Top Flite B-25J Mitchell Project

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Page 7

       The next step in cowl preparation was to epoxy the cowl ring in place. I used 5 minute epoxy with microballons to form a nice fillet on the rear of the cowl ring.

       After the epoxy had cured I used my Dremel tool to grind out any epoxy that would interfere with the cowl mounting tabs locations as seen below.

       You will notice in the photos above and below that a notch was cut into the cowl ring to clear the intake manifold on the right side of the cowl ring(s). This is necessary to remove the cowl after the dummy radial is glued into place (to clear the Saito cylinder heads).

       After some fit checking of the dummy radial engine it was very clear that the stock Saito mufflers were not going to work as they were made. The angle at which the mufflers come out of the cylinder heads expose them in front of the radial engine face. This would not be the look I was looking for so a change had to be made.

       One way to remedy the exhaust problem would be to increase the angle the pipes come out of the cylinder heads by bending the header tubes. This would clear the dummy radial engine and preserve the scale appearance.

       The big question now was how to bend the brass tubing without kinking it severely. After some thought I figured the best way to bend the headers was to fill them with lead and bend them right on the cylinder head. The “lead mandrel” method as I call it keeps the tubing round as it is being bent, eliminating kinks. To get started I heated up the headers with my propane torch to “bake” the paint off the header ends. This will help heat conduction to melt the lead that will be added to the header tubes. A wire brush was used to remove the charred paint afterward.

       To add the lead to the inside of the header tubes I needed a way to plug the end of the tube to hold in the molten lead. I used a scrap aluminum plate to tie wire the exhaust pipes to and hold them in place while the lead was added to the header tubes (one tube at a time).

       I prepared four spools of electronics solder to be added to the four exhaust pipes. I weighed out the spools to about 8 grams of lead. In reality any lead soldering wire will work but the “non-rosin core” solder will work the best.

       To add the lead to the tubes I heated up the headers with my propane torch and fed electronics solder wire into the end of the exhaust pipes as seen below (one pipe at a time).

       Once the lead “mandrel” cooled I removed the exhaust pipe from the aluminum plate and cleaned the threads on the locknut from any grime.

       To bend the header tubes I used one of the Saito cylinder heads as a bending handle. I just bolted the mufflers (one at a time) to one of the cylinder heads and then used my hand to gently bend the brass tubing into place. I was initially worried that I would mess up the threads of the cylinder head but in reality the head is pretty strong. The key is to use grease on the locknut threads and make sure the locknut is seated completely before bending (as seen below).

       The absolute limit to the amount of bend you can put into this header without breaking it is about 80 degrees. I bent all of my headers to about 70 degrees from perpendicular to the flat end flare of the tube (as seen below).

       You can clearly see the difference below from the stock header on the right. This additional bend will allow the dummy radial to sit close to the Saito engine and keep a scale appearance.

       The last step in the bending process is to remove the lead solder from the tubes. I simply heated them up with the torch and shook out the lead. A bottle brush was then used to clean out any debris from the header followed by compressed air to blast out any remaining dust.

       A coat of black spray paint was applied to the headers after bending to finish the job.

       It was now time to install the exhaust pipes to the engines. I used a small amount of Permatex anti-seize compound on the locknut threads to keep the aluminum threads from sticking to the steel locknuts (which can happen over time). I then installed the aluminum header gasket washers into the cylinder heads before screwing in the header locknuts. Medium torque on these locknuts will hold them in sufficiently. I positioned the exhaust tube that has the pressure tap on it to the right side of the engine (viewed from the top rear of the engine). This can be seen in later photos.

       After the exhaust pipes were installed it was time to figure out how to secure them. No doubt these flexible pipes would vibrate and move wildly if not secured so a flexible but strong clamping system must be employed here.

       To secure the exhaust pipes into place I chose to use some loop straps I found at McMaster-Carr (part #3177T72). These straps feature a stainless steel band with a silicone rubber cushion capable of resisting high temperatures.

       To secure the straps to the engine I decided to drill some holes in the steel motor mount plates as seen below. I secured the straps with some 4-40 cap screws and “nylock” nuts.

       Notice the position of the exhaust pressure tap as mentioned earlier.

       The next step was to prepare and trim the dummy radial engines for installation into the cowls. I removed two of the dummy cylinders from each radial to allow for air cooling of the Saito engines as seen below. Notice that the front of the radial has an “up” side identified by the raised square on the nose of the radial.

       I trimmed the dummy radial to clear the right exhaust header as seen below. Luckily the left cylinder header clears the radial without modification.

       I installed the dummy engine pushrods by drilling holes in either side of the pushrod lands with a 1.15 mm carbide drill. I then pushed in the rods with a pair of forceps.

       Once I was satisfied with the fit of the radial in the cowl and on the engine I glued it into place with medium CA glue. I also glued in the pushrods from the back side of the radial.

       Once fitted to the engine the radial/cowl assembly looked astonishingly scale. Even the Saito blended in seamlessly to the dummy radial engine, I was extremely pleased with the outcome.

       The last step in mounting the cowl was to cut out where the exhaust tubes exited the cowl as seen below. An 1/8” gap is needed around the pipes to allow for vibration (this will help prevent damage due to the pipes scraping the cowl). *** Be sure to read later in this article for additional steps to insure the mufflers are properly supported to prevent vibration ***

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