What To Do About The Blaster Swing Arm
The factory Blaster swing arm is too weak to withstand any sort of aggressive riding. Any Blaster that I have been around that has been ridden aggressively winds up with a cracked swing arm. Failure is accelerated when the machine has been widened, or is jumped extensively with the rider landing on the gas. The primary location for cracks to form is shown with an arrow in the photo below.
Cracks that have formed in this area cannot be permanently fixed by welding. After all, if the material thickness in this area was sufficient, the cracks wouldn't have formed to begin with. Welding a crack in thin material isn't going to suddenly make it thicker.
The best way to fix the weak swing arm problem is to buy a new one. But, I have a problem paying 600+ dollars for an item that contains at most $30 worth of material that can be made in a jig in 10 min or less. I understand that people need to make money, but selling an item for 1000% of the manufacturing costs is really pushing it. For this reason, I make my own swing arms which mimic closely the factory piece (See photo below.)
Making a new, stronger swing arm using mostly factory components is relatively straightforward. The only material needed is a few feet of rectangular stock (1"x 2"x 0.125"x ~36"), and a few inches of thick walled pipe (2.000" OD x ~1.600 ID x 12" long). The first step in the process is to measure the height of the machine. This will be used later when mocking up the new swing arm. After height measurements are made, remove the factory swing arm from the machine and clean it up. The axle should also be removed from the carrier, as it is much easier to get everything straight later with it removed. Make sure to measure the length of the factory arm as a baseline to be used later. Also measure the offset of the rear carrier mounting plates in relation to the leftmost point of the front attachment points. This measurement is needed to ensure the chain lines up correctly on the new arm. If you don't have extra Blasters or swing arms sitting around be sure take extra measurements and pictures of the factory everything before proceeding.
Next, the factory swing arm will need to be CAREFULLY cut apart. I have found it best to use a plasma cutter for this, but a die grinder with a cutoff wheel and some patience will also suffice. Cut the rear carrier mounting plates, chain guide mounting bracket, shock mount, and front attachment points from the factory piece. Make sure to clean up the parts by grinding away any factory weld left on these parts.
After cutting apart the factory arm, the next step is to cut new beams from the rectangular stock. The length of these pieces is determined by the desired length of the new arm. My personal preference is +2" over the factory length. Any more than this and it ruins the character of a Blaster in my opinion. In the pictures to follow, the swing arm being built is +4.5" over the factory piece. This arm is for the Blaster 500 and was necessary to ride the thing without it flipping over backwards all the time. The beams will need to be slotted in the rear to allow the rear carrier attachment plates to slide into them. The arms will also need to have semicircular relief in the front to allow for front attachment tubes to be welded. It is best to do this by drilling a hole in the center of the stock with a hole saw, and then cutting through the center of the hole. This beats notching the beams with the saw after they are cut to length. The front of the beams will also need to slightly tapered around the front mounting tubes. If this is not done, the swing arm beams will contact the engine cases on the right hand side. At this point, the project should appear is in the photo below:
The next part of the process, truing and tacking together the new swing arm is the most difficult, and also the most important. In order to ensure the carrier mounting points are the correct width apart, they should be assembled to the carrier before any welding. Be sure both plates are mounting to the carrier at the same point in the adjustment slot, and the factory adjustment mechanism is also installed. Also put the swing arm mounting bolt through the front mounting tubes to be sure their bores are correctly aligned. Measure everything and make sure its straight and correctly positioned. This includes making sure the carrier is perpendicular to the beams and parallel in both planes to the front mounting tubes. Remember, even a 1/16th of an inch here results in errors of 1/2 or more at the end of the axles. To help with this process I weld mounting tabs to piece of sheet steel to support the front attachment tubes, and use a V block to support the rear axle carrier. During this process do not worry about placing the rear shock mount/cross brace. It needs to be placed with the new swing arm mounted on the machine in order to ensure the correct ride height. After everything is straight, and heavily tack welded, weld the arm together. Take care not to get the front attachment tubes too hot, as their inner surface is the race for a roller bearing.
The last structural piece to be welded to the arm is the rear cross brace/shock mounting tab. To do this, mount the arm to the machine, with the axle and rear wheels installed. Support the machine at the desired height (remember measuring it at the beginning), and tack weld the rear cross brace and shock mount in position. Make sure the cross brace is square with respect to the swing arm beams and finish weld the piece.
To finish the arm, install the chain on the machine remembering to run it through the guide. Attach the mounting bracket to the guide, position the guide correctly, and weld the mounting bracket to the swing arm beam.
At this point, the new swing arm is finished, and the only task remaining is to apply a coat of paint.