Nuclear reactor lid bolt.
Fullsize image: https://diqn32j8nouaz.cloudfront.net...t_fullsize.jpg
https://diqn32j8nouaz.cloudfront.net...r_lid_bolt.jpg
Previously:
https://www.homemadetools.net/forum/...458#post142767
https://www.homemadetools.net/forum/...978#post137684
https://www.homemadetools.net/forum/...2374#post96156
https://www.homemadetools.net/forum/...371#post104108
