraft thermostat housing decides to leak down the road...