Tee Dee needle thread pitch

Thanks Roddie,

The thread pitch is 80 TPI.  Nominal diameter is .083" for the horse shoe type back plate needle. The TD .049/.051 and the tanked Bees diameter is .100" nominal, although the needle tapers are different. Found that out trying different needles in different spray bars.
The Cox instruction sheet does not mention running Tee Dees on bladder pressure although I have never run one on a plane with a hard tank. This was back in the 1980's running 1/2 A combat.  Never had problems with the stock setup using surgical tubing as a bladder. We used Cox racing fuel in those days.  I have bench run them on a hard tank though.  I do have a couple of new in package Kustom Kraftsmanship fine thread needle valve assemblies and pressurized back plates that was from my LHS and was wondering if I should try them out.

Cool! Cox used at least two different fine-thread needles/NVA's in their early product engines. I've asked questions about them here in the past, but haven't gotten any replies.

Here's two that I have..




and the back-plates they came out of..



I'm assuming that the long-tapered needle came out of the back-plate with the longer nipple on its spray-bar but I'm not sure. I don't want to install the wrong ones.. and chance damaging the seats.

You'll notice that the shorter needle is bent a little. I'm also assuming that it was the earlier of the two.. and looks like a more precision piece with a rounded-point. The back-plate with the shorter "nipple" is probably the one that it fits. I wonder if Cox; in a cost-saving measure, redesigned the needle to be cheaper to "machine".. and at the same time.. added some length to the nipple for better fuel-line retention? That's just my own theory..

Sorry for the lousy image. You can enlarge image for a better view  Top needle and cutaway spray bar is the 128 TPI .
Bottom is the 80 TPI assembly
It takes 12 turns to clear inlet hole on the 128TPI assy.
6 turns to clear on the 80 TPI assy.  
Tried different needles with similar results.
Planning on experimenting with different tapers on the 80 TPI needles to see if I can get more turns out from the needle seat. Notice the extra threads sticking out on the needle that could allow that. Maybe you could do some experimenting too.

Agustin Jr.

coxaddict wrote:

Sorry for the lousy image. You can enlarge image for a better view  Top needle and cutaway spray bar is the 128 TPI .
Bottom is the 80 TPI assembly
It takes 12 turns to clear inlet hole on the 128TPI assy.
6 turns to clear on the 80 TPI assy.  
Tried different needles with similar results.
Planning on experimenting with different tapers on the 80 TPI needles to see if I can get more turns out from the needle seat. Notice the extra threads sticking out on the needle that could allow that. Maybe you could do some experimenting too.

Agustin Jr.

My previous photo was actually of two 128 TPI needles.. although the long one looks similar to an 80 TPI needle from a product back-plate.

Here's a photo of a coarse-thread (left) and that fine-thread (right)



That fine-threaded one will also probably require the 12 turns (approx.) to clear the inlet-hole.. as you mentioned.

Someday I'd like to try running a pressure-bladder. The fine-threaded needles/NVA's would likely be easier for "me" to tune, than the coarse ones. I have no experience swapping NVA's in/out of carbs.. so I made my own mounts for the back-plates that use those needles.



Below photo shows the mount-comparison between "postage-stamp" (190 product back-plate/fine needle) and "horseshoe" back-plate (coarse-needle)..



My mounts are "taller" (vertically) and wider (horizontally) than the horseshoe-mount.. but will fit vertically on a radial-firewall designed for a "bee" engine.

Your mounts are really nice. What material did you use to make them?

coxaddict wrote:Your mounts are really nice.  What material did you use to make them?

Thank You Augustin! They are a light-gage aluminum.. (.060" I believe) Goldberg Models made a radial-firewall mount for the old Cox "Postage-Stamp" plastic backplate.. and they can still be sourced.

The mounts that I made were influenced by Dale Kirn's "290-Special" 1/2A control-line balsa airplane plan that adapted the early Cox product-engine to have a new "life" after the plastic airplane that the engine was originally in.. broke and was non-repairable..  which was inevitable. The well-made engines usually languished in a box.. or worse; were thrown away along with the broken airplane.

Dale's model-plan called for modifying the backplate (sawing-off the two protrusions on the backside) to accept a pair of light-gage sheet-steel mounts that were drilled to accept the case-screws.. and bent so as to be mounted to a non-conventional nose-block on the model.





I though that this looked a lot more complicated than it needed to be. The backplate need-not be sawn/cut to accomplish making mounts for it. I designed two plates with two 90-degree opposing bends which mounts flush onto a conventional radial firewall in the same space as a .049 "Bee-engine" would occupy. The mounts fit between the molded protrusions on the backside.. and allow clearance for the air-intake.




I made a few versions of the plates on a CNC router machine until I got them to where I was satisfied after the "bends" were made. I made a fixture/jig for accurately forming the two 90 degree bends. A slotted PVC block to be held in a bench-vise. The prototype-block worked.. but showed signs of fatigue after bending only a few plates. The bending is done with an aluminum-block and small hammer. This placed a lot of pressure on the PVC block's slot.




I redesigned the block with a slot widened to accept aluminum "jaws" of the same stock as the mounts.




This allowed the bending of several mount-sets before the fixture-block showed signs of deformation.



These mounts were machined with a 1/8" endmill completely.. therefore all of the bores have a .125" ID. If you made them at home.. (and you could very easily with basic tools..) you'd want to drill the case-holes a bit smaller.. but that depends on how close you can get the spacing to 17/32" using hand-tools. The case-screws are 2-56.. and a 2-56 screw has a .082" OD.. so drilling the holes with a standard 3/32" drill should suffice.. if you mark the hole-positions accurately.