Upper Thermostat Housing Question: 35mm vs 40mm

[RangerJoey]

Hello,
I'm flushing my radiator and figured I'd replace a few parts while drained. I picked up a Stant 195F thermostat, new O-Ring and Gasket. But I'm confused about the upper thermostat housing. On my '99 4.0L, RockAuto, Amazon, and even the part manufacturers themselves, (like Dorman), list a 35mm and a 40mm option that's compatible. I have no idea what the OEM size is, but when I look at the new upper radiator hose, the inner diameter is listed as 37mm. What gives?

I've had good experience before with Dorman, and their parts are:
902-1012 (40mm)
902-1007 (35mm)

One thing I think I've noticed is that the 35mm images aren't black. The 40mm units only look like they come in black, (which is what mine is. Well, that and a bunch of brown rust! )

Btw, I've watched a million videos on replacing this. About 99.9% is for the 4.0L SOHV. I feel so bad for you guys! While still a tight squeeze, this '99 isn't a comparison at all!

 

[G8orFord]

It's not too bad on the SOHC for just a t-stat change. I was able to change the entire upper and lower housing without loosening the intake but the sensors just have to be in place first. To just change the sensors it's easier to loosen the intake manifold and lift it a bit.

I can't help you on the housing size other than buy both and return the one that doesn't fit.