After paying A$68 to learn how delicate the thermostat housing is I thought it would be interesting to section it to understand how it worked.
After changing out the water pump seal I followed the great YouTube instruction posted by ScooterWest on how to bleed the system. Unfortunately after buttoning up the bleed screw I had a very small leak coming out of the hole so I did my usual trick and turned it another 1/4 turn...bang goes $68 bucks.
Turns out the brass bleed screw will seat on a moulded (plastic) step in the thermostat housing. Over tighten it as I have and you destroy the seat and allow coolant to get into the hole just below the o-ring and it comes straight out the nipple. What looked like a very small leak could empty 500mm of coolant in about 4 hours.
The manual says tighten to 3nm. For those without a torque wrench that goes down that low, this is about finger tight + about 1/10th of a turn. This is not like a brake bleed screw that seals against an aluminum or steel seat, when I stuffed this one I got no real feel of resistance and I was using light two fingers of pressure on the spanner.
Hope this saves at least one person my fate. The good news is that it seems I wasn't the first one to make this mistake, when I rang the dealer he just laughed and fired back, "yeah mate, got plenty in stock"