Quick update... pics coming later     Carbon and oil slick has stopped. 2 NEW Gates 3338 180°F tstats are on their way
Which filter housing is the Primary Oil Filter?
Thermal camera misfired (loose nut behind the wheel) but I know what the engine temps are ... Hottest 170°F head cyl5-6 governor-side - 165°F generator-sides (sensor for water temp) ... coldest head cyl1 158°F generator-side .
Exhaust temp cyl1-6  220°F+ @ head  ... just under 200°F @ muffler
Gates 3338
1 Cade in Mhina
1 made in US (well that's what they said when it was parked)
Oil change 6¾ gal. filters 1x Cat 1R0659, 1X Wix 11561 10W40Delo

Dumb question

Removed the thermostats and pulled a hot water test on them. The thermostats were marked 175*.. One starting opening at 143* and the other at 145*. and they were wide open by 157*.. Service manual said the thermostats should start opening at 166* by 175* be mostly open and wide open at 183*. 

Update on 8T temp: I know it takes me a long time to anything anymore but I finally got the new 180° thermostats in the 8T. I warmed it up on 1/2 throttle for 15-20 min. Temp came up to 160° nicely and then at full throttle I put it to work for about another ten minutes before I ran out of fuel. ... Temp gauge started to come up a little more 170°-ish , The stack started to melt the oil off the muffler. I might have fixed the problem but I need to get out and put it to work. Snow is coming all next week so plenty of work time coming. I have a short video of checking thermostats in the traditional method in a pan of heating water and thermometer.  I'll be posting later.I do have a reversible fan I'm thinking of putting on the 8T and a duel Cat heater core, once I get the heat and amperage on step. The Generator is putting out 13.5v now - not sure how many Amps yet

Warming up before thermostats
image shows states open at low temp and exhaust has a lower temp
: :
: bottom rad hose
After thermostats changedimage shows thermostats not open yet and exhaust temp is much higher:
:
:  bottom rad hose 
 

How well did those Gates 33388 thermostats fit? I vaguely remember checking them out some time ago and am thinking the flange diameter was slightly off?
Also that thermostat is missing the lower part that controls the coolant bypass circuit.

I found with a little bit of help they worked adequately. The Gates 33388 dose have a small 1/16" hole for a bypass in the seat C.; If you like one can drill an 1/8" hole in the lid of the valve to allow more coolant to bypass.Drill a hole mid way between seat and spring in lid so not to interfere with seat nor spring. Also the brass flange has a bit thinner OD at A. which can be made up by a thin o-ring to help center the thermostat in the housing. As well the brass fling at B. is 0.136 thinner and can also be made up by a thick o-ring to fill the gap - when torquing down housing be sure not to block bypass in seat with o-ring or gasket..

Ooo the lower bypass ....  lower part that controls the coolant bypass circuit .... after looking at the manual again I see what you mean. I'll keep doing some research and get back with you on that .....  so far i read the bypass circuit dose have a flow but a diminished flow after thermostats open. The coolant was not drained out of the thermostat housing so I did not get a look inside the housing. I did stick my finger around to see if there was a slot for the bottom of the OEM thermostats to set in but there was nothing of the sort.
Thermostat p/n  5H957 (8T14759 - 8T14774 & 8T14776-up) later version of the 8F881-T which I believe has the part that controls the coolant bypass circuit in the housing 2H72491

  

 
 

 Still looking but this is what I have come up with so far


 

QUESTION???
If my heater core comes off the side of the thermostat housing (see pic) and returns to the bottom of the radiator hose, isn't that bypassing my thermostats? I may need to reroute  the heater core inlet connection.
 

The bypass I'm referring to is the re-circulation back to the water pump.
Do you have a copy of the "Servicemen's Reference Book" for the 4-1/2" bore  6-cylinder covering applications of the D318. Form #30238-2. It gives a good explanation how the cooling system works.

I do have for the D315 service manual which is why I know what you are talking about.
 
This OEM thermostat has no skirt (D318 8T parts manual FORM UE032887) p/n 5H957

 
This OEM thermostat has a skirt (D315 GENERAL service manual FORM FE030522-02)

 
This illustration has the skirt blocks the bypass (D315 GENERAL service manual FORM FE030522-02)

 
in illustration shows p/n 5H957  NO skirt nor bypass* .(955 parts manual)  FORM UE032563 
*Bypass may be the bevel on the bottom of the brass valve and seals agents the housing ????
Don't want to but I will pull it apart to clarify the issue ... give me a week.
 

my guess would be any passage of coolant...is a bypass......before total warm up (nice try)
-automotive wise it's within the pump..usually