66 gto taillight fin covers
 

Is there supposed to be a gasket or something on the outside tailpanel . The cast fin covers don't fit tight in the middle . I know there's a gasket on inside housing?
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No, there is nothing on the outside between the taillight louvers and the tail panel. They often don't fit perfectly flat until you screw them down and they conform to the tail panel.

Yes, on each pin on the tail lamp molding there should be a washer seal #7583551.

Ah, I see you are correct Kenth. I didn't see anything in the Ames catalogue for what OP is inquiring about, so I figured there wasn't anything there. My mistake. Surprised Ames doesn't carry them, as I imagine a simple rubber washer from a hardware store would work just fine (i.e., seems like it would be easy for Ames to source and sell).

However, I don't think the rubber washers will solve OP's issue of the louvers not sitting flat in the middle. They just need to be tightened down.

Those Louvered Taillight covers are a PITA if they get tweaked from uneven and/or overtightening. They have to be near dead flat to look/seal correctly.

I was searching for similar pin seals for the quarter panel rear finish trim and everything I found was too brittle to use, so I rolled small amounts of the 3M strip caulk into balls and then lightly flattened them around the pins and it worked well.

I have found these are the closest to original mastic washer seals.

https://www.amkproducts.com/bulk-fas...forlookup=8430

IIRC i used the smaller ones for my taillight covers.

HTH