Originally Posted by XR4EVER
There really aren't any weak links in the 400. The subframe will only be a problem if you carry a lot of weight back there over really rough terrain over extended periods, but it won't hurt to have a spare in your possession. Just keep everything lubed. You might want to pick up spare clutch and throttle cables and pre-route them and zip-tie them in place, just in case.
+1 more or less. The subframe can crack after heavy use but is easily welded, as it is mild steel. But it makes sense to have a spare subframe if one can get one cheap.
The swing arm pivot bolt is known to get stuck, and the frame pipes can rust under the engine.
That is about is as far as I know.