How To Make Needle Valves and Spray Bars

Created: October 25, 2004
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Making needle-valve/ spray-bar assemblies--NVA's for short is a tedious but relatively simple process that can be carried out on even the smallest and most modest of lathes. I know model engineers who keep a little Taig, Sherline, or old round-bed Drummond just for operations like this. No great accuracy is required, though good precision is.

First, let's review what we require in an NVA. Most importantly, it needs to meter the fuel supply in a consistent and reliable way, supplying the fuel as tiny atomized droplets. We achieve this by ensuring that the metering occurs at the inlet, not the outlet. And it must supply only fuel. If it leaks air, the mixture will probably undergo large and unpredictable changes as the needle is adjusted, making the engine hard to tune, or even prone to sudden stopping.

Lastly, if it's a replacement for an engine that's come to you without one (the most likely scenario--countless old engines are in this state), you'll almost certainly want it to look as close to original as possible. If the engine is to run, you also want it to be soundly made and capable of accurately metering fuel! This is not so simple and was the topic of discussion in the February 2003 Issue of Model Engine News. I suggest you click on the link and review why an accurate needle seat is essential to harmonious operation.

If you have, or can borrow an engine that still has a complete, original NVA, problem solved--make a simple sketch of the important dimensions and away you go. If not, check out old magazine engine reviews and other sources for a drawing or photo. The early reviews in the Aeromodeller and Model Aviation are good sources of relatively accurate, full-size 3-view drawings of many old English, Australian, and even some American engines. They won't give you the thread size, but we can make an educated guess on this. Determine the spray-bar diameter by poking number drills into the venturi hole (which hopefully has not been drilled out by some ancient butcher). Now look at a table of threads appropriate to the engine source. English and Australian engines up to the 70's will generally use a British Association (BA) thread. After this they will probably be metric. There are exceptions--some used 1/8" Whitworth, a 40 TPI thread, but most were BA. American engines will be UNC, or UNF. The venturi spray-bar hole will be close to the major thread diameter and you'll probably get it right, or close enough for government work.

Now armed with a simple rough-sketch, we can make a start. Unfortunately, magazine reviews seldom mention what the NVA's were made from. But if in doubt, brass is usually a good guess. A few, like the AM series, used aluminium for both needle thimble and spray-bar. Some DC engines like the Merlin and Spitfire had an aluminium thimble on a brass spray-bar, but most were all brass. The common "stock" sizes for thimbles and spraybars are 1/4" and 3/16" round brass, and 1/4" AF hexagonal (AF is short for "across the flats"). It's a good idea to keep a stock of this, and other AF hex sizes "in the rack" as most NVA's do not use stock nuts, rather they are made from a smaller size of hex stock than the size common for the thread--and nothing stands out worse that a giant, plated nut in place of a smaller, elegant, plain brass one. I've frequently made replacement nuts by drilling out and re-tapping a smaller size nickel-plated brass nut, thinning it down, then carefully filing the plating off the hex sides.

Enough yabbering; on with the machining. For this exercise, we'll be making an NVA for an ME Heron, a 1 cc diesel from the Isle of Mann, built in the 60's and available in air and water cooled versions:

Bend over the end, make the nut, and the job is done. That's all there is to it. My workshop notebook has all sorts of NVA drawings scattered through it. Some day, in my copious free time, I plan to clean these up and make them available via these web pages. So much to do... so little time...



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