What air ducts are required, if I want to replace all of the air ducts in my Sierra?
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Technical Editor:
Here is a link to an FAQ that also addresses this question. This FAQ also has an Addendum that addresses the aft cabin exhaust duct and overhead air plenum feed duct (and blower duct if so equipped).
https://www.beechaeroclub.org/modules.php?op=modload&name=FAQ&file=index&myfaq=yes&id_cat=5#458
The following text applies specifically to the 1977 and later Sierras.
The long underfloor ducts in the 1977 and later Sierra (and Sundowner) are 132″ long. There are some additional shorter ducts used at the forward end, under the panel. There are no sidewall or overhead ducts.
NOTE: The following ducts are used in all 1977 and later Sierras and Sundowners.
111728-8C8-264 This is the long duct to the rear seat lower spar cover heating outlets (two ducts required); 1.5″ ID by 132″ long. You will have extreme difficulty getting the old duct out of the forward cabin area, and even more difficulty trying to get new 1.5” duct installed. I strongly recommend that you use 1.25” SCEET (red silicone) from Custom Ducts instead, for this particular application. It will go in far easier, and won’t cause as many interference and corrosion problems due to an overly-cramped installation. The smoother inner liner will flow as much or more air as the 1.5” CEET duct. The Custom Ducts SCEET will come with 1.5″ molded-on cuffs, making it easy to connect to the 1.5″ air fittings. The SCEET will never absorb moisture, regardless of age.
111728-8C8-14 This is the very short 1.5” ID by 7” long duct which connects the firewall outlet to the right-side splitting Tee.
111728-8B8-10 This is the very short 1.25” ID by 5” long duct which connects the splitting Tee to the inline Wye feeding the two center-pedestal outlets.
111728-8B8-19 This is the short 1.25” ID by 9.5” long duct which connects the inline Wye to the RH center-pedestal outlet.
111728-8B8-46 This is the moderate-length 1.25” ID by 23” long duct which connects the inline Wye to the LH center-pedestal outlet.
111728-8B8-33 This is the short 1.25” ID by 16.5” long duct which connects the cheek air box to the RH and LH Wemac in the corners of the instrument panel.
111728-8D8-23 This is the short 2.00” ID by 11.5” long duct which connects the cheek air box to the RH and LH air inlet boxes beneath the sides of the windshield. Custom Ducts makes this part with slightly oversize cuffs, making it much easier to install on the air boxes without damaging the boxes.