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Top Flite B-25J Mitchell Project

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Page 8

       To attach the cowl hatches to the cowl a series of 1/16” birch plywood strips needed to be cut out. These plywood strips will support the back side of the cowl hatch pieces that I cut out earlier. I made my strips about 15 mm wide by 74 mm long. Notice that the strips were cut perpendicular to the grain, this will help the pieces bend to the contour of the cowl.

       The plywood strips were epoxied into place with 5 minute epoxy and microballons. (Be sure to roughen up the inside of the cowl with a Dremel tool to allow the epoxy to stick) I let the plywood overhang the cowl edge about 6 mm to allow for 2-56 blind nuts to be installed in them. It is important to scrape away any left over epoxy from the face of the plywood before it cures completely to allow the hatches to fit flush.

       A small plywood tab was also added to the front of the hatch openings. This tab will help keep the shape of the hatch and reduce warping.

       Because the hatches are contoured it was necessary to bevel the plywood strips to allow the hatches to mount flush. Be very careful to not scratch the cowl paint with the sanding drum (notice the protective tape).

       To hold the cowl hatches in place a set of 2-56 blind nuts and screws will be used. A set of plywood doubler plates were made to help hold the 2-56 blind nuts in place. The doublers were glued in place at about 15 mm inside of the hatch edges. Notice the pencil marks for the centers of the hole locations.

       The hatches were taped into place and the screw holes were laid out with pencil marks. The screw holes were positioned at 15 mm in from the outer edge and 3 mm in from the inner edge as seen below. A 3/32” drill bit was used to drill out the screw holes and pilot the blind nut holes (all the way through).

       I decided to use some 2-56 X 1/4” button head cap screws that I got from McMaster-Carr (Cat #92949A077), They have the “rivet” like appearance that I was looking for. The blind nuts I used were from Dubro (Cat #133) and come in packs of four.

       I used a 1/8” drill bit to open up the holes in the plywood to accommodate the blind nuts. I then used a 2-56 screw and washer to “set” each of the blind nuts into the wood. Thin CA was used under the blind nuts to hold them in place.

       The hatches were screwed into place and adjusted for a snug fit.

       The hatches now needed to be cut out to clear the valve covers. I started with small holes and worked my way up to the final cut out. I left about 1/8 to 1/4” gap around the valve cover to allow for engine vibration.

       At this point I decided to finish up the cowl by adding the fueler valve, crankcase breather fitting and the needle valve extensions. I reinforced the valve location by adding a small piece of 6 ounce glass to the inside of the cowl with epoxy as seen below.

       I like to use Dubro’s Kwik-Fill fueling valve (Cat #334) on most of my glow fuel models. It is a very convenient way to fill your model without using plugs or messing with dangling fuel lines.

       These 3/32” bulkhead fittings from Fourmost Products are a great way to attach the crankcase breather to an outside port on the cowl. This will allow me to easily treat the engines with “after run oil” at the end of a flying day.

       Both fittings are installed in the reinforced fiberglass patch as seen below. Notice that the filler is located on the lower inboard side of the cowls. This will help hide the fittings while being viewed statically.

       I decided to permanently mount the carburetor scoop to the wing at this time. I used the stock hardware as the manual suggested.

       The carburetor needle valves need extension handles to be able to reach them from outside the cowl. I fabricated a couple of wire extensions to just reach outside the cowl ring. They install easily into the needle valves as they already have a hole with a set screw in them for this purpose. The silicone fuel tubing around the needle extension will help reduce vibration caused by the engine.

       Blue threadlocker was used to help keep the set screws tight.

       The hole drilled for the needle extension wire was drilled smaller than the silicone tubing for a tight fit.

       The fuel tubing system could now be installed. I also installed the plug wires in place at this time.

       The removable cowl hatches will make servicing the engines much easier than removing the cowl assembly every time. Notice the crankcase breather tube attached to the bulkhead fitting. It is important to make sure both engines have the same length of fuel tubing attached to the crankcase breathers. This will help insure both engines will have similar back pressure in the crankcase and hopefully run identically.

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