Cracked Rotax cylinder water jacket... WELD?

I have just removed my kart from my trailer and discovered a crack either side of the cylinder head on the water jacket. I filled the radiator with kart specific coolant 50/50 mix and a few days ago we had some bery cold temps in the -2 and -3’s. I assumed it wouldnt freeze since the container said protection down to -35.

Whats the best method of repair? The barrel is barely used maybe 5 hours so i dont want to buy a new one. Ive heard of jb weld but dont know if that will bond well to aluminium. Of course a tig weld would suffice but i think the crack extends up to the top mating surface where the gasket sits.

Can solder work? The max temps on the head shouldnt exceed 100c so would solder not melt ?

The engine is still on the kart atm so i will post pics of it when i find the time

This isn’t exactly advice but if it were me I would cut a shallow V along the entire length of the crack and fill it in with JB Weld.
I’m not saying that it will be the cure but it’s worth a try. I would NOT weld it.

I would weld it, Tig make sure to heat in oven prior to welding

curious why you prefer JB over welding. My old Rotax went to a guy in Springfield IL who had the cylinder welded up, but have never gone back to him to see how that worked. Many years ago, had to have an aluminum cold plate that had Freon passages repaired. Wound up flying in a guy in from Minneapolis that had the skills. No leaks after the repair.

I said in my post that my idea may not be the ultimate cure. I’m leery of welding due to fear of warping and/or distortion.

I have repaired a Rotax cylinder with JB Weld a number of years ago and it worked fine. I had to use a hammer to get the crack on the water jacket wall to move back in line so that the O-Ring gasket would seal and the outer head/water jacket would line up as before the freezing. JB weld sticks to pretty much anything and can withstand all the temperature changes that your cylinder will go through. I applied the JB Weld on the inside and outside of the cracked wall. The application on the outside only needs to be in the crack and not beyond so it won’t show much after you put everything back together.
John K

When the coolant was added to the water system, was the engine then run up to operating temperature?

I’ve built a water jacket on a 2 stroke outboard before. If you want something a bit more “advanced”, you also have a choice of Devcon epoxy line from embedded Aluminum, steel, or Titanium. I used the AL.