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Cox Engine of The Month
September-2021
crankbndr's

"Cherry Bomb" .051 engine

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Post  chevyiron420 Sat Nov 04, 2017 6:52 pm

This is the first video I have ever attempted. Yesterday I got 14500 rpm out of it but not today. I think its leaking air around the needle valve. I do have the original cylinder back on, and found a piston\rod with a pretty good fit. Its running on 10%-26% fuel and a 6x3 cox prop.
https://www.youtube.com/watch?v=OcCFEhqDWSI
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Post  roddie Sat Nov 04, 2017 9:23 pm

Hey Phil! Congrats on your first video man!! Thumbs Up I'd never heard a Wen-Mac run before. She seemed to run pretty steady. Actually makes me want to run my recent rebuild. My needles for the rotary-valve engines all have "flex-cable" extensions.. and both needles seem "loose".. like they'd likely leak air. I only have two of those needles.. and one is noticeably bent.

Do you have any "Dubro" small (1/2A size) silicone fuel tubing.. or "any" silicone fuel-tubing small enough to fit snugly over the threads of the needle? If you suspect an air-leak there.. that would likely fix it. That's a place to start anyway. Is 14.5K what you're used to seeing on that fuel blend and prop?

You had previously mentioned to me, that the heavy starter/flywheel assy. on these engines can be removed.. and a Cox drive-plate fitted to significantly reduce weight. That starter-flywheel weighs nearly .25oz. and the band-spring (which would be discarded) would definitely put the assy. over .25oz. The Wen-Mac/Testors .049 engines have a 6-32 threaded crankshaft.. correct? This would entail enlarging (drilling-out) the Cox .049's drive-plate.. right? Do you know which size drill would be used.. to best enable the crankshaft's splines to be cut into a Cox drive-plate? It would seem a worthwhile mod to make for a lighter-weight engine. These engines probably don't make near the power of a Cox Medallion.. but they don't suffer from the gremlins of reed-valve induction either. It would be interesting to "strip one down" and build a small model such as a Goldberg "Stuntman 23" or a Sig "Skyray".. to see how it performs.

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Post  1/2A Nut Sat Nov 04, 2017 9:54 pm



there you go Smile
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Post  MauricioB Sat Nov 04, 2017 11:19 pm

@chevyiron420 wrote:This is the first video I have ever attempted. Yesterday I got 14500 rpm out of it but not today. I think its leaking air around the needle valve. I do have the original cylinder back on, and found a piston\rod with a pretty good fit. Its running on 10%-26% fuel and a 6x3 cox prop.
https://www.youtube.com/watch?v=OcCFEhqDWSI

What a pleasure to see the engine work, congratulations .... there are a couple of beers! Beer Cheers Thumbs Up
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Post  chevyiron420 Sat Nov 04, 2017 11:48 pm

@roddie wrote:Hey Phil! Congrats on your first video man!! Thumbs Up I'd never heard a Wen-Mac run before. She seemed to run pretty steady. Actually makes me want to run my recent rebuild. My needles for the rotary-valve engines all have "flex-cable" extensions.. and both needles seem "loose".. like they'd likely leak air. I only have two of those needles.. and one is noticeably bent.

Do you have any "Dubro" small (1/2A size) silicone fuel tubing.. or "any" silicone fuel-tubing small enough to fit snugly over the threads of the needle? If you suspect an air-leak there.. that would likely fix it. That's a place to start anyway. Is 14.5K what you're used to seeing on that fuel blend and prop?

You had previously mentioned to me, that the heavy starter/flywheel assy. on these engines can be removed.. and a Cox drive-plate fitted to significantly reduce weight. That starter-flywheel weighs nearly .25oz. and the band-spring (which would be discarded) would definitely put the assy. over .25oz. The Wen-Mac/Testors .049 engines have a 6-32 threaded crankshaft.. correct? This would entail enlarging (drilling-out) the Cox .049's drive-plate.. right? Do you know which size drill would be used.. to best enable the crankshaft's splines to be cut into a Cox drive-plate? It would seem a worthwhile mod to make for a lighter-weight engine. These engines probably don't make near the power of a Cox Medallion.. but they don't suffer from the gremlins of reed-valve induction either. It would be interesting to "strip one down" and build a small model such as a Goldberg "Stuntman 23" or a Sig "Skyray".. to see how it performs.

Thanks roddie, yeah I have some fuel line that fits on the needles well. I was just tired and wanted to shut down for a wile.
Its not difficult to remove the starters but I normally leave them on. I like using them. I will use a cox drive washer on a car engine that has no rotomatic on it. The crank threads are 6x32 but the splines are bigger than that. I measured the spline and chose a number drill a hair under that. I dont have my drill bits here or I would check it for you. I think either of the models you spoke of would be good choices. Here is my Baby Ring even though I would not radial mount again.

wen mac mkXIII video Img_0216
wen mac mkXIII video Img_0215
Yeah roddie I think that 14000 rpm is fairly typical with that fuel and prop, even though I have one that goes much faster.
Phil
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Post  Kim Sun Nov 05, 2017 4:44 am

Cool video and photos Phil!!
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Post  roddie Sun Nov 05, 2017 10:12 am

Hi again Phil,  So this was originally a car engine?

wen mac mkXIII video Img_0210

Yea.. regarding installing a Cox drive-plate; I could manage that. I have a vernier/caliper and a full #drill index.

What's involved with removing the roto-matic starter "housing" from the airplane-engine? Citing my engines; I see two pins protruding from the front of the case. Is the housing merely pressed-on to these pins? The Testors style has horizontally-oriented pins (protruding from the beam-mount lugs..) and the Wen-Mac has vertically-oriented pins.. with the upper pin sticking into the venturi.. Huh...

Here's a Testors example..

wen mac mkXIII video Roto-m10

and here's the Wen-Mac example..

wen mac mkXIII video Roto-m11

Are these housings (either type) "pressed-on"? I suppose that once the flywheel is pressed-on to the crank; the housing can't really move forward off the two pins. The above Wen-Mac example is disassembled.. and the housing feels fairly secure.
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Post  chevyiron420 Sun Nov 05, 2017 12:06 pm

roddie, yep that was a car engine. I think the drill bit size for drilling the cox drive washer is a #13 .185, you might start with a #14 and see if it will start on your shaft first.
Now, the spring housing is not pressed on, its just held on with two little rivets. They look like tiny nails with spiral splines. All you do is get a small screw driver under the housing and wiggle and pry till the rivets start to back out. If you bend the housing a little lay it flat on something substantial like a vise and put a piece of round stock in it, or a upside down socket and tap it back in shape. The late model wen macs and the testors have the rivets horizontal. The wen macs are marked with a little bar under the 049. The early engines have the vertical rivets. If you remove the starter from one with the rivet in the carb, plug the hole or you wont be able to choke the engine.
Have you got your testors mccoy together? I would love to see a video of it running. I bet it will out run my wen mac mkXIII. This Site Rocks!
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Post  chevyiron420 Sun Nov 05, 2017 1:13 pm

I knew the needle didnt feel right yesterday, so I swapped it for another one. man what a difference! Same fuel, same prop.
wen mac mkXIII video Img_0217
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Post  roddie Sun Nov 05, 2017 4:35 pm

@chevyiron420 wrote:roddie, yep that was a car engine. I think the drill bit size for drilling the cox drive washer is a #13  .185, you might start with a #14 and see if it will start on your shaft first.
Now, the spring housing is not pressed on, its just held on with two little rivets. They look like tiny nails with spiral splines. All you do is get a small screw driver under the housing and wiggle and pry till the rivets start to back out. If you bend the housing a little lay it flat on something substantial like a vise and put a piece of round stock in it, or a upside down socket and tap it back in shape. The late model wen macs and the testors have the rivets horizontal. The wen macs are marked with a little bar under the 049. The early engines have the vertical rivets. If you remove the starter from one with the rivet in the carb, plug the hole or you wont be able to choke the engine.
Have you got your testors mccoy together? I would love to see a video of it running. I bet it will out run my wen mac mkXIII. This Site Rocks!

Geez.. I just lost a lengthy post into oblivion by dumb-thumbing the wrong key on my laptop... DAMMIT! Mad Mad Mad

In short... DAMMIT! .. Laughing The Testors/McCoy is back together.. but I may check my reed-valve engines to see if I can find a better piston/rod fit. The one that's in there isn't real bad.. but I'd like one with less play.. having replaced a broken crank.

Then there's this Wen-Mac Mark XI which I have a good feeling about. It was in the box that Bob (rsv1cox) sent.

wen mac mkXIII video Wenmac23
wen mac mkXIII video Wenmac24

I have a surplus of various 6 x 3 propellers.. and will enlarge the hub-holes of a couple to try on these engines.
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