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.

Hey guys,

Been a while but iv managed to get the cylinder off today to have a better look. Unfortunately it has extended up towards the top gasket surface which is quite annoying.

Instead of getting it tig welded which would require very precise pre heating and cool down i may order some aluminium brazing rods and braze the sides. As for the to gasket surface i may have to braze it and carefully grind it smooth with a grinding wheel or sandpaper. When reassembling i may add some liquid gasket underneath the head gasket.

Also from the photos you can see the ice had pushed it out slighty but i dont want to make things worse by trying to press it back in.

Any tips are helpful.

Was your cylinder similar to mine ?

The crack in mine was worse. The frozen water had pushed the wall out further to make a wider crack which I carefully tapped back into place with a hammer until it was even. If you are going to go that route, just take your time and make sure the gasket surface on top is completely flat. The JB Weld will withstand the heat and there isn’t that much water pressure inside the water jacket to push its way out through the repair.

Aluminum brazing is even more temperature- and conditions-sensitive than aluminum welding.

This is an obvious job for JB Weld. It’s not a particularly highly stressed area of the outside of the water jacket and I think you can carefully align the sides with a C-clamp and use a matchstick to spread JB Weld on it.

See if anyone is willing to give you a rotax cylinder, ask on some of the facebook karting pages.

Hi guys newbie posting here, although I do read a lot of information on here and came across this when looking for different rotax max barrels.

My barrel cracked on both sides during the winter after 2020 club season.

I have had mine repaired by weld and is just as good as before now problems or issues.

I’m going back to the guy to collect my other engine from him over the next week so I could ask for more information as to how he has made the repair. I have also attached photos of my repair!