Looks like I need some new engine compartment ducts. What should I do?
Search strings: Custom Ducts, air ducts, firewall-forward ducts, engine compartment ducts
Technical Editor:
I strongly advise you to purchase the 3″ SCEET with cuffed ends, from Jon Kinney at Custom Duct, 800-863-2620. CD will give you a BAC member discount. Jon not only makes the cabin air and heat ducts you have probably been reading so much about; he also makes the engine compartment ducts. Because of his premium preferred material and the cuffed ends, they will easily outlast standard replacement ducting five to one. In my experience, they will at least go to TBO, and may well still be serviceable on the rebuilt engine. Here is the CD link on the BAC website:
http://www.customducts.net/index.html
Some added thoughts:
– Part of the reason why the CD items hold up so well is because the ends don’t get torn up during R&R. The molded cuffs make it easy to get the ducts on and off the fittings without damage.
– If you request it from Jon, he can send you a few feet of reinforced silicone tape or sheet to use as chafe guard, when you install the new ducts. You can tie-wrap the chafe sheet over the new duct where needed for added chafe protection. You can easily and inexpensively replace worn chafe guard during Annual Inspections or routine maintenance, helping the main ducts to last almost indefinitely.
– You can either measure your existing duct lengths, or go by the ‘book numbers’ to order. My advice is to measure what you have, if they seemed to fit well as they were routed. Sometimes ducting gets re-routed to accommodate different configurations such as a different air filter. You can buy as many or as few new ducts as you wish. Just measure the diameter and length, and send it to Jon.
– The Sundowner lower cowling has a fresh air inlet duct connection on the aft RH side. The cowl port is fiberglass, and tends to be too large for standard 3″ duct. This is particularly true if someone has made improper repairs. The ideal replacement is a special duct from Jon. He makes a duct that is 3″ by 19″ by 3.125″, for this exact purpose. The 3.125″ end will slide right over the cowling port, while the 3″ end connects to the heater selector air box on the center lower firewall. This special duct takes all the hassle out of disconnecting/reconnecting the RH side of the lower cowling.
– The Sundowners have a long 3″ duct that runs from the lower cowling nose air filter, back to the carburetor air box. It is supported by two large Adel clamps on the LH side of the lower cowling. This duct makes it a pain to drop the cowling. The solution is to buy or make a 4″ long ‘nipple’ out of 3″ thinwall aluminum tubing. You can buy a 12″ length of 3″ thinwall from Aircraft Spruce for about $13 and shipping. Or I can sell you a cut and primed length for $15 and postage. You make the new forward duct long enough to connect to the filter box, run through the first (forward) Adel clamp, and aft to about one inch beyond the aft Adel clamp. You install the 3″ nipple in the aft end of the duct, the use the Adel clamp to fasten the duct and nipple to the cowling. You put several flat washers between the clamp and cowling, so that there is about 1/8″ of space between the nipple and the cowling. You make the aft duct the proper length to run from the carb air box forward to the new 3″ nipple, and fasten it to the nipple. From then on, you can disconnect the LH side of the lower cowl by simply removing the forward end of the aft duct from the new nipple.
– You don’t have to have them; but I sell special 3.125″ SS thumbscrew clamps, for $5, that enable you to R&R the lower cowling ducts without tools. Since they use fingers only, there is much less risk of damaging the duct cuffs or nipples (especially the fiberglass nipple on the RH side), by crushing them when using screw-type clamps. I had a minimum order of hundreds of these made, a few years ago, in order to get them for this very purpose. While they are ‘standard hardware’, no one stocks them in the size that we need.