Things have been progressing well on the 1975 DT400B. Gotten new headlight, tires, brakes, sprocket and chain knocked out. Bike runs well and starts on the first kick. Just hit my first surprise today and wanted to get some opinions on what to do...

While replacing the front sprocket, I shot a little brake cleaner to loosen up 45 years of crud. To my surprise, the paint wiped off of the cases effortlessly. I’m guessing the cases were stripped and shot with a rattle can years ago, and it appears it was an effort to hide some damage. My guess is that a chain broke causing damage to the edge of the left case. Bike is a strong runner and not leaking fluids. Prior owner actually put over 6k miles on it 3 years ago before buying a VStrom and letting the Yamaha sit (after tank liner failed).

Should I leave well enough alone and ride it? Could this be repaired correctly? Maybe build a 2nd motor to have on hand when this one gets tired? How bad does it look?

If it aint broke.....

Yep, as above, just carry on riding. Unusual place for chain damage though being on top rather than forward.

Thought the same thing about the location of the damage. Impossible to say. The bike came to Texas by way of eBay a few years ago. It lived in Iowa since new and I’m the 3rd owner.

Numbers match and it appears to have been well cared for. Sprockets were pretty bad, but I guess it’s possible that he just repaired the chain and sold it.

Aluminum piece and JB weld repaired this, no leaks, no problems.