• Pre-1979, only the 1977 XLCR had the box arm with Disc. ¹⁾
• 1979 - 1981 had the box arm with disc and it will fit pre-79 frames. ²⁾
• A 79-81 swing arm will bolt on a 72 frame. Things to consider when changing swingarms are master cylinder placement, operation and the fact that the 79-81 swing arm shock mounting will change the angle of your shocks. ³⁾
• Consider if you even want a disc on the rear. It won't work any better than the drum, which is already powerful enough to lock up the wheel. Put your effort into improving the front brake if you want to stop better. ⁴⁾ You would do better by adapting a later 39 MM Evo fork for brakes and handling. ⁵⁾

• If you can do it without removing the staked nut on the left side is best. It serves to center the swingarm in the frame and you can bung up the threads on that nut really easily. ⁶⁾

• For the swing arm its pretty easy when you know how it works. The pivot bolt when tightened presses the inner bearing and race,bearing spacer, and dust shield onto both sides of the frame and the actual adjustment is done by tightening bearing locknut on the right side till you have the desired preload once the preload is set then comes the lock-ring so the right bearing locknut cant turn. ⁷⁾
• When you install the locknut do not stake it as advised in the FSM. A little [not a lot] of blue locktite is all that it needs, and whoever needs to remove it one day will be thankful. ⁸⁾

Article and Pics by ericfreeman of the XLFORUM ⁹⁾
Click on a pic to enlarge:

Here are all the parts laid out as they will fit into the swingarm 10) →	The bearing outer races are already installed in the swingarm 11)	Inspect the swingarm inner surfaces to ensure they are clean. You can see the bearing outer race already installed in the other side. 12)

Use some 800 wet/dry sandpaper to smooth the surface 13)→	Press the outer races into place using an appropriate tool.14)	Grease the tapered roller bearing. I used a very sticky marine grease that won't wash out. It's not for bearings that rotate at high speed like wheel bearings but will be fine for the swingarm that just oscillates through a small arc. 15)

Install the bearing in the outer race16)→	Pack the area around the bearing with more grease and fit the spacer on the bearing, with the smaller diameter section facing up so it will fit into the hole in the dust cover.17)	Install the dust seal, ensuring it fits flush with the swingarm boss. The flat side of the spacer should protrude a little through the dust seal hole. This spacer will seat against the frame when the bearing preload is set.18)

Here's the outer view of the leftside assembly, with the pivot bolt nut and bearing locknut in place. I'll wait to lock the bearing locknut into place until I trial-fit the swingarm to ensure I can properly adjust the left bearing preload.19)→	Repeat the procedure on the other side. Here's what it looks like with both bearings installed.20)	This is the entire setup assembled to keep all the parts together prior to fitting the swingarm in the frame.21)

Click on a pic to enlarge:

1980 Swingarm bolt assembly 42)