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Cox reed valve thickness? Empty
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Cox reed valve thickness?

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Cox reed valve thickness? Empty Cox reed valve thickness?

Post  wmazz on Mon 11 Feb 2019 - 20:19

I just finished rebuilding a .020 pee wee, and the kit came with
a Mylar (I believe) reed. It had a mangled copper reed, and the
e-clip looked like it was upside down.

I have read about all sorts of reed valves for Cox engines, but
I have often wondered if large 2-strokes and Cox engines like
the same characteristics for reeds??

For low rpm, I would use a thin reed for good throttle response.

For overall performance I would use a medium thickness reed.

For peak HP, I would use a thick reed.

I am asking because IMO, the copper reed would have been
better suited for sustained high rpm over a light weight mylar
reed?


Bill M.

BTW, making venturi tube gaskets for the pee-wee was a pita!
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Post  Marleysky on Mon 11 Feb 2019 - 22:02

There has been pages of discussion on reeds, materials, thicknesses. I don’t think anyone has done a standardized laboratory type test on the differentiations between the various materials available. I myself use the “ suck-blow” test to verify the reed seals and call it good.  If the engine starts and runs, I consider it a WIN!  
As far as making the Venturi gaskets, there is a great write up on how to make your own on a bamboo stick and a exacto knife, using silicone tube. Ive purchased the “factory” versions for the 049, and the Pee Wee rebuild kits include one for the 020 size. I’m sure our sponser’s appreciate our business.
http://www.exmodelengines.com/product.php?productid=17767&cat=249&page=1

Yes, the original star shaped copper (berilium?) reeds were intended for extended 5 to 8 oz runs.  I don’t think many used throttle control, as you were doing good if you actually got the plane to fly without crashing, and introducing dirt into a engine that trives on being extremely clean to stay running!!
My nickels worth Wink
Also, I am not familiar with the larger 2stroke engines using the reed valve. Most larger engines utilize the rotary intake for air/fuel mixing.
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Post  wmazz on Tue 12 Feb 2019 - 15:54

Marleysky wrote:
There has been pages of discussion on reeds, materials, thicknesses. I don’t think anyone
has done a standardized laboratory type test on the differentiations between the various
materials available.

My thoughts were:

A thin light weight reed would best suit a Texaco engine.

A medium thickness/weight reed would suit general purpose engines to ~18,000rpm

And a thick or heavy reed would better suit high rpm engines.

Marleysky wrote:
As far as making the Venturi gaskets, there is a great write up on how to make your own on
a bamboo stick and a exacto knife, using silicone tube.

I used that tutorial to make my gasket.

Marleysky wrote:
Ive purchased the “factory” versions for the 049, and the Pee Wee rebuild kits include one for
the 020 size. I’m sure our sponser’s appreciate our business.

I recently made purchases from Ex Model Engines, and Cox Int.

Marleysky wrote:
Most larger engines utilize the rotary intake for air/fuel mixing.

Are you a "Ski Doo" man, or were you thinking about regular model airplane engines?


Bill M.
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