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Mar 31, 2013, 01:36 PM
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RknRusty's Avatar
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Home made fine thread needle valve for a Big Mig

This came up in my Shoestring Stunter thread but I figured it was better to continue it here so a search would be more likely to find it. I know I searched a lot before I came up with this. One of our coxengineforum members set me on the right track, so I can't claim genius here.

For running pressure bladders on my 1/2A planes I'm always in search of fine thread NVs. There are lots of ways to do it including just pressing the valve through the screw holes in the case and dispensing with the venturi altogether. But I like having a screen, so here's what I did.

I started with an old Cox needle valve from a postage stamp backplate, some of which had 128tpi threaded needles, like the old Stuka engine and a couple of others. For $10 bucks you can also buy this valve from and modify it like this.

EDIT: Jan 19, 2019
I just visited Texas Timers. Although Hank still has the needle valve page, it appears he can't get them any more. A few months ago the price shot way up, and now his message states there will not be any more in stock. I do like his bladder fittings and Luer Lok syringe adapters, so I bought a few extras before they disappear.

The Cox NV is about 1/8" wide and fits snugly into a 5/32" brass tube. And as luck would have it, the 5/32 tube fits firmly into the spraybar hole in the Big Mig venturi.

I tried soldering the Cox NV into the 5/32 tube, but it was sloppy, so I cleaned it up and JB Welded it in there. I made the hole in the tube bigger than the spray hole in the Cox spraybar so there is no interference.

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I roughed up the brass and applied some JB and pressed it into the venturi, carefully making sure the spray hole is pointed in the right direction. I made index marks on the brass and on the venturi so I would know if it turned while I was assembling it. For extra security to keep it from slipping and turning, I drilled and tapped a 2-56 hole through the back side of the venturi for a setscrew to hold it tight. I bored that hole on the fuel inlet side of the spray hole so as not to distort the threaded tube of the spraybar when I tighten it. I also will shorten the needle and solder a 4-40 nut on top of the stub to keep it out of harms way. I'm famous for inverted landings.

It works great. Here's the finished product:

In the first picture you can sort of see the index marks I made to align the holes.
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Description: NVA
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Description: NVA

Here it is on the engine. Plenty of room for the setscrew.
JB Weld is also holding it in place. so I think it's secure. I'll let it set up for a day before I run the engine.
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Description: NVA on Big Mig

Here's one of my my shortened needles:
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EDIT: Jan 19, 2019
If you need help making and using bladders for your 1/2A planes, I made a couple of videos that you can watch on my 'Tube channel. Here's the link:

Back then, I made the bladder videos a little more complicated than it really needed to be, but you'll get the idea.

I hope you find this useful.
Last edited by RknRusty; Jan 19, 2019 at 08:42 PM.
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Mar 31, 2013, 04:34 PM
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If you like inverted landings, maybe you should have a look at the Fora and other F2D needle valves. They are so short that they don't even extend past the crankcase, so there is nothing to break off. Kind of hard to adjust on a little .049 in a hurry like when using a bladder, but ok if forceps or something similar like a shutoff is used.
Mar 31, 2013, 09:30 PM
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Thank You Rusty.!
So Which exact Cox NV did you use?
Remembering that the Cox in Williams lake 'guy' Didn't Know either..
Hopefully so I can order a couple . with Confidence.. of their being correct.

As digression I have a Very short (shorter than on a Fora :-) NV wheel on my VA, from Rutherford, engine. Good thing.. lotsa scary revs there to chew ones' fingers. Also easy enough to diy fab a shorty ... if necessary
Mounting/fitting a nv etc I can deal with .. It's just the 'worth buying' parts which is confusing me at the moment.
Mar 31, 2013, 11:34 PM
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RknRusty's Avatar
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There is nowhere to order the 128tpi Cox NVs. You just have to get lucky and know what you're looking for. But the good news is, go to and click on needle valves in the left column. On that page you'll see a price list including Non-Tee Dee engines. I believe that is identical to the old rare Cox 128tpi NV. They are $10 bucks each. If you have any questions, e-mail them, his name is Hank.

The order form is at the bottom of this main Needle valve page in the picture(I can't link you directly to it because it's a prehistoric old html frames page).

Click the third "Here" link to see a picture of it. I'm pretty sure it's the same as the Cox NV. Hank Nystrom is the guy and he can tell you all the dimensions if they aren't on the picture page. Hank is a good honest guy who is an avid flyer himself.

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Apr 01, 2013, 09:22 AM
Registered User
It looks like you could solder on a piece of tubing to the needle valve where the hole is. Put a piece of aluminum wire or something in the hole so it doesn't get plugged. That will give a remote needle. It would need to be done very carefully for sure. That is how the old Kirnkraft 128 TPI remote needle was. Then all you would need is a nipple on the carb that would need to be made up. I soldered a remote needle to a metal tank and it worked well on a CS motor, a Big Mig would be similar. and your hand is not even close to the motor.
Apr 01, 2013, 10:54 AM
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RknRusty's Avatar
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Or you could solder it into a wheel collar lining up the setscrew hole with the spraybar hole, then thread the tip of a tube to screw into the old setscrew hole. Or maybe a pressure nipple would have the right threads and you could drill it's hole out bigger. Lots of ways to do it. And you could bore and tap another hole in the collar so you could use one of them for some sort of mounting screw. You're right, Aspeed, I would like to have a remote NV on hand. Especially for the Thunder Tiger or my MP Jet, those are both really close to the prop.

The Big Mig venturi angles the needle safely away from the prop, so my method works well enough to suit me for this one.
Apr 01, 2013, 12:03 PM
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Some pics. Put the TD in to show what I do on the plastic to repair or insure they don't crack there. It shows a piece of brass tubing that another piece of 1/16" is soldered to to make an adaptor that goes over the venturi. I can't find one that is made up.
Apr 01, 2013, 12:29 PM
Registered User
Great info.. Gentlemen..
Yess. the remote NV is clever and too often overlooked.
I once looked into the ones fitted to Toy Car engines.. Piccos or similar iirc?
V nice units and affordable. Can't remember (increasingly common :-) why I didn't buy some then.. perhaps ? I wasn't sure of the thread pitch suitability and the not overly friendly counter Guy barely knew what the remote NV actually was.. focused on selling the entire engine rather than wee spare parts for it.
Should revisit that one.. one day.

Am ordering the T Timers bits.. thanks again. ooops not 'till April 9th tho.

Aspeed : Elegant solution to the cracked venturi boss issue :-).
The Bespoke Ali replacement one occaisionally attracts, but I'm not enamoured of it's square edges or it's pricings.
Last edited by Bare; Apr 01, 2013 at 12:38 PM.
Apr 04, 2013, 09:40 AM
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RknRusty's Avatar
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Last night I put the new venturi on my best spare Big Mig to test run it. After I made a new bladder and plugged it in to the fuel nipple, it promptly fell off. The barb on the nipple isn't big enough to hold the line tightly. I didn't bury any of it, the whole thing is out in the open, but it wouldn't even reliably hold the small clear fuel tube that I never use for anything. I was, needless to say, aggravated.

So this morning I fiddled with a couple of fixes and came up with this aluminum tube to hold it in place. I'll just have to fasten it to the airplane, but it took care of the problem:
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I think a metal clamp pinned under each mounting screw should be sufficient for flying.
EDIT: That was a stupid idea. Just use some fine copper wire and twist-tie it onto the nipple.

It took a few tries to get the needle set, but it runs fine right about 3 turns and not overly sensitive. The engine is a good one. It spins an MA 5x3 at about 24.4k with 3 shims in the head. So that should do the trick. Anyone that tries this, I hope you have a valve with a better barb on the nipple.
Last edited by RknRusty; Sep 12, 2017 at 02:57 AM.
Apr 06, 2013, 11:11 AM
Registered User
Hmmm... took a look at your NV (photo somewhere above) and yours 'seems' to have little nipple or barb. There is very little raised ridge and the reduced diameter bit of tubing seems inordinately short.
Texas timer ones look 'better' in this.
But I won't be 100% convinced until the rascals arrive.
Could you slice off a wee ring of brass tubing and carefully solder it on to increase the barb's diameter .. then file it a bit to allow tubing to go on?
Or perhaps remove the barb, solder on a bit of tube to extend the reduced piece then do the sliced off ring as a fresh barb at the end ot te extended tube? Fiddly But one way to solve the issue permanently?
Another choice would be to build a remote NV, which would then take the bladder pressure and the faulty nipple would not be pressurised??
Apr 06, 2013, 01:15 PM
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Andrew0820's Avatar
Originally Posted by Bare
........ Or perhaps remove the barb, solder on a bit of tube to extend the reduced piece then do the sliced off ring as a fresh barb at the end ot te extended tube? Fiddly But one way to solve the issue permanently?
There certainly doesn't appear to be much left to clamp so an extension is probably the best permanent solution. It's hard to tell if there is enough room to get a couple of wraps of waxed dental floss around the tubing between the barb and carb body. However, I'm inclined to think that it might simply slip off given the amount of tubing.

One other option is to use a collet, much like the crimp collets used to secure the fittings on hydraulic hose. It might be possible to use a short piece of aluminum or brass tubing that can be slipped over the tubing --- you would want a close, but not real tight fit. Push the fuel tubing over the barb so that it is tightly against the carb body, then slide the brass or aluminum over the tubing until it is also tightly against the carb. You might need to lube the fuel tubing a bit to get the outer tubing pushed in. Once seated, it might compress the tubing enough over the barb to retain it.

Just a thought.

Apr 06, 2013, 06:07 PM
Registered User
Or you could get a tighter piece of tubing and add a joiner piece with a nipple that is a bit bigger to make it easier. I think this piece has been on for 35 years?
Apr 06, 2013, 06:30 PM
Registered User
kinda fun to have tricky problem to solve :-)
My Shindaiwa Lawn equip has these lovely split ring coil fuel tube retainers that one winds over the fuel tubing and fitting nipples.. that do a v good job of retaining the hoses.. reliably.
Unlikely to find such small enough for 1/2A fuel tubing though.
DIY is a possible..would take some serious fiddling to aquire the skill needed to wind a decent/useable coil though.
As digression; Shindy's are Impressive in engines and engineering. I was amazed at the detail design work.. But then they do sell in the hundreds of thousands.. annually. to 'do it for a living yardners'... for genuine reason.
Apr 06, 2013, 08:53 PM
coriolan's Avatar
Originally Posted by aspeed
Or you could get a tighter piece of tubing and add a joiner piece with a nipple that is a bit bigger to make it easier. I think this piece has been on for 35 years?
Didn't know the "Stels" was that old
Apr 06, 2013, 09:27 PM
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RknRusty's Avatar
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I managed to get a piece of the soft wire out of a bread bag twist tie around a fuel tube and secure it pretty tightly behind the barb. I wouldn't want to have to re-do it in the field, I'll just replace it in the shop before I go fly each time.
EDIT: I never replaced it like I said above. 4-1/2 years later, here in 2017, it's still on the nipple secure as ever. I plan to fly this engine on m y Baby Streak at 1/2A Day at my club in Huntersville next Saturday.
Last edited by RknRusty; Sep 12, 2017 at 03:02 AM.

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