AC gasket
I've been working on the roof quite a bit to fix the big split and some minor cracks.
I took the opportunity while I was up there to do the AC gaskets and was that a voyage of discovery ! My units are the original Coleman's with the heater element and metal covers outside.
After lifting the front unit clear of the hole I removed the crushed gasket and the two pieces of 1/4" plywood???
The penny dropped when I crushed the new 1" gasket 50% as instructed, the inside cover wouldn't let the switch stems line up with the holes.
That's why the 1/4" shims one each front and back maybe the original gasket was applied like foam ? so crush the gasket some more after first removing the heater assembly. Truly a voyage of discovery.