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Post  roddie Mon Mar 25, 2024 9:43 pm

I bought some .750" OD Aluminum tubing.. hoping it would be a candidate for my muffler design. It's just a 12" length piece.. which is good because it won't work for what I originally hoped.

The tubing might work-out for making cylinder-sleeves as exhaust throttles.

Shown below with your "garden variety" Cox .049 cylinder inserted..


my attempt at making an exhaust throttle for Cox .049 Exhaus10

.5885" pipe ID..

my attempt at making an exhaust throttle for Cox .049 Exhaus19

.5935" cylinder-neck OD on crankcase...

my attempt at making an exhaust throttle for Cox .049 Exhaus20


Deburring the pipe's "bottom" to fit the case.. along with a cross-drilled "relief-port".. might work. We'll see...
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Post  balogh Tue Mar 26, 2024 12:55 am

Roddie, a very interesting project. Remember though when designing the ring internal diameter, that aluminum has about 2 times higher thermal expansion coefficient than steel, therefore when fitted cold, the throttle should be sitting on the cylinder almost tight, so as when running  hot, and expanding more in diameter than the cylinder, there should still be a very small radial gap that, if excessive, would compromise sealing for an efficient throttle.

I make my home made exhaust throttles out of thin aluminum coil cut from beer cans, wound around the cylinder neck with free ends allowing free expansion with no diamater increase, and use a steel circlip to keep them tight on the cylinder neck even if hot..a photo below, showing my March 2024 COX engine of the month winner queenie features such a DIY exhaust throttle on a thin wall No1 cylinder with a bottom flange on it, that would not allow the use of a normal throttle.

my attempt at making an exhaust throttle for Cox .049 20240318


I keep watching your posts with excitement.
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Post  Greenman38633 Tue Mar 26, 2024 6:10 am

Nice work Guys. Way beyond my skills
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Post  rdw777 Tue Mar 26, 2024 1:44 pm

Good start Roddie, Will be interested to see what you come up with!!…. Throttled engines are fun to play with Very Happy
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Post  davidll1984 Wed Mar 27, 2024 6:38 am

Even a little loose the ring still changes the rpm just less efficient for very low rpm I like the idea of an simple rc silencer Thumbs Up
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Post  roddie Wed Mar 27, 2024 8:20 pm

I measured, marked/scribed and cut a ring from my 12" length pipe.. to the approx. length of 7/16".. using my new "Rigid" cutting-wheel plumber's tool.


my attempt at making an exhaust throttle for Cox .049 Exhaus21


my attempt at making an exhaust throttle for Cox .049 Exhaus23


my attempt at making an exhaust throttle for Cox .049 Exhaus22


This gets it fairly close..


my attempt at making an exhaust throttle for Cox .049 Exhaus24


Then I used my deburring tool to gradually produce a beveled slip-fit.


my attempt at making an exhaust throttle for Cox .049 Exhaus25


Not sure how I want to proceed..    Cut a groove for a clip-ring I guess.


my attempt at making an exhaust throttle for Cox .049 Exhaus27


my attempt at making an exhaust throttle for Cox .049 Exhaus26
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Post  roddie Thu Mar 28, 2024 1:39 pm

my attempt at making an exhaust throttle for Cox .049 Exhaus28

So.... the ID of the clip-ring is .692" and the OD of the barrel is .748".......... That's a difference of .056". A groove in the barrel that's .028" deep should then measure-out to that .692" diameter. If I go halfway; with a .014" deep groove.. that should be adequate for the clip-ring to seat and hold position.

I'm planning to make a mandrel for the barrel; that I can "chuck" in my VSR drill and either file a groove.. or use a Dremel/cut-off wheel. I'm leaning toward trying one of my V-cutting riffler-files.


my attempt at making an exhaust throttle for Cox .049 Riffle10


I bought those file and rasp sets 30 years ago at a local job-lot outlet... on the cheap.. and have used them countless times for different things.
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Post  roddie Thu Mar 28, 2024 6:22 pm

The mandrel... I made from one of my 1/4" square-drive socket-wrenches.. and some D/S foam tape.. (the same concept I used to finish-sand my aluminum muffler-rings..)


my attempt at making an exhaust throttle for Cox .049 20240323


my attempt at making an exhaust throttle for Cox .049 20240322


my attempt at making an exhaust throttle for Cox .049 20240321


I'll admit; I wrapped X3 sockets before I arrived at the size that worked.. peeled the adhesive-liner from the D/S tape... and applied the talcum-powder (anti-sticky) treatment to the exposed adhesive.. but the fit was so tight.. that I couldn't get the barrel to go on all the way. I ended up using triple antibiotic ointment for its petroleum-jelly base to get the barrel all the way on. A comedy of errors..  but I managed to cut a groove which was deep enough for the clip-ring I'd picked-out.


my attempt at making an exhaust throttle for Cox .049 20240326



my attempt at making an exhaust throttle for Cox .049 20240324


my attempt at making an exhaust throttle for Cox .049 20240325


my attempt at making an exhaust throttle for Cox .049 20240327


It's a redneck way to do it.. Laughing I know.. but I don't have access to the machinery that I used to when I was employed..
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Post  roddie Thu Mar 28, 2024 7:00 pm

So now......... I'm wondering if I can simply drill a "cross-hole" for the exhaust-relief.. or does it need to be a pair of "slots".. 180 degrees apart? I blocked the crankcase openings (crankshaft and backplate) and with cylinder and barrel installed; used my mouth to blow-through the top of the cylinder. Some very slight leakage is present.. but I think I have something that will work.

I'm hoping that this will work on the stepped-wall cylinders.. as well as the "two-slit" type. We'll see..
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Post  balogh Fri Mar 29, 2024 12:07 am

Very well done, Roddie, looking forward to seeing the end product. I would suggest a rectangular relief to be cut instead of 2 holes drilled, even if this is less difficult..the latter will block the exhaust even if fully open..
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Post  getback Fri Mar 29, 2024 8:12 am

Redneck or not you are getting it done with Good progress , like the file set handy to have , drill two holes side by side and file out the rest to get the oval hole is what i would do but its ur puppy . Popcorn Thumbs Up
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Post  rdw777 Fri Mar 29, 2024 10:15 am

Great work Roddie!….Just for reference on the round hole type: I have one on an .049 I use in my Zoomslot Saucer…. Idle’s great when fully closed but will not kill the engine…. The hole size is a #23 dill …. Obviously smaller area than the cylinder’s exhaust port but doesn’t seem to take away top end….. Photo for reference of the hole to the engine’s port…. The lower left corner of the port is the last to close (I don’t know if that makes any difference). The engine has SPI…I used it as a model for a homemade throttle for an .020…

I have a rectangular port factory throttle on an .020 that works good as well… Just don’t have one in .049 to measure for comparison…


my attempt at making an exhaust throttle for Cox .049 74c31810
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Post  roddie Fri Mar 29, 2024 10:28 am

Thanks guys, The "drilled-hole" (#23 drill, thanks Robert) is what I'm going to try first.
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Post  roddie Thu Apr 04, 2024 2:59 pm

I haven't drilled my #23 cross-holes yet.. but I found that I'll need to "bevel" the bottom of my tube to clear any of the back-ends of the Cox reed-valve .049/.051 engines.

tank-bowl type..

my attempt at making an exhaust throttle for Cox .049 Exhaus31

horseshoe type..

my attempt at making an exhaust throttle for Cox .049 Exhaus30

postage-stamp type..

my attempt at making an exhaust throttle for Cox .049 Exhaus29
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