Blue spots on exhaust cans

someone tell me what this was caused from.
The thing I see at first is that the spots occur in basically the same spot on each can and are vitually the same size. What the heck would cause this? it's a nice piece up for sale too, with this possibly interesting flaw. I'm thinking the internals rotted and the external metal got hot?? I've had 5 xj700s and more exhausts and I have never seen this.

http://cgi.ebay.com/ebaymotors/1985-Yam ... ccessories

 

Perhaps the engine was running way to lean which overheated the exhaust at the baffle. That would be my best guess.


grz Thijs

 

Your initial diagnosis is correct. Either rotted or fractured, allowing hot exhaust to come directly in contact with the outer skin. The SR500 commonly fractured the inside of its head pipe about 2/3 of the way down causing the same thing. Old British bikes and older BMWs had single-wall pipes, they always turned blue (or at least a golden-bronze color.) That's why the Japanese started using double-walled pipes so they don't change colors.