This thread is for tried and true transaxle locking methods. If you have a method not listed here, contact me. All questions should be asked in their respective threads.
PIN LOCK
This method requires welding the spider gears to the pin in the bull gear. Fairly easy, although welding them wrong will result in axle binding.
This method requires welding the spider gears to a square piece of steel (block), allowing the pin to remove and joining both axles together as a solid piece. Slightly more technical, but stronger.
Solid axle shaft, no spider gears. Requires some hand fabrication with a grinder. No welding required, but is optional. (Done by Doc Sprocket)
HYDRO LOCKER
This method is to lock a HYDROSTATIC transaxle using a method similar to the Locker Block. *NOTE: Not all hydros are the same. Examine yours to see if it is compatible.*
This method doesnt require welding, as all you do is add two extra spider gears to your bull gear and a grinder. **NOTE that not all transaxles are compatible with this method. Check your bull gear design to see if it will work. Most if not all transaxles will work with this method. The extra spiders MUST a.) be 16 tooth and b.) match, so you will need a second "donor" transaxle of the same model. If you have a 15 tooth transaxle it wont work, however the 15 and 16 tooth gears both share the same splines so you can swap them.
Adding extra gears isnt too difficult, just cut the head off of a 5/8 bolt to act as a spacer between your added gear and the bull gear. Grind at the head until it fits firmly in between the bull and spider, as you dont want it to be too loose so that it hops around, but not too tight that it cant be removed. You may need to grind the bolt head on the edges too to clear the bull gear guide.
Example courtesy of Philip Lytle.
If you have a bullgear with a square center, you can use spider gears from a matching transaxle and grind at the backs until the spiders fit firmly into the bull gear.
Zinc fumes are EXTREMELY dangerous and can result in severe injury or death. Do not attempt unless you are in a well ventilated area, wearing full covering clothing, and wearing a respirator.
This method doesnt require welding, as it encases the differential in Zinc which stops the gears from turning.
Not sure you can lock an 820 with extra gears, have to go look at one.
Most of these methods do not directly attach one axle half to another. This means they can flex some in the differential area, and the transaxle case is what keeps the two halves in line. If you are a rough driver, this will split the case in half. A solid axle through the case helps, or completely welding the spider gears, diff carrier, and axles together. Be careful to weld them straight, will bind in the case if they are crooked.
You can also install a straight through go kart axle on an 820. Requires boring out the diff hubs to 1in, and broaching a keyway in them. Here is a video with some of that in it.