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Author Topic: Cox TD fine needle conversion  (Read 1804 times)

Offline Mark Mc

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Cox TD fine needle conversion
« on: March 20, 2021, 02:37:26 AM »
This came back up over on the CEF, and I realized I never posted it on this board.  So I figured I'd throw it up here for those who don't frequent the CEF.  I'm just copying and pasting over, so sorry for the large pics.  I tried to find where I had them saved online so I could minimize them, but they're not where I thought.  Maybe someone here will find it of interest.

When I was digging out the parts for a Babe Bee for another person, I came across a baggie of parts.  These are leftovers from an RCG buy from a couple of years ago for an abused TD .051 and other broken parts.  I fixed the mostly there TD .051 and threw the rest of the parts in the baggie for later.  Lately I’ve been in a pressurized TD mood, so I figured I’d get to this as soon as I finished what I was doing for the other fellers.  The case is the infamous #2 case for the .051, and everything else was just parts.  I had to destroy the red venturi body to get it off, but it was broken anyway.  Here’s what I had to work with:




The crank, as you can see, was broken somehow.  The venturi looks like someone used a pair of pliers on it.  I don’t have a spare venturi so I needed to fix the smashed one.  Thinking for a while, I figured that if I use this prop nut and tap it a few times into the venturi would return it to a round(ish) shape.





Then I dug out the other spares I had, a new venturi body, a spare needle valve assembly, a spare #4 piston/cylinder set, and a used but good high compression glow head.  Look though I might, I could not find a crankshaft.  Ah, well.  I’ve been looking for a reason to build a TD with a left handed crank so I could build some twin engine planes, so this is the time.




Since this is a FrankenTD, I started thinking about drilling out the venturi since I plan on using bladder pressure.  I thought about it for a couple of hours, but decided against it for now.  But I did have a fine NVA from a different project, and decided I’d do the conversion on the stock NVA.  I looked up the previous thread there on the CEF about converting a TD with a postage stamp fine needle here: https://www.coxengineforum.com/t11875-fine-thread-nv-assembly-for-a-td.  It's the old method of pulling the coarse NVA out, pressing in a piece of tubing, and epoxying in the spraybar.




I have everything I need to do the conversion, except using JB Weld instead of epoxy as described in the article.  But, again I thought about it for a while.  Do I really need to go to all that trouble?  That stock needle is awfully thick.  Even.  Almost the same as the fine NVA barrel?  Time for the calipers.





Well whaddayknow. The needle diameter is 0.11 inch, and the barrel of the fine NVA is 0.112.  Hmmm…  The knurled portion of the NVA is slightly larger in diameter at 0.119 inch.  Hmmm²…  Why can’t I just drill out the existing NVA body and press in the fine needle spraybar?  Not even mess with the 5/32 tube and JB Weld?  I have that expensive drill index with every size bit known to man that I’ve never used.  Looking at the index tells me that a #32 drill bit at 0.116 should be just about perfect.

I cut off the fuel nipple to expose the small hole, but left the threaded side of the NVA so I could use that as a drill guide so as not to drill the hole off center.  I drilled the hole stepping up the size of drill bit until I got to the #32 bit.  Came out just perfect for the fine NVA.  Then I cut off the threaded side of the stock NVA.  Just the right size.







Then I pressed the fine spraybar into the stock NVA, and made sure it was seated using an auto-center punch.  Lining up the hole in the fine spraybar with the hole in the stock NVA requires a lot of care.  After drilling out the stock NVA and looking inside, it answered a question I’d had for a long time.  I wondered how they drilled that tiny hole straight through the stock NVA exactly centered.  The answer is that they drill it off centered.  The factory drill bit comes in at an angle, so that with the hole centered in the inner face of the NVA, the other side of the hole is off center in the barrel.  Not a problem with the stock needle, but a big problem for me.  If I pressed the fine spraybar with the hole facing directly centered with the hole in the inner face of the stock NVA, the holes would not align and the fuel wouldn’t flow.  I had to be careful to press the fine spraybar in at the proper offset for the holes to line up. 

After seating the fine NVA in the old, stock NVA, it was time to see if I’d screwed up and wasted a good NVA.  I made up a bladder tube and hooked it up to the new NVA.  With the needle closed, I filled the bladder with air and then watched to see if there was any leakage.  Huzza!!!  No leak!  But, would fuel flow when I open the needle?  I opened the needle and the air flowed out of the bladder through the NVA setup.  But, did it leak when the needle was open?  I filled the bladder again with air, and then put my thumb and finger on each side of the NVA, then opened the needle.  A very small leak.  Okay, no biggie. A little leak around the needle threads is expected.  So I dug out a product engine backplate spray bar seal (https://coxengines.ca/cox-.049-.074-spray-bar-seal-10.html) from my spares and inserted it on the NVA barrel.  A second leak check with the needle open showed no leak.  Huzzahs and Hosannas!





Now to find a left-handed prop to run.  Digging around showed that the only 1/2A sized left handed (pusher) props I have are 1) the Curtiss Pusher prop, 2) a couple of speed props, 3) a Cox 6x2 plastic prop, and 4) some three bladed pusher props.  The 6x2 is the only one I could use for sport flying, so I’ll try that while I search for a better option for matching left and right handed props.






Mark

Offline Larry Renger

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Re: Cox TD fine needle conversion
« Reply #1 on: March 20, 2021, 05:26:14 PM »
Tornado made matching L and R props in many sizes.

A great project that would not be too hard to copy. Thanks! 👍
Think S.M.A.L.L. y'all and, it's all good, CL, FF and RC!

DesignMan
 BTW, Dracula Sucks!  A closed mouth gathers no feet!

Offline kevin king

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Re: Cox TD fine needle conversion
« Reply #2 on: December 20, 2021, 01:57:20 PM »
Mark, where did you get the bladder fitting and what kind of bladder are you using?   

Offline 944_Jim

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Re: Cox TD fine needle conversion
« Reply #3 on: December 20, 2021, 06:45:48 PM »
Great job, Mark! I wish we could do Greenies here too. You would have my vote.

Offline Mark Mc

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Re: Cox TD fine needle conversion
« Reply #4 on: December 20, 2021, 11:08:33 PM »
Kevin,  the bladder material and fittings come from Texas Timers (http://texastimers.com/).  The stuff they sell has always worked very well for me.  The bladder tube is what they are selling for $1.50 a foot.  I use only about an inch, maybe an inch and a half, for a decent run time with that tube.  It expands more than you’d think. I usually replace the tubing every couple of weeks, as I don’t want to let it get too weak and rupture on me.  Just buy a couple of feet of fuel tubing and it will last you a season.  At $1.00, the fuel tube fittings are cheap, so I ordered three of them to keep a spare bladder set on hand at any one time in my field box so that I can quickly replace a bad tube.  I also bought three syringe connectors so that I keep two syringes ready to go in my field box, one 10cc and one 30cc syringe.  I use the small fuel line that they sell at $2.50 a foot.  A little expensive, maybe.  But it is the only fuel tube that I find reliably stays attached to the small fuel nipple on the TD NVA.  I also bought three check valves.  They last a long time, and you never know when they will become unavailable.  They are the most expensive part of the bladder setup, but it makes it very easy to refill the bladder for the next run, vs. unplugging the fuel line from the NVA to fill the bladder.  I put the check valve on the side of the bladder tube opposite the fuel tube fitting so that I fill the bladder from the back.  This is described on the bladder page of the Texas Timers website.  I, myself, have not used the Tee fitting, but that’s all up to preference.  If I was using a bladder on a combat ship or with the bladder enclosed somehow, I guess a Tee fitting would be useful.  The one time I put a bladder inside a full fuselage, I just did my normal check valve on the back of the bladder tube and ran a long fill tube out the back of the bladder enclosure.

But, I am trying to figure out just what size surgical tubing it is that TT sells, so that when Hank finally retires, I’ll know what size surgical tubing to order from Amazon when I need more.  I’d order some to try out, but you can’t just order a couple of feet off of Amazon.  You have to order a whole roll, and it’d go bad long before I used a fraction of it.  I’ll have to find a local medical supply store and see if I could buy just a couple of feet when I have some time/remember to do it.


Motorman,

At different times, Cox used two different thicknesses of crank web on the TDs.  The #2 case is for the thick web, so if using a crank with a thin web the crank has more fore/aft play in it.  You have to use another or thicker washer behind the drive plate to reduce the end play.

Mark

Offline kevin king

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Re: Cox TD fine needle conversion
« Reply #5 on: December 24, 2021, 03:53:44 AM »
"But, I am trying to figure out just what size surgical tubing it is that TT sells"

1/32" Wall thickness.
3/16" Diameter.
5/32" Inside diameter. Or very close to that.
After looking at the size chart on this site, the Inside diameter  may be 1/8". Its not a fun product to measure that's for sure. Yikes!
https://www.latex-tubing.com/SizingChart.html   
 PS. I will let you find the fuel tubing. 😁

Kevin                                                                 
« Last Edit: December 24, 2021, 04:35:11 AM by kevin king »


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