Making a Custom Tongue Muffler

[Dave Hull]

Here is a simple-to-build muffler project if you are having trouble finding exactly what you want--or if you just want something to build in the shop. It can be made with a hacksaw, files, a drill (even better a drill press) and a number 43 drill bit (.089") and a 4-40UNC tap. The body of the muffler is made from DuBro extruded aluminum exhaust extension stock. I got mine thru Smith Brothers Hobbies in Northridge, CA. It fits many mid-size engines from Fox .35 Stunts to OS Max .40/.46, etc. Be sure to check your engine’s exhaust port before starting work!

First, I determine how far I want the muffler to extend past the fuselage (full-body) to avoid blistering the paint with the exhaust gases. On my "cowl-less igNoblarf" (aka, Iggy) the overall muffler is 1.600" long for a Fox .35 Stunt. On this particular muffler that included the thickness of both end caps. There wasn't anything critical about the length as there was no attempt at exhaust pulse wavelength tuning. After cutting off the extrusion to length, I filed it FLAT and then lapped both ends on some wet-or-dry sandpaper. Be sure you place the paper on the flattest surface available. A ground cast iron tablesaw or other machine tool surface is ideal.

[Dave Hull]

The interface plate is made from a piece of .062 aluminum sheet. The end cap is either .032 or .040" aluminum. Neither is threaded so the alloy and heat treatment isn't too critical, but better is always better!

Both ends of the muffler body are threaded. I would aim for exactly the bolt pattern of the engine case on both ends. That way, you are less likely to start putting the exhaust holes on the wrong end. If you choose a different thread for your screws, you can look up the required drill size in a drill chart. Generally, you will find these are based on 75% of full thread depth. That's pretty standard. When tapping with small taps like this, be sure to use a lubricant. There are specialty lubricants for aluminum like TapMagic, but plain old kerosene works well. Go slow, clear the chips often and remember--two steps forward, one step back. Snapping off a tap can take all the fun out of a project.

Once the end caps are screwed on, any final blending of the outer shape can be done to match up with the extruded main body. Some careful file work and it will look good. The web at the top and bottom may be thin, depending on your engine. You can always leave the interface plate wider than the muffler stock to ensure it is robust. You can also fine tune the opening in the interface plate so that the edges all match up, reducing turbulence and flow losses.

I used regular 4-40 socket head cap screws to attach the muffler to the engine. If you are making this for a metric engine, you might want to use metric screws to keep things straight and avoid cross-threading mistakes. I used low profile, alloy steel (black oxide finish) button head 4-40UNC screws for the end cap. I had them and thought they looked good. And less stuff to catch on while you are wiping down your plane at the end of the day. Since these screws are semi-permanent, putting some Loctite on them won't hurt.

On most engines, I start with 9 holes of 3/32" diameter down near the end of the body. This seems to be a pretty good compromise between power loss and noise reduction. As an alternative, you could put the holes in the end cap, but if you do, you might want to use .062" material there. Be sure you consider which side you put the holes on. Most helpers/launchers would likely prefer that the noisy side points down or away. Also consider where the fog of castor oil will go. But short of converting to electric, or by becoming a "tuna pipe man" you are going to spend time cleaning up oil, whether it's on the top or the bottom.

[Dave Hull]

If you plan on running exhaust pressure you can thread the body for a nipple fitting. These come in different sizes and with different threads. I pick out my fitting before I start drilling the pilot hole so that I don't end up with any closer of a relationship with Murphy than I already have....

I like to use the "trailing edge" of the muffler for the fitting location. That seems to be most convenient and has the advantage of thicker stock for better threading. It is located just inboard of the screw holes for the end caps. No reason to interconnect those two holes. You can file a flat spot on the muffler to provide a better seat for the shoulder on the fitting.

Here is one of these mufflicators installated on Iggy, the "Lost Nose igNoblarf." It works fine.

The weight on these isn't too bad--and is certainly lighter than most (all?) stock mufflers. More material could be removed from the extruded body, but it wasn't worth it to me on the ones I've built so far. If you want to try it, start by taking away the front and back edges between the screws. There's some meat to be had there!

Depending on your application, I'm sure the basic design can be modified to suit most engines but it becomes more challenging if the DuBro extrusion is not compatible with your engine.

[Paul Taylor]

Nice job Dave!!!

I have cut the stack off a stock LA muffler. Used JB weld to hold the end cap on and put in a pressure tap all with a hack saw, file and drill.  It has held up for years but doesn’t look as nice as yours.

👍🏼👍🏼👍🏼

[Dan McEntee]

   I have been making tongue mufflers like this since the late 80's when I first started, and used some variations. Sometimes I would weld up the end if I wanted a rounded off look ( because I could and had access to the equipment ) and then just drill holes. On some engines, the extruded material does just fit against the exhaust of the engine, but I like the idea of the end piece there to help with the fit. Everything can be shaped smooth t help exhaust flow as much as possible. Sometime I would just space the end cap out with washers to create a gap, and the washer thickness or number of washers sets the gap width and the back pressure. Sometimes I would file an angled slot at the end cap end for the exhaust to exit, and like the holes, can be top, bottom, or both. I would file a matching angle on the end plate if it was for a larger engine. It's easy to polish the stack on these if you wanted to do so, just do the individual pieces and then assemble.  I saw one Jim Lee made for a SIG Mustang that was a short stack and then a lengths of tubing coming out of the blank end for the exhaust. These were attached with JB-Weld also I think. Lots of options and I think JB-Weld now comes in a version that can take higher heat. Imagine a length of the extrusion, with the end open and a length of tubing JB-Welded on the end with one end of the tube plugged. Changes in the size of the tube and it's length will get you different results and back pressures. The exit end could be squeezed into a fish tail shape like some chopper exhausts are, and that can alter and affect back pressure and still quiet things down quite a bit

   One issue with aluminum mufflers is stripped threads. Steel screws combined with vibration and occasionally getting over torqued will eventually lead to stripped threads. It's the same with stock mufflers or home made tongue mufflers. When first installing a muffler, use threads that reach as far into the threaded hole as possible, but don't let the screw bottom out or get into the tapered threads at the bottom of the hole if you didn't use a bottoming tap.  In terms of length, make sure the screw goes in at least 3/8" or more and if making a muffler, drill and tap the holes in at least 1/2", more if possible. The more threads that become engaged between the bolt and the threaded hole, the more secure it will be against vibration and and last longer. It doesn't take as much torque to keep the bolt tight because there are more threads doing the job. Learn to judge how much torque you apply to make things just tight enough. Lock washers help keep things secure also.

     If you have a muffler with stripped threads there are ways to repair them. on many stock mufflers, the threaded portion of the flange is more like an ear and the threaded hole comes out the other side. I have taken these and used a steel nut or even a blind nut to repair the threads. you might have to grind one side of the nut flat, or grind the flange of the blind nut flat on one side, and also grind a bit of clearance on the muffler stack to allow things to align. when you are satisfied with how things fit and are aligned, use a bolt that is just long enough to protrude a tiny bit through the nut and smear JB-Weld over the nut to the aluminum to hold it in place. make it as smooth and pleasing to your eye as possible, and allow 24 hours  for it to cure. Then dress things down with a file if needed. Now you have steel against steel, and over torqueing isn't much of an issue any more. While you are at it, might as well do both sides, one right after the other to help assure alignment, because the other threaded hole has been through the same punishment and are likely to be loose also. This method can be used on many types of threaded holes. On a length of extension stock like originally used in a home made muffler, you can go back from the face of the mounting surface about 1/4" and using a Dremel cut off wheel, cut an appropriate slot and fit the nut into that, making it as neat as you can and covering with JB-Weld to hold it in place.

    Another way to replace stripped threads is to use a threaded insert. In my job as a machine maintenance and repair tech, I used these quite a bit. When you have operators that can't tell a 3/8"-16 hole from a M10 thread, and other similar cross overs, threads got pulled out a lot!! There are commercial inserts available for general purpose work and also the venerable "Helicoil" (which I don't care for) but they are made with the side body being quite thick and take up some room. I had many instances where these would not work because of how the part was made and other clearance issues, but the repair still had to be made. I started taking the next size bigger bolt or screw, and drill the center of it for the tap size hole of the threads I was replacing, and then tap that hole. Next, just cut a slot on one end to fit a screw driver and finally cut to length. Clean up the ends with a file and test fit in the correct size nut, then tap out the stripped hole for the new insert you just made. Install the insert with some lock tight and then dimple the surrounding surface with a center punch if you can to help lock it in. I have never had one of these fail and I have used them in all kinds of machines from large web printing presses, to folders, bindery equipment, to even small engines, tools and appliances. It saves a ton of money and time!! Some thread sizes run really close to each other, but you just need to be precise with the original center hole when you drill it. If you have access to a lathe, this gets easier but still requires care.

    Using this method for models is pretty much the same, and for the most part I have used on 4-40 threads in mufflers and on a couple of occasions in an engine case. I have used 6-32 bolts, and 8-32, which gives you a little leeway if you don't have access to a lathe to drill the center hole and are forced to use a hand drill or drill press. In that case I don't try to go too deep with the center hole, just deep enough for the threads to clear the length of the inset I need. I install these the same way and use lock tight or coat the threads with JB-Weld or anything that will take the heat encountered. The neat feature of this method, and really also the before mentioned method, is that they can be drilled out and replaced if needed, but I have never had to do that so far. The steel threads can take so much more punishment that I haven't worn one out. The last time I did this was with a spinner adapter nut. Quality spinners and their mounting hardware are getting scarce so I tried one from China off eBay. It came with an adapter nut that I simply could not use on the application I had. The screw that held the spinner on was much smaller than normal also, and would not fit the center hole of a Great Planes or Tru-Turn adapter nut, which is 13-32 I think. I was able to drill out the center hole of the spinner to take a 8-32 socket head screw, and then made an insert for the GP adapter nut with 8-32 threads on the inside. I put just a touch of CA glue on the insert to hold it in place and so far nothing has even vibrated loose. this is another area when you have to be careful not to over torque the screw.  There are lots of places that this method can be used to make a repair or make an adapter.

   Type at you later,
    Dan McEntee

   

[Dan McEntee]

Nice job Dave!!!

I have cut the stack off a stock LA muffler. Used JB weld to hold the end cap on and put in a pressure tap all with a hack saw, file and drill.  It has held up for years but doesn’t look as nice as yours.

👍🏼👍🏼👍🏼

     The later generation of Super Tigre engines used that tube type manifold that the huge canister attaches to. My R/C buddies that use the ST.40s for club racing and have to use the stock muffler have found that there is a sweet spot for that location, where you get max RPM. I have welded up may of these manifold and while doing some once, I had the idea that the manifold could be plugged on the open end and the holes drill to make a tongue muffler for the ST.51s I have. I made up one but have never tested it yet, but have seen photos of other guys that had the same idea and have been flying them, and report they work quite well. I generally prefer a Tom Lay of Scott Dinger style tube muffler but reworking the manifold is a light weight option. The mounting ears on these can be repaired with the same methods I had in my other post, or easily welded up when broken off, if you can get some one to do it.

   There is an old saying, "If you can use what you have got, then you won't need what you have not!"  This is getting more and more true as things become less and less available these days!!

   Type at you later,
    Dan McEntee