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"Traded an electric charger for this "Complete" (original) Cox Thimble Drome Olympic .15 engine. Right corner is photo as received."



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Finally got to this FrankenTD Empty Finally got to this FrankenTD

Post  batjac on Wed Sep 16, 2020 11:44 am

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:

Finally got to this FrankenTD DSCN7244


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.

Finally got to this FrankenTD DSCN7245
Finally got to this FrankenTD DSCN7255


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.

Finally got to this FrankenTD DSCN7263


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 here 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

Finally got to this FrankenTD DSCN7276


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.

Finally got to this FrankenTD DSCN7281
Finally got to this FrankenTD DSCN7283


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.  Hmmm ²…  Why can’t I just drill out the existing NVA body and press in the fine NVA?  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.

Finally got to this FrankenTD DSCN7287
Finally got to this FrankenTD DSCN7291
Finally got to this FrankenTD DSCN7297



Then I pressed the fine NVA into the stock NVA, and made sure it was seated using an auto-center punch.  Lining up the hole in the fine NVA 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 NVA 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 NVA 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!

Finally got to this FrankenTD DSCN7306
Finally got to this FrankenTD DSCN7311


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.

Finally got to this FrankenTD DSCN7316
Finally got to this FrankenTD DSCN7318



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