63 ots heater duct donut seal

where does the donut get glued? on the heater body or the heater duct?

On a Series II, and I believe all other models, it is attached the duct that is bolted to the bonnet.

Well my donut Is glued to the duct . Look pretty tacky glued to the heater body don’t you think
My heater barely keeps the dew off the windshield
Engine heat keeps your feet warm

The alignment is tricky. I used a ring of plasticine to determine the best location, and it’s still not right.

I laid the donut on the heater box with double sided tape on the duct side so when you closed the bonnet .the donut stuck to the duct in the correct position . Pretty close

On my 69 S2 using advice in a post from Erica Moss, I used the adhesive backed foam insulation on a roll. This may not be the correct color or thickness…example only:

BUT! She also advised checking to see how well that bonnet-mounted duct round opening fit up with the round opening on the heater box. I always seemed to have a bit of engine smell/fumes inside the car, so thought this might be something to investigate. Her advice was to check that alignment and then put in some sort of gasket material to move the duct (in my case) further, more towards the firewall. During the alignment check process, I also found that the duct was not making contact with the heater box when the bonnet was closed. I cut a length of that foam insulation and stuck it around the perimeter of the duct opening. It got a bit mashed as the thickness I used was a bit more than was needed, but it ended much of the unneeded engine fumes

On both my 3.8 and 4.2 series 1 cars the donut is attached to the duct.