DM.... That plug is not a factory part which is why you cant find it in the manual.
It is a hex head, brass pipe plug (tapered NSP threads)
That plug caps off the bypass line to the rear of the head... It was added after the fact as some kind of a Thermostat delete idea (a bad one IMO)
If you run a thermostat in that head with the plug in it... you will be blowing hoses and taxing the he!! out of the water pump when the thermo stat is closed....
That hole that the plug is in goes straight back to the rear of the head where the approx 3/8" I.D. hose goes down to the back of the pump.... that is the water pump bypass.... This hole does not lead into the jacket of the cylinder head cooling [other than where it comes out into the thermo cavity where that plug is] , it is simply a path to the rear of the head.
I think that RMX put this in to combat cooling problems on the early radiator deletes that retained the "T" in the coolant lines to the radiator and they just ran a dead loop.... OR still had the front radiator left in the sled... they run cooler without if you plumb it like I have shown in the diagram below.
I have a 2006 cylinder head in my hands right now (well I put it down to type)
The factory designed cooling system on the 900's, If you delete the radiator and the "T", this system will cool even the Baddest 975's and 1140's out there while still running the thermostat for optimum engine temps.
Is there a hose on the back of the engine from the water pump to the head???
If there is, I suggest removing the plug out of the thermo housing and putting in the stock thermostat... no worries with cooling.
Hope this helps.
