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How to arrange Thermal Hopper Reed Valve Components? Empty

How to arrange Thermal Hopper Reed Valve Components?

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How to arrange Thermal Hopper Reed Valve Components? Empty How to arrange Thermal Hopper Reed Valve Components?

Post  DougW 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 Admin on Thu Sep 06, 2018 11:51 pm; edited 5 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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Post  Mudhen Sat May 13, 2017 9:44 pm

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Last edited by Mudhen on Thu Nov 18, 2021 12:17 pm; edited 1 time in total
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Post  Cribbs74 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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How to arrange Thermal Hopper Reed Valve Components? Empty Thanks Mudhen!

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

This is good thread, stickied!

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Post  Mudhen Sun May 14, 2017 12:03 pm

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Last edited by Mudhen on Thu Nov 18, 2021 12:16 pm; edited 1 time in total
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How to arrange Thermal Hopper Reed Valve Components? Empty Wow! Very comprehensive.

Post  DougW 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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How to arrange Thermal Hopper Reed Valve Components? Empty Bug Reeds

Post  jmendoza Fri Sep 17, 2021 1:07 pm

The Bugs came with 2 reeds, a thick, and a thin. Thin reed under the thick reed. The Reed stop was there to hold the reeds close to the opening in the venturi so they would not distort, and to prevent the reeds from opening too much and interfering with the crank-pin and con rod big end. The hole in the reed stop are to allow flow and was oriented with the tip facing 12:00.

As to some engines having two thin reeds, this is most likely an assembly error, as the thin reeds sometimes stuck together because they were stamped from several sheets of brass stacked on top of each other and were difficult to separate. Interestingly, Gen 2 Bugs came with a single reed which was as thick as the thin and thick reed combined. It is tricky to get the reed retainer on properly without putting a kink or ding in the thin reed and then it will not seal correctly. So, it was found that a thicker reed was easier and less problematic (wears longer).

The direction the reed stop is oriented does not matter because, if you stop and think about it, the flow from the venturi past the reeds is 180 degrees out of phase with the flow from the case into the cylinder bypass. Meaning that the reed is closed when the piston moves down to uncover the bypass/transfer ports when the charge flow through them.
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