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Builder's Tip: Heat Muff
By Bill Evans, EAA 794228, EAA Chapter 266
May 2017 - The three photos are of a sample handmade heat muff made by Bush Caddy when Sean Gilmore had it. Sean gave it to me to use as a sample.
It’s a nine-sided, semiround shape that when closed becomes eight-sided. A PK screw or three could be used to close it. The nine sides are each 1 inch wide and the straight sections are 7 inches long. The folds for the nine sides are about 40-45 degrees. When closed, the ends fit a 11/2-inch exhaust pipe.
Handmade heat muff
The tapered and folded ends are 1 inch long and taper from 1- to 1/2-inch wide. It needs 2- by 2-inch inlet/outlet stub tubes. They are rolled and welded from the same material as the case, say .010 to .020 stainless steel; 304 is good.
Heat muff end detail
The stub tubes are notched, then welded onto the case on opposite sides and opposite ends, not as the photo shows. Once the stub tubes are tacked in place, the inside of the tube is cut out by any means available. Heat is transferred by using a drill press or lathe to wrap .032 lockwire around a 5/16-inch rod into a coil. You’ll need about 5 feet of the coil. The coil is wrapped around the engine exhaust at a straight section where it’s about 1.5 inches in diameter.
The heat muff is pried open and slipped over the exhaust and coil. Then band clamps are used to close, and two clamps retain the heat muff closed at the ends. Stainless steel screws can be used to close the casing and retain it. Alternatively, the muff could be installed on the pipe, and a wire could be used to pull the coil inside the muff for heat transfer.
A layer of gasket material for the joint and high-temp sealant can be used to seal the edges of the muff. It’s suitable for carb heat and cabin heat/deice when used with a NACA duct and 2-inch scat hose. They make a black version of scat hose for hot air; I might use that instead of red.