Had the same problem myself
This is the result of twisting stresses on the beam length of the swingarm.
Very common on hard ridden Blasters due to uneven loads on each wheel or beam of the swingarm.
I haven't seen it result in a sudden or catastrophic failure, anyone?
I welded my crack up, it didn't seem like chrome-moly (the frame is) and took weld well.
You can put a thin plate over it. I'd recommend tacks rather than a continuous weld on the plate.
After all the rhetoric, I might need nomex shorts for my next comments.
The stock arm is the lightest design I have seen, which is what you want in suspension parts, right?
Less unsprung weight is better, as long as you have the strength. The stock arm does well.
The whole point of "engineering" is "just enough" strength and no catastrophes. Stock does this.
My son has a 4" aftermarket swingarm that failed. No big deal, we spotted the cracks on the cross tube.
So what does he run now? A modified stock arm... Hey, hey, HEY! Stop throwing that stuff at me!!!
Seriously, as AWK08 says, it is pretty hard to weld sheetmetal and expect strength.
All the bending moment occurs between the two opposing forces.
The wheels and carrier is forced up and the shock mount forces down on the arm.
Anything between these points better be hellish strong, especially if you are adding leverage.
The strongest metal on these arms is the 1/4" carrier mount at the end of the swingarm;
Not only is that 1/4" plate strong, it is mild steel and easy to weld to, especially over the 4" length.
So I tried it on my KTM 250 powered Blaster.
This was a 2" extension. Welding the heavy steel to heavy steel is not hard for a good welder and can be trusted. It is very open for inspection and the weld is in the least stressed plane.
Egad that's an ugly looking weld! We welded it in short stitches to avoid bending.
Later ones we did a continual weld over a couple tacks that looks a lot better.
The horizontal plate is to get rid of any side to side bending. A tube could have been used too.
Does it work?
This was done back in 2012 and still holding together on my KTM powered monster.
I am mainly a trail rider, but I wheelie over logs and rocks and I have enough power to wheelie at any speed and not loose rpm on any hill. So far no cracks or fatigue.
My son and I have 4 or 5 Blasters on the go around here. He of course wanted more, so he made up a 3" extension and with its success made a 4" extension. Picture that inserted plate above being square. It is still running with no problems.
All the things that Awk08 and Ken and others have said against modified swingarms are true. Poor design and poor welds (embarrassed to show mine, not a work of art) will result in broken parts and possible serious injury. I am agreeing with them. Just showing what is possible with a little thought.
Steve
Smallest displacement, guess who is quickest...