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  1. #1
    1QwkSport2.5r's Avatar
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    Making muffler bridge adapter..

    How feasible would it be to make a bridge adapter from a 37mm bolt spacing to 35mm bolt spacing? I took some measurements and am finding it to be so tight, I dont think it would work. I'm trying to see if I can use a .46 muffler on a .30 engine - mainly for noise. Buying a new muffler isnt in the cards at the moment, so I'm trying to see if something I have will work.

    edit: I do not want to elongate the holes in the engine case to fit the wider bolt spacing. This is only a temporary thing.
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  2. #2
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    RE: Making muffler bridge adapter..

    You simply use two pieces of metal bolted together with the holes drilled to match the engine and muffler.
    Like shown here, these are two pieces bolted together as the holes for the engine and muffler that would be too close together otherwise.

    Fox Eagle IV .60 with a OS .61SF sized Performance Specialties Muffler.


    Same setup shown on a Fox Eagle III .60 engine


    here is a Rossi .60 engine with a Performance Specialties OS .61 SF mullfer adapted to it:



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  3. #3
    1QwkSport2.5r's Avatar
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    RE: Making muffler bridge adapter..

    You were the guy this was aimed at, but the problem is the bolt spacing on the engine is 35mm and the bolt spacing on the muffler is 37mm. The problem I see is the screws going through the engine into the adapter would hit the heads of the screws going into the muffler. That is, unless the threads into the second piece of the adapter were shallow...

    Being only 2mm off, I dont know how I could make it work and have enough screw threads holding it all together without it coming apart from vibration.... I have some 3/16" alum. I'll cut a few pieces for and see if it will work or not. So far, I have my doubts since the bolt spacing difference is only 2mm. If one were farther apart than the other, it might then work.... I'll see if I can get a picture to post of the situation.

    Here's a pic (sorry its a little fuzzy..) I have another muffler I could use that has a much larger bolt spacing, but the exhaust outlet is much bigger which I dont think will provide enough backpressure to the tank. This muffler is a stock muffler from my TT .46 Pro. (The other muffler I have is from one of my K&B Sportster .65's which has a much wider spacing)


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  4. #4
    earlwb's Avatar
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    RE: Making muffler bridge adapter..

    I don't think you pictured the method yet, One side of the piece of aluminum matches up to the through hole mount and is threaded to fit the engine. The other piece uses two short screws that match up with the muffler. Outside of the two are two more holes that one side is threaded so that you can bolt the two halves together to make the bridge adapter.
    Instead of using 6 short screws, you use four short screws and two longer screws for the engine.
    I haven't had to do it myself yet. But I will eventually.

    Like this quick sketch shows


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  5. #5
    1QwkSport2.5r's Avatar
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    RE: Making muffler bridge adapter..


    ORIGINAL: earlwb

    I don't think you pictured the method yet, One side of the piece of aluminum matches up to the through hole mount and is threaded to fit the engine. The other piece uses two short screws that match up with the muffler. Outside of the two are two more holes that one side is threaded so that you can bolt the two halves together to make the bridge adapter.
    Instead of using 6 short screws, you use four short screws and two longer screws for the engine.
    I haven't had to do it myself yet. But I will eventually.


    I jumped the gun, and hadnt thought about it enough. I was looking at it as using 4 screws instead of 6, not considering bolting the two halves together and seperately bolting into the engine and muffler. I was originally thinking of 2 screws into the muffler and adapter, and 2 screws through the engine and into the adapter holding it all together and to the engine. Going at it this way would just need the muffler offset enough so the screws dont interfere with one another.

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  6. #6
    earlwb's Avatar
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    RE: Making muffler bridge adapter..

    I added a pic to my last response.

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    RE: Making muffler bridge adapter..

    I like those old Enya mufflers for that.
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  8. #8
    1QwkSport2.5r's Avatar
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    RE: Making muffler bridge adapter..


    ORIGINAL: earlwb

    I added a pic to my last response.

    After my last response, I see how yours are set up. I originally wasnt going to use 6 screws, but I have some laying around that will work perfectly, so I'll make an adapter up.
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  9. #9
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    RE: Making muffler bridge adapter..

    I forgot to mention that one could thread some aluminum rod for the muffler holes, screw in the rods using red locktite. The rods are now plugs. Let it sit overnight so the red locktite cures real good. Then redrill a hole or two where you want them, and then tap the hole or holes to match the engine's screw holes. A number of engine manufacturers have done this before and it is a common repair technique too. The plug doesn't have to go all the way to the bottom either, just real close. You want it flush with the mounting surface so that it is easy to make a punch mark to drill the new offset hole and tap it.
    Since you are about 2mm off, you could do it to one hole so the muffler threaded holes match the engine's through holes.

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  10. #10
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    RE: Making muffler bridge adapter..

    Offset the muffler to the rear. The front muffler bolt would go through the adapter and screw into the muffler. That should allow enough space for the rear bolts to miss each other.
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    RE: Making muffler bridge adapter..

    Earls is purtier but I did likewise.
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  12. #12
    1QwkSport2.5r's Avatar
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    RE: Making muffler bridge adapter..

    I made the port cutouts and drilled/tapped the holes in my adapter, but I'm wondering what the best screw/nut combination will work the best to hold the two halves of the adapter together. I was going to tap one half out to maybe 6-32 and use either a double nut or nylon stop nut. I didnt know if the heat of the exhaust would soften the nylon and cause the nut to loosen or not. I have a 4-40 tap, but I thought 2 4-40 screws wouldnt be strong enough to stay tight. Earl - what do you think?
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  13. #13
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    RE: Making muffler bridge adapter..

    Mine all use 6-32 screws so far and some metric but I think those were 3.5mm or 4mm. But 4-40 should work OK, if it doesn't then you can drill and tap it for 6-32 or the next metric size larger.. I usually use the Allen hex head screws. If you have the room a lockwasher on each screw works good. I have ground the heads down a little on some screws so I could use a lockwasher on them. Blue locktite (non-permanent) too.
    The lockwasher seems to do wonders for keeping the screws from loosening up. But no lockwasher and screw only works. I have noticed that the helicopter guys do not use lockwashers or washers on the engine mount screws, they just bolt them on up to the motor mounts. Some of the heli engines also state to not use lockwashers too.
    After you run the engine, you may or may not need to tweak the screws a little. I only had the two outside screws that hold the two metal pieces together come loose on me, and that was on the test stand. But I hadn't used lockwashers or locktite on them as I was testing it out.
    Usually the nylock nuts do not work well on the muffler/exhaust area as the nylon insert tends to melt on you. But any other type works though.

    I'll likely be making up some bridge adapters for smaller engines, but I haven't gone smaller than a .45 so far.

    Here is a bridge adapter I made for a Fox .45 for a Performance Specialties muffler. I have another one I made for a Fox .45 to K&B muffler, but I can't find the pics for it at this time.




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  14. #14
    1QwkSport2.5r's Avatar
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    RE: Making muffler bridge adapter..

    Well, it isnt a precision piece, but it does work. I'm going to go test it out shortly.

    Stock TT .46 Pro muffler on Enya .30ss:


    edit: Testing revealed some better performance numbers on the little Enya 30SS. I put my numbers in the Tach forum. Well worth the work. I might try my .46 Jett Stream muffler to see if that gives any better performance over the TT muffler.
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  15. #15
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    RE: Making muffler bridge adapter..

    I would think that a .46 muffler on a .30 engine would help a lot.
    Your adapter looks really good. I would say you did a nice job on it too.

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  16. #16
    1QwkSport2.5r's Avatar
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    RE: Making muffler bridge adapter..


    ORIGINAL: earlwb

    I would think that a .46 muffler on a .30 engine would help a lot.
    Your adapter looks really good. I would say you did a nice job on it too.

    Thanks, Earl. It didn't turn out too bad, and the gain of 1540rpm on the 9x5 prop put a smile on my face. No drill press, endmill, or lathe either. I knew the TT muffler was going to give it some kick, but I didn't expect that much. I didn't see as huge of a gain on the 10x6 as I did with the 9x5 or 9x6. Seems as though 9" props are it's forte.

    I'll maybe test the Jett muffler and compare results. This muffler is a lot bigger than the TT muffler so I wonder if this will benefit the larger props more than the smaller ones. I'll have to find out.
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  17. #17
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    RE: Making muffler bridge adapter..

    People are sometimes amazed and surprised to see how much RPMs or power is lost with some mufflers.
    I have seen as much as a 3,000 rpm drop in performance with some mufflers. Then we have other mufflers that have little to no drop in RPMs when used, and a few mufflers that actually increase the RPMs too.

    When I did some experiments to satisfy some of my curiosity, I found that a Fox Eagle IV .60 would turn about the same RPMs using the same prop as a Rossi engine and the same muffler. Both engines really came alive using a Performance Specialties tuned muffler (a Nelson Muffler). Both engines were within a 100 RPMs of each other, which pretty much makes them equal then. The stock OEM Fox muffler, although it works OK, it is more noisy than I like and it restricts the engine power a lot.

    Using a 9x5 or 9x6 prop on a .30 is a pretty reasonable assumption to make. The older design .29's through .35 engines typically all ran the 9x5 through 9x6 props. They usually didn't like lugging down so much with the 10x6 props. Where a .40 would run a 10x6 prop OK.  But then the modern big bore .25 engines that were bored out for .32, .35 and .36 displacements or even larger too, would turn a 10x6 prop really well.

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  18. #18
    1QwkSport2.5r's Avatar
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    RE: Making muffler bridge adapter..


    ORIGINAL: earlwb

    People are sometimes amazed and surprised to see how much RPMs or power is lost with some mufflers.
    I have seen as much as a 3,000 rpm drop in performance with some mufflers. Then we have other mufflers that have little to no drop in RPMs when used, and a few mufflers that actually increase the RPMs too.

    When I did some experiments to satisfy some of my curiosity, I found that a Fox Eagle IV .60 would turn about the same RPMs using the same prop as a Rossi engine and the same muffler. Both engines really came alive using a Performance Specialties tuned muffler (a Nelson Muffler). Both engines were within a 100 RPMs of each other, which pretty much makes them equal then. The stock OEM Fox muffler, although it works OK, it is more noisy than I like and it restricts the engine power a lot.

    Using a 9x5 or 9x6 prop on a .30 is a pretty reasonable assumption to make. The older design .29's through .35 engines typically all ran the 9x5 through 9x6 props. They usually didn't like lugging down so much with the 10x6 props. Where a .40 would run a 10x6 prop OK.* But then the modern big bore .25 engines that were bored out for .32, .35 and .36 displacements or even larger too, would turn a 10x6 prop really well.

    Well, comparing the size of the stock (loud) muffler that came with the Enya to the TT muffler, theres a big difference. The enya muffler isnt as small as the ST S.29 muffler, but it had "power robber" written all over it. This little engine has a little more bark to it, I think.

    I tested a full range of props on the stock muffler after I finished break-in and then I ran the same series of props to compare against with the bigger muffler. When I fly it, I'll keep it on a 9x5 or 9x6 most likely. My TT .46 on the stock muffler only turns the 10x6 2400rpm faster than the .30 Enya does on the TT muffler.

    I will be getting a ST S.29 ringed next month thats coming with a full tuned pipe. I might do a few experiments with that pipe too between the two engines. I still have an airboat that needs to break the sound barrier one of these days.
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  19. #19
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    RE: Making muffler bridge adapter..

    I had a Dumas large airboat that I but a Fox .60 Hawk engine on. At something over 1/2 throttle, if the boat hit a small wavelet, the nose would lift up and it would go airborne and flip all around like a leaf and wind up upside down in the water. It never occurred to me to try to put some control surfaces on it, as it might have been flyable. I could see maybe some elevons on it. A few years ago we had that RC toy car, boat, plane toy that was popular for awhile with some people. That was when I got the idea about maybe the Dumas big airboat needed elevons.

    Later after running the big Dumas boat for a while, I scratch built a wedge shaped airboat that was larger, and it worked pretty good. It wouldn't go airborne on me, but it was slower to accelerate up to speed. But it did go pretty fast though.

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  20. #20
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    RE: Making muffler bridge adapter..

    ORIGINAL: earlwb

    I had a Dumas large airboat that I but a Fox .60 Hawk engine on. At something over 1/2 throttle, if the boat hit a small wavelet, the nose would lift up and it would go airborne and flip all around like a leaf and wind up upside down in the water. It never occurred to me to try to put some control surfaces on it, as it might have been flyable. I could see maybe some elevons on it. A few years ago we had that RC toy car, boat, plane toy that was popular for awhile with some people. That was when I got the idea about maybe the Dumas big airboat needed elevons.

    Later after running the big Dumas boat for a while,* I scratch built a wedge shaped airboat that was larger, and it worked pretty good. It wouldn't go airborne on me, but it was slower to accelerate up to speed. But it did go pretty fast though.

    My flatbottom boat actually runs really nice and level with very little hopping when balanced properly. I found a good balance between where the prop is in relation to the length of the hull and having enough pressure on the bow to ride level. At that time, I was using one of my K&B Sportster .65's on it and had a cruddy rudder on it so steering suffered some. Now that I have a much better rudder on it and the ST S90K engine, I have to re-set the balance of the prop to length ratio. I have an adjustable motor mount that allows me to slide the engine back and forth so I can tweak the balance. When its set right, it gets out of the hole with a quickness and has a good top speed. Once I find the sweet spot for the prop on the S90K, I'm sure I'll see the speed get better. I might add a wing or trim tabs to it but I want to see what it can do without it first.

    Dumas swamp buggies arent all that stable for the size engines they are supposedly designed around. They aren't wide enough I dont think. For the more hardcore airboaters, the standard rules of thumb is hull width = 1.5 times prop diameter and the prop should be located 1/3 the total length of hull from the transom. Wet rudders provide a huge improvement in control over air rudders, generally.
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