Good signature kittylover!!

Hi Dan,
No need to take your unit apart. I have a 5Hxxxx water pump housing and 5H0957 regulator here.
After going through four different part number water pump housing changes  and four different regulator part number changes the latest 5H items have done away with the bypass circuit and skirted regulator. So no reason your Gates unit won't work even if some dimensions are a little off.
As near as I can tell  the 8F8817 regulator in a 2Hxxxx housing also does not use the skirted regulator.
Apparently the literature we've been looking at is for earlier model D318"s
Looking at the size of the cavity in the housing without bypass/recirculation I don't think it would matter what regulator was dangling in there.

Again Thank you so much Old Magnet
I pulled a test on the old 5H957 thermostats and they popped again at 143°F ~ 62°C & 145°F ~ 63°C ... ~ = about
175°F 79°C Tstats wide open at ~ 184°F ~ 84°C
175°F 79°C Tstats ½ open at ~ 175°F ~ 79°C
175°F 79°C Tststs starts to open at ~ 166°F ~ 74°C
So the thermostats' (5H957) springs are out of spec and are opening early and closing late causing engine to run cold.
First two pics show thermostats - one open and one closed and same height
third pic shows part number and manufacture

Thanks kittylover123 for noticing that I am trying. However, the question I was trying to ask was ... In the pic that shows the thermostat housing and there is an arrow pointing to a port that goes to the heater. Could the draw from the port going to the heater be too much flow bypassing and not allowing the engine to heat properly? I'm wondering also, if the way the cab heating system is plumbed if it could be thermal shocking the thermostats killing the springs? I'll look for another port in the cooling system to draw from and hook up the heater and block heater and still have good circulation.

The spring has little to do with lost operation. The driver for the thermostat is a wax plug sealed in the base of the valve push rod. As temperature increases the wax expands to open the valve. Lost function is most likely due to loss of wax or change in its expansion rate along with age or possible binding.
The really early thermostats were bellows type charged with an organic liquid with boiling point below desired temperature. These had frequent failures due to leaking bellows.

I just opened up an early original water pump (with bellows type thermostat) that has the recirculation passage and there is a skirt at the top of the bellows to somewhat limit the recirculation flow. Can't say it is a very good design and no wonder it was soon dumped.
Here's what it looks like. Requires orientating the vertical straps to align with the protruding recirculation port and would only have minimum affect. I don't know what the cat part number would be but it has stamped "Motorstat" and the number "10" and the manufacturer is "The Bishop & Babcock Manufacturing Co." Cleveland Ohio.
 

Recirculation Port
 

Thank you so much Old Magnet. Your help as been greatly appreciated. I find the information about the wax and bellows expansion intriguing.
I tried to get the thermostat out of my ol' 955E Traxcavator a few years back and found it a bit tough because of all the sediment and rust clinkers. I was expecting the 8T's cooling system to look like your pictures and my 955E. I was pleasantly surprised when they pulled out by hand and to find the cooling system on the 8T clean and green down to -35+°F. The gasket had been resealed with silicone and someone had been in there in its recent past (10yrs er so). The thermostats must have been going out slowly over time and finally let go after they cycled a few times.
I looked at the cooling circulation chart and think pulling from the port on the side of the thermostat housing and dumping back into the lower radiator hose should be fine once the engine comes up to temp. The flow of the coolant could not thermal shock the thermostats because the flow pulls the heated coolant across the thermostats not the other way around.

Thanks everyone.

 

Dan that is how the 8T I run is plumbed for the cab heater. For the engine tank heater, I pull coolant from the block drain hole and return into a T placed into the thermostat housing line for the cab heater. I am anal about flow, so I put a 1/4 turn ball valve in the tank heater line that is opened when plugged in. That way it cant flow the wrong way when needing cab heat. It works great for us.

Thanks kracked1 for the information. Just the conformation I needed. I live in Alaska and tank heaters I have messed with from the time I was in dippers. I don't have a tank heater on the 8T yet but I should have one in the my cash somewhere. I've been putting some thought into where to put the tank heater. I prefer the percolating L style or the freeze plug type to the percolating inline style. I have several 955 traxcavators with D315's that draw from the block drain like you have and returns to the elbow between the pony and diesel. Both ways looks like the heat is circulated sufficiently through the system but your way sounds easier and no brazing up the elbow. I don't see a need for the extra valve because all percolating type heaters have a check valve already builtin unless you wish to draw or push flow a certain direction. If it works for you don't mess with it cuz then it will get the idea that you need more to do ... kinda like the wife.
Again Thanks for your help

178*F wooohooo
Thanks everyone

I'm looking at the thermostat pictures and I recognize it since this was also used on overseas CAT clones and distant cousins. The problem with the old style thermostat seemed to be at least based on my experience. The get stuck after sitting for years or being run with water forever and then left to try rot inside the housing. Also people liked to get rid of them in the old day's since it is a place where frees damadge can occure when you are running the engine with water. 
Here is a example of a Soviet thermostat TC-6 that was a very wide use one many applications. That includes also on Lipetsk model KD-35 and Chelyabinsk model S-80

This one was used on Moskvich cars model 400 to 408. Looks kind of the same as the one that the CAT D2 uses?

I think you can convert it to a modern thermostat should not be that difficult. But I can not remember who did a video about it. Both thermostat that examples I showed open at 70 degrees Celsius. 
 

Stumbled across this info in my files...