Fitting The Duct Interface

Today’s work activities continued on with the fitting of the air duct … a spacing of 5/16" is required between the aft edges of the air duct and the frame around the radiator (this measurement is made without the duct interface in place). This can be a little difficult to mark, but discovered a #6 nut was 5/16"  ... so used one to mark the air duct for more trimming.

Prior to installing the duct interface, the gap between the frame around the radiator and aft edges of the air duct needs to be 5/16". A #6 nut happens to be 5/16", so one was used to make the measurements.


After sanding away the excess material from the aft edges of the air duct, the duct interface was set in place to check the fit. A decision was made to offset the duct interface a little inboard so a little more material was removed from the air duct so the duct interface could be moved inboard a little more.

Test fitting the rectangular duct interface after offsetting the frame a little more inboard. The duct interface is the  termination point for the air duct.


At this point, the duct interface is to be placed over the end of the air duct and the bottom cowl set back on the fuselage and a ¼" spacing should be obtainable between the duct interface and the frame around the radiator. Prior to installing the lower cowl a ¼" thick paint stirring stick was cut into strips and taped on the frame around the radiator so the desired ¼" gap could be checked and visualized.

Pieces of a ¼" paint stirring stick were taped to the frame on the radiator to establish the desired gap between the radiator and the duct interface.


The goal was to be able to see just a little light between the duct interface and the blocks of wood before closing the gap by moving the interface aft against the wood spacers. There was a tiny gap all around except for the left side of the radiator. In that area, the duct interface was pushing on the spacer blocks … which upon further investigation, revealed the left side of the duct interface was touching the lower cowl and will require a little sanding. Of note for fellow builders … when the bottom cowl was sitting on the ground during the test fitting of the duct interface, this was not an issue. However, when on the fuselage with the hinge pins installed, the shape of the bottom cowl is pulled in a little tighter which created the loss of clearance between the duct interface and the bottom cowl.

Duct interface in place and if one were to look straight down, there is a small gap visible between the blocks of wood and the duct interface except for the outboard edge … this is because the duct interface is pushing on the lower cowl and requires a little sanding to ease the pressure.


Once the duct interface clearance is resolved on the outboard edge, the duct will be drilled and Clecoed to hold its position for the bonding process. The reason it is necessary to create an even ¼" gap between the duct interface and the radiator frame is because a gap gasket will be installed later to make a seal between the lower cowl assembly and the radiator to prevent the loss of air flow through the radiator.