Some observations on changing air conditioner gaskets
My coach is 22 yrs old, and I've started taking care of things like re-doing all the joint seals (caps, gutters, roof feed thrus) and painting the fiberglas roof (it was chalking a lot).
So it was time to change the AC gaskets, paint the roof around them, etc.
I have Duo-therms, and there must be a typo in the manual, as it said they were 150 lb each. more like 50 lbs. Regardless, I had a friend with a forklift at his shop, so we forked them off the coach and into my truck.
When you remove ACs you need to tuck the power and freeze sensor cables somewhere in the AC chassis so they don't snag on removal.
With the ACs off it was time for a little refurbishment. Rinsed out 20 years of dirt from the coils and chassis. Removed the duct covers to find the foam insulation had completely deteriorated. Replaced that with some foil backed Dynamat, which serves to insulate and damp noise. There were also chunks of foam inside the cold section that needed to be re-secured. Taped all the joints with foil tape, flipped the units over, scraped off all the old gaskets and glue, and painted the chassis, as well as the covers (using the same rust-o-leum deck paint as the roof) to slow the degradation from UV. Installed the new gaskets, and taped all the seams in the coach's ductwork as well as putting some foam gasket between the mating surfaces.
Here's where things sort of went wrong. The instructions say to either torque the bolts to 50 in-lbs or compress the gaskets to 1/2". I used the 1/2 inch measurement and referenced it from the inside using a 6" rule. A few weeks later I noticed one corner was a lot higher than the others. I made 1/2" hard spacers, loosened the bolts, and put the spacers under the corners between the AC chassis and the roof, and tried to tighten the bolts to even stuff out.
When I turned the unit back on there was a lot of rubbing going on between the fan and the cold-side ductwork. I removed the units to find the new gaskets over-compressed and they would not recover. When the bolts were tightened too far it crushed the coach ductwork into the AC unit ductwork and bent the cold side fan housing a bit. Getting it close to normal again was a pain. Did it really bad on the front AC and less so on the rear one. Fixing it required taking all the aluminum tape and the cold side duct covers off, and replacing it once everything was fixed.
The fix involved putting some low-density rubber gasket material (Frost King) on the roof of the coach and putting the units back. It's a little mickey-mouse but I didn't want to go through the whole nut-roll of getting the ACs off the roof to replace the gaskets again. I don't believe it will leak.
Bottom line - when tightening the bolts to compress the gaskets I’d say that anything between 3/4 and 1/2 inch is fine. Going all the way to 1/2 seems to lead to problems. I’d make 9/16 spacers next time.
One thing I learned tightening the bolts again - it's possible to twist the AC chassis and induce a "wubba wubba" noise that comes from the compressor making contact with the bolts that go through its vibration isolators. You can make the noise come and go by tightening and loosening bolts. You can also induce the cold side fan rub and make it go away. Tightening the bolts with the AC running is the way to go.
'99 Beaver Monterey 30', Cat 3126/300