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Cox Engine of The Month
November-2019
jmcalata's

"Pee Wee .020 throttled combinated"



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Post  1/2A Nut on Thu Feb 21, 2019 11:28 pm

I think the trick here is what can be done today by most folks without having to
rob rare parts or do any difficult mods. The piston is purpose built back in the day
for more rpm but failed to hold up to that stress. So today one should look at this
as a weight savings project to use on a smaller plane such as a .020 project that
needs the extra boost of power without having to make the engine scream.
In that case one could still have quite a beast of a power plant for 7g more than
a .020. Same thing can go in hand with a CL project that has a issue with a
CG too far forward instead of adding weight in the tail this would be an option.
Then again a purpose built plane for a extremely low weight .049 could bring
forth a interesting and fun plane or car or boat project.


Last edited by 1/2A Nut on Thu Feb 21, 2019 11:35 pm; edited 1 time in total
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Post  Jason_WI on Thu Feb 21, 2019 11:33 pm

Could save another gram with a narrow neck crankcase. Also the early baby bee crankshaft had a thinner web.
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Post  1/2A Nut on Fri Feb 22, 2019 1:04 am

Yes good idea every effort adds up that's the trick.
On page 1 I updated some pics can save .2g going with a Killer Bee crank.

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Post  1/2A Nut on Sun Feb 24, 2019 1:02 am

Horse shoe backplate will allow me to bench test the engine as shown.
Least amount of mass to spin with a 4 x 4.5 prop which would be a
considerable boost in speed and power if swapping out a Pee Wee
engine on a existing project plane. To be fare one will have to include
the weight of a fuel tank in my case I would use a 15cc / 4.1g custom
made tank.

For me I would also want a aluminum throttle sleeve for
RC use figure another 2.5g / .515" dia cylinder will need a lapped in
.52 ID sleeve / tight fit to compensate for expansion when hot.
.15% nitro 20% castor would be plenty.

Cox aluminum piston? - Page 2 Imag9321

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Post  balogh on Sun Feb 24, 2019 1:07 am

Brad how does the compression feel with the aluminum piston? Is the theory correct that to consider the higher thermal expansion of aluminum than steel the piston fit when cold was set loose?
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Post  1/2A Nut on Sun Feb 24, 2019 2:59 am

The cylinder / piston fit is popping over in the "good" category.
It has been oiled with after run oil that aids in compression.

I will have to do a rich break in. I need to put a muffler on it next
keep the noise down during the bench test.

Yes like all model engines with aluminum pistons when running
the compression should improve. I am using a stock low compression
glow plug in anticipation of that happening. I think folks found the
compression was lost after a few runs but in retrospect the compression
was just fine once running with a expanded piston.


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Post  getback on Sun Feb 24, 2019 6:25 am

Brad do you think the engine would benefit with using extra castor ?
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Post  1/2A Nut on Sun Feb 24, 2019 12:32 pm

18 - 22% should be enough depending on the desired rpm and time of year.
Summer time in TX if high rpm is desired would go 20% nitro / 22% castor oil.

18% castor and 6% synthetic has some benefits keeping the engine cleaner
as the thinner oil will seep everywhere washing out the engine reducing
baked on castor build up.
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Post  1/2A Nut on Sun Feb 24, 2019 5:10 pm

Results:
First rich run, aluminum was dusting off and hardening it's surface to the new piston.
Cox aluminum piston? - Page 2 Alumin10

Second tank the aluminum particle mixed goo was reduced quite a bit as it becomes more seated.
At about half way through the tank I started to lean the mix got too greedy with the rpm allowing the piston
to expand too much too soon and caused the engine to stop. Throttle is set low enough to keep the noise down.
Needle was at 3.4 turns which started to "race" the piston. 47g with slide type exhaust throttle.


Uploaded on Feb 24, 2019

Grish 5x4
Cox Aluminum Piston Bench Test Break In .049
1oz rich run prior to this one aluminum surface was
dusting off small particles as it seated into it's thermal
state. If leaned up too much it expands to the next level
of heated state subject to binding enough to shut down.
Will require patience with some rich runs till fully broken in.

Compression seems good still bit less than when new. I imagine
another 2oz of rich running to baby the piston into it's best tuned
thermal state. You want to compress the outer skin of the piston's
molecules into a hardened condition, if rushed you scrub off too much
material too fast. The reward is a piston that will last. Plus you don't
want to stress the ball socket during break in, it is aluminum after all.


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Post  getback on Mon Feb 25, 2019 8:43 am

Thanks Brad , information taken in , i have 3 pistons and 1 rod coming , i am pretty sure somewhere in all my stuff i have some rods maybe even alum. one ? Small Cox Logo This Site Rocks!
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Post  GallopingGhostler on Mon Feb 25, 2019 10:35 am

1/2A Nut wrote:Grish 5x4. Cox Aluminum Piston Bench Test Break In .049 1oz rich run prior to this one aluminum surface was dusting off small particles as it seated into it's thermal state. If leaned up too much it expands to the next level of heated state subject to binding enough to shut down. Will require patience with some rich runs till fully broken in.

Compression seems good still bit less than when new. I imagine another 2oz of rich running to baby the piston into it's best tuned thermal state. You want to compress the outer skin of the piston's molecules into a hardened condition, if rushed you scrub off too much
material too fast. The reward is a piston that will last. Plus you don't want to stress the ball socket during break in, it is aluminum after all.

Thanks for sharing this Brad and for documenting your experiences. Regarding thermal expansion got me a little curious about the theoretical empirical dynamics behind the metallurgy. (I say theoretical because values given are in general and not toward the specific grades of materials involved, and arbitrary assumptions made for the sake of discussion so values can be quantified.)

https://www.engineeringtoolbox.com/linear-expansion-coefficients-d_95.html

Thermal Expansion Coefficients in 10-6 in./(in. Deg-R):

  • Steel, 6.11 - 6.94
  • Cast Iron, Gray 6
  • Aluminum, 11.7 - 13.3
  • Brass, 10 - 10.6

Degrees are in Rankine, to convert Fahrenheit, add 459.67.

Cox .049 bore is 0.406 in. Hypothetically, say steady state methanol combustion temperature with 15% - 25% nitro is 500 deg-F (where most synthetic oils vaporize and combust), then we multiply these coefficient values by 1e-6 * [0.406 x (500 + 459.67)] = 0.00039

Arbitrarily rounding values for back of the envelope calculations:

Aluminium (12) = 0.005 in.
Steel (6.2) = 0.002 in.
Cast Iron (6) = 0.002.

We see that at 500 deg F, we have about 3 thousandths of an inch piston expansion toward cylinder walls. Of course, we have not also taken into account that because the piston head is thinner, it doesn't have the heat sink effect of the thicker cylinder, which will not expand as much due to its slightly lower temperature. Piston top will be hotter.

It is not hard to see that one may have to start the engine with lower compression, so that by the time it reaches steady state temperature, the aluminium piston has expanded to the point where it can now maintain a proper compression seal.

Now we can surmise that Leroy Cox went to a steel piston for obvious reasons. The ABC engines work well with aluminium pistons because the brass is a lot closer to aluminium than steel for the sleeve.
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Post  1/2A Nut on Mon Feb 25, 2019 10:38 am

Thank you Eric, it has been fun to play with, something different.
The aluminum rod would surely require some flywheel tweaking
to get it to balance and run smooth.

Yes  Thumbs Up good info GG the engine temp variance between steel and
aluminum is requiring me to move with caution giving the piston the
opportunity to scrub off aluminum at a pace that wont over shoot best fit.

A digital meter to read temp would be a good tool to have on
tap during this process. After full break in I will see if my
calipers show much change in dia. range is  0.001 to 6"

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