Any one has idea what are the normal (acceptable) differential working temperatures of our Ford 8.8 differential with 4.10 gears.
In hard driving and hot days mine is about 170oF and that is on the place where the pinion bearings are. Front cover is about 155oF.

 

That seems about right. I'm not certain on the characteristics of the much heavier weight oil in the differential, but engine oil doesn't start evaporating moisture accumulated while sitting until around 160 degrees.

at that temperature it certainly isn't the ring and pinion, or any of the clutch or bearing pieces that are going to heat fatigue. Those are all hardened steel and can tolerate high temperature.

It's the oil I'm not sure about, but I wouldn't think that's high at all.

Why are you testing this out of curiosity?

 

I assembled diff yesterday with some mechanic who did that couple of times but he is definitely non expert in Ford 8.8” differentials at all. What we had is a couple videos from Youtube, Ford differential assembly instruction and specifications (preloads, backlash etc) and Ratech tool to measure pinion shim thickness.
The one thing he was very curios was pinion pre load I said that in ford speck is16 in-lb to 29 in-lb of rotating preload. I said as well that because it is totally new bearings it is generally suggested to put preload on higher levels of the spec as they will settle any way. So we ended on about 22 in-lb.

He said it is too much in his opinion. Anyway we put it on and everything works quite fine I have just a littlie tiny small whine in about 60-100 mph. And he said that it can be because of that too high in his opinion preload on the pinion. Other guy said that if that would be too high then the temperature of differential would easily go to at least 194 F - 212 F

So I guess that 170F I got means that preload is just fine.

 

I would call a reputable mechanic and ask about operating temperature. Better safe than sorry.