Is the surface that the thermostat sits within under the thermostat housing on the 5.9 magnum part of the block or is it a cover or piece that is a removable? My friend has an issue where his is leaking and the surface is all pitted out

 

Isn't the thermostat in the intake manifold? The top part is easily replaceable, it just bolts on. The base though, is part of the intake... can either try and fill and sand, or, get a different manifold.

 

Yeah upon some research it’s on the manifold and nobody sells it, what would you recommend filling with for sanding? luckily there’s like 8 2nd gen Dakota/durango at the pick n pull nearby so I’m gonna try there on the morning for a potential donor, not opposed to saving a buck and some time if filling and sanding is a solid fix

 

Something that will bond to aluminum, and can tolerate the temps. Probably some variety of JB Weld out there that would do the trick. Clean the surface REALLY well before applying whatever it is you use.

Thinkin' about it though, have you looked INSIDE the intake? Is it sucking oil in there? If it is, it's gotta come off anyway, so, might just as well replace it. (and fix the plenum gasket on the replacement manifold....)

 

Pics would help. The t stat sits in a little grove, The gasket sits above it holds it in place. As long as it's not deteriorated past where the t stat sits it should be fine. Silicone may work but let it set up before adding coolant.