Where exactly do the O-rings fit when installing the recirculation fitting on a InfuSsion mash tun?

The included red O-rings should arrive slipped over the threads that will ultimately be on the inside of the mash tun.

Please note, that there is an O-ring groove between the lock nut and the bulkhead's main body.  So essentially the o-ring stays exactly where it is now, just remove the extra and set it aside for safe keeping, then you're ready to install the bulkhead. In turn, the o-ring will actually make its seal on the inner mash tun wall.