Anyone with a simple press can re-arch the spring. I would only do that once and then replace when it loses its arch again. The spring will break if re-arched more than a couple of times. I did it myself a couple of times before it broke causing a fair amount of damage to the rudder. I suspect that the original MK IV spring is nothing more than cold rolled steel. I am now using the long leaf, one leaf, of a Super Cub spring which is holding up great.
Ok, did you notice that is says all Rotax 912 engines. My question is are you talking about Mobil 1 or are you talking about Mobil 1 4T oil the two are very different oils. Really makes no difference to me what oil you choose but I would like to think that you are making an informed choice.
You may want to do a little more research. The way I read it Aero Shell Sport Plus 4 is the only oil currently recommended by Rotax. Mobil 1 covers a lot of oils. The one that was previously deemed as acceptable was Mobil 1 4T which is motorcycle oil. It has the additives necessary for the Rotax gearbox, But is not suitable if you are running 100LL. http://docusearch.flyrotax.com/files/pdf/d06038.pdf
Actually those hoses are the hot water coming from the heads. The 1" hose is the outlet and that should be routed to the rad. The 1" hose coming from the other end of the radiator then goes to the water pump which Is pumped out to the bottom of the heads thru the heads and back to the expansion tank. So now hopefully my last post will make more sense the tee's are then 1" x 5/8". So if you want to use copper a 1x1x1/2" sweat tee will work for a cheap solution but does not allow much clamping room. The wye's from UPI are more expensive but were worth it to me. UPI's part # for the Y is Z17550 it is a little hard to find without that. https://www.universalpartsinc.com/
Tee to the core after the expansion tank before the radiator and tee back in after the radiator before the 1" returns to the pump. I used y's facing to optimism flow directions but I know several others using tees that work just fine. The core is connected parallel to the radiator if that make more sense.
I had the factory heat muff on the muffler and after many attempts with no noticeable heat went to the hot water heat on my 912. Works great now other than all the drafts I have not address. Built my own for about $150. Fifteen dollar heater core from NAPA $60 fan from UPI. Guess I have less than a $100 into it The middle px is of the shroud before the fan was attached