I have a Sierra. The muffler seems to keep rubbing on the lower cowling, on the left forward side. New engine mount cushions don’t seem to make any difference. What causes this? How can I fix it? (Search strings: cowling interference, muffler interference, muffler chafing)
Editor:
I have never been able to explain (with certainty) the rub in the left front of the lower cowl (from the muffler), on many Sierras. Changing the mount cushions has never solved it for me. I suspect that the Sierra muffler is simply located too closely to the cowling in that corner, and it hits every time you start up and shut down (when the engine excursions are at their greatest). It is also possible that replacement exhaust parts were not jigged properly, and do not have the correct fit. The Sundowner mufflers are much smaller in width. I have never seen heat damage in that spot, so I don’t think that the muffler is rubbing there during cruise flight.
Here is how I have fixed them before:
– Look in places like Wag Aero, Spruce, and Sun-N-Fun until you find a reasonably-priced, small aluminum blister, with a flange around it. These are often made and sold for streamlining protruding hardware, cowling clearance bumps, etc. You want one no more than about 3/8” to ½” high, maybe 2” wide by 4” long, sort of a blunt and shallow teardrop shape.
– Position the blister inside the cowling so that the deepest part is located at the deepest rub point, and mark the flange outline on the cowling..
– Reduce the outline by the width of the flange, and cut the hole in the cowling. The blister should fit neatly, with the flange flush against the inside cowling surface.
– Drill a few strategically-placed holes through the cowling and flange. Countersink the outside, and plan on mounting the blister using #4-40 100-degree, Phillips flat-head (flush) machine screws, with the tiny MS21042, 4-40 thread, heat-resistant flange-head nuts. You don’t want any of the hardware sticking out very far beyond the flange, where something could vibrate against it. You can trim the screws off almost flush with the nuts, using a Dremel with a cut-off wheel.
– Sand the gelcoat off the outside of the lower cowling, for the distance you wish to ‘blend in’ the blister.
– Mix up some blue Super-Fil (or the equivalent) from Spruce. DO NOT use automotive fillers/Bondo. Bed the flange in a thin film of the filler, and install the screws. Use the remaining Super-Fil to blend the contours on the outside of the cowling.
– After final finish sanding and fill-priming, paint the outside using a high-quality, high-solids paint, as closely matched as you can get. Make sure that it is properly applied using a spray gun, not spray cans, to get proper fill and coverage.
Once this has been done properly, almost no one will even know that a blister has been added to the cowling. Since the aluminum blister is so thin, and actually sticks out past the original cowling outer surface a little, the muffler will never hit it again on the inside.