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How to arrange Thermal Hopper Reed Valve Components?

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Help! How to arrange Thermal Hopper Reed Valve Components?

Post  DougW on Sat May 13, 2017 5:41 pm

My second-hand pre-1955 engine might have had its reeds re-arranged incorrectly.

The instructions don't have a drawing. They say to remember carefully the way the parts were arranged when dis-assembling to clean.

The Aeromodeller review

http://www.mh-aerotools.de/airfoils/documents/cox_test_thermalhopper_049_aeromodeller_february_1954.pdf

has an exploded view of the reed assembly which shows the reed backplate at the back, behind the reeds. The reeds point downwards.

The Model Aircraft review
http://www.mh-aerotools.de/airfoils/documents/cox_test_thermalhopper_model_aircraft_august_1955.pdf

has a cutaway drawing which appears to show the reeds pointing upwards, and though it is not clear, the reed backplate may be shown in front of the reeds.

My engine has the backplate in front and sideways. the exploded view shows it flat, but my backplate is not flat, the flap with a hole is formed so that it sticks out beyond the plane of the backplate's base a bit. In the exploded view the hole looks as if it lets air/fuel in, such that the hole would be useless if the plate were in front as it is. If the backplate is supposed to be at the back, I am not sure whether the flap should project forward or aft. The 4th part, a retaining ring for the other three, has a very small indentation at 3 o'clock looking aft. It appears to be a keying feature.

The other bits of info I found were that the rear reed that bears on the air intake is thinner, for a better seal. The second reed is stronger. And somewhere it was said that there is a stop to prevent the reeds opening too much.


Last edited by DougW on Mon May 22, 2017 7:55 am; edited 4 times in total (Reason for editing : Added more info on my engine's reed setup. 2nd Edit May 22/17 I made errors in pasting my post's link to the Aeromodeller review and it did not work.)
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Help! Re: How to arrange Thermal Hopper Reed Valve Components?

Post  Mudhen on Sat May 13, 2017 9:44 pm

The reed assembly has four parts:
1. The Reed Housing, (P/N 306.) This is the cylindrical part into which the reeds and the reed back plate are contained.
2. Reed Back Plate, (P/N 307.) This is the aluminum disc with the “flap with a hole.”
3. Reed Set, (P/N 308.) There are two reeds, one thick, one thin.
If you look on the inside the reed housing, you will notice a small “lip,” or “nipple.” This is the “key” over which the reeds and back plate must align. (Both reeds and back plate have a small cut-out for this fit.)
The direction of the reeds whether upright or sideways makes no difference. I’ve heard upright allows for a smoother fuel induction, but I don’t really know. Looking down into the reed  housing, you’ll need to place the reed back plate in first, with the flap pointing away from the reeds, (toward the inside of the crankcase,) and aligned with the key. Next, insert the thicker reed, then the thin reed. (The thin reed was used to seal against the carburetor body, and the thicker reed was for spring return. If you intend to run the engine, only the thicker reed is needed. After the parts are set in the housing, I usually will put a small dollop of Vaseline to hold the parts while pressing the assembly onto the housing. If you run the engine the Vaseline will easily flush out.
Hope this helps.
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Help! Re: How to arrange Thermal Hopper Reed Valve Components?

Post  Cribbs74 on Sat May 13, 2017 10:39 pm

Actually it helps immensely. Early reeds are somewhat a mystery. I have had some trouble in the past with the early reeds. This info would have helped instead of trial and error.

I vote for a stickey!
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Help! Thanks Mudhen!

Post  DougW on Sun May 14, 2017 9:26 am

Your description agrees with the way my engine is and, so far as I can tell, with the Model Airplane cutaway. The Aeromodeller exploded drawing disagrees. I wonder whether the engine could even run like that. It shows the back plate flat. I wonder whether the aft reed could seal the intake with the back plate at the back, with the raised part facing back, perhaps with the hole going over the intake tube so that the base could rest flush. With the raised part facing forward, I can't imagine how the reed could seal the intake during downstroke. I wonder if there is anything to properly limit reed deflection if the back plate is not at the front.

I would sorely like to take the reed assembly apart to check these surmises but have seen advice that too many dis-assemblies could wear out the interference press fit. It is suggested to do it only for essential cleaning when soaking won't suffice.

If you are right and Aeromodeller is wrong, there may be a lot of confused engines from people who followed its drawing. But perhaps they just won't run and the owners are forced to get it right.

Your engine runs well, right ?

I am inclined to trust you.  I am also inclined to trust the previous owner of mine because the engine was clean and not dinged, yet had been run a fair amount it appeared.

[Edit 3: You say that the thin reed is not needed if one is going to run the engine. Really? Is it just a storage seal to keep out dust? Is it better to remove it for running, or is it just that the engine can run without it if one does not have one?]

In re-assembling ball-socket Tee Dees I have several times lost track of which way the con rod had been over the crankpin despite trying. There did not seem to be any wear patterns visible that would give a clue. Maybe it does not matter. Once I read that one could reverse a con rod if it was scratching the back plate while running despite being clear statically. But I digress. Just to commiserate with anyone who got mixed up over reeds. One can get excited when one has just finally managed to get a stuck cylinder unscrewed.

On fuel induction vs angular location of reeds, the cutaway gives the impression that in that orientation (vertical, sticking up from the bottom)  the fuel/air vapour would be shot right up the cylinder bypasses. But that's just theory, not sure it could be detected with a tachometer. [Edit 2: There might be a swirl in the crankcase that would mean the best reed position should be a bit before, and that would depend on which way, CCW or CW, the engine was run. Getting pretty far out on a limb with speculation.] Another question which occurred to me is that the Tee Dee cylinder can seat in various angular positions depending on how the threading ends. Would this affect induction ? Also the exhaust port relation to external flow.

[Edit 4: I agree with Cribb74 on the value of your contribution. Thanks again!  :-) ]

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"Socrates' attitude to the laws of Athens is set out in the 'Crito', which shows him unwilling to flee and so escape sentence. Though the laws be unjust they must be obeyed lest the rule of law fall into disrepute. He fails to see that this might happen precisely because of injustice." Bertrand Russell, Wisdom of the West, p.71


Last edited by DougW on Sun May 14, 2017 10:32 am; edited 3 times in total (Reason for editing : 1.To add B.R. quote 2.Induction question 3.Thin reed not essential? 4.Cribb74)
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Help! Re: How to arrange Thermal Hopper Reed Valve Components?

Post  Admin on Sun May 14, 2017 11:51 am

This is good thread, stickied!

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Help! Re: How to arrange Thermal Hopper Reed Valve Components?

Post  Mudhen on Sun May 14, 2017 12:03 pm

The two-reed concept began when Cox vice president, Mark Mier, co-designed the .045 engine for the O-45 Special Series tether cars.
Mark’s inspiration for the engine was a Kiekhaefer Mercury outboard engine he used for water skiing. While I don’t know for sure, I think the Mercury engine was also dual reed.
From the .045 through the Strato-Bug, the twin reed concept remained. While spending over a year to design and test the Space Bug, there seemed to be no reason to abandon the idea.
It may not have occurred to anyone that two reeds were unnecessary. The reason for the dual reeds was that the thin reed sealed against the carb body while the thicker reed provided spring return tension. The reed back plate limited the reeds movement. (That is why the flat portion extends away from the reeds.)
It wasn’t until Bill Selzer designed the Babe Bee that the twin-reed concept was dropped in favor of a single reed.

I’ve restored several of these engines and discovered some unusual anomalies. Some engines were equipped with two thick reeds and others with two thin reeds.
The thicker reed was marked with a blue dye to help visually confirm the difference. Yet, engines still left the factory with mismatched reeds.
I’ve also run the early engines, (except for a Thermal Hopper,) and they all ran fine with a thick reed. I even had a Strato Bug wind up to 18,000 rpm.
Another issue which pertains to the Space Bug and Thermal Hopper is that the thin reed sealed against an aluminum lip machined into the carburetor body. At 250 revs/second, it doesn’t take long for the thin reed to fail. The lip cuts a nice curved slit into the reed. As far as sealing is concerned, there is no difference because whether one reed is used, or two, the reed sealing against the carb has a liquid “gasket” that forms on each intake stroke.

If you want to restore the engine for display, then keep the two reeds. If you want to run the engine, the thick reed is all you need.
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Help! Wow! Very comprehensive.

Post  DougW on Sun May 14, 2017 4:25 pm

Dear Mr. Mudhen,

Now I understand as well as believe. Your account of the history is quite something interesting as well as locking in the thin reed concepts.

Maybe the hole in the limit piece #307 relieves gas pressure on it and improves flow.

You have probably saved me from taking it all apart and re-assembling it backwards.

I am not quite sure whether to pro-actively remove the thin reed like a wisdom tooth or to wait for it to fail. Would one notice that it had? Would it for a while protect the thick reed from the aluminum lip in the carb body?
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Help! Re: How to arrange Thermal Hopper Reed Valve Components?

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