I inspected mine, no cracks found. Plenty of hours on the airframe too.....
Rotax blue head 582 ( and 618 ) thermostats are a two stage thermostat.... The engines are designed with two coolant paths, one that bypasses the radiator all together, and another that goes through the radiator. The thermostat controls which path the coolant takes, making it a more complicated piece and therefore more expensive. The idea is that when you start the engine, the coolant only circulates through the engine itself, which speeds up the warm up of the coolant... once the coolant inside the engine is up to a certain temp, the thermostat will slowly open allowing the coolant to go through the radiator... of course as that coolant enters the engine the thermostat closes until that water heats up, then reopens, more flows in, closes, heats up, etc.... till all the coolant is at operating temp.
It does this to keep the temp of the coolant going through the engine stable.
My Yamaha sled engine conversion uses a very simular two stage thermostat and dual path coolant routing.
My experience with several blue head 582 and 618 engines, is they tend to run at around 125-145 degrees once fully up to temp. I think Penguin is wanting to run his hotter. I believe Rotax says one thing in some of their manuals, yet the way they designed the engine and thermostat it does another... I have seen the same temps out of these engines mounting in pusher or tractor config, and with small rotax " twin radiator " set ups, smallish single piece radiators, and of course the radiator common on most gyros, the VW rabbit radiator.
Even my 120+ HP four stroke yamaha engine, fully warmed up, the coolant never gets above 145 degrees.
I have no explaination about these cracks a few have found. Perhaps since most of the people who found them live in florida, a state known for corrosion of aircraft due to all the humidity salty air, it may be a issue of corrosion? Could be improperly welded from RFD as well? I don't know....
I do know that on my last piper, it had steel wing struts and part of the annual inspections was to remove the struts and check the inside of the tubing for corrosion, and before putting it back on the plane, the mechanic poured Linn seed oil into the strut and the strut was rotated all different directions to coat the inside of the tube with this oil. Ive often wondered if we shouldn't be doing something like that to the insides of all our steel tubes on our gyros....