Thread Standards Concerns

Reader Contribution by Staff
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Restorers of old engines and machinery inevitably run into problems involving thread or fastener standards, the most common being the 1/2-12 UNC standard that appears to have been used before World War I. I have not seen this standard on anything built after about 1918, when it changed to 1/2-13 UNC. In the British Whitworth threads, the 1/2-12 BSW continued until Whitworth was effectively abandoned in favor of metric in the early 1970s. Yet even standard sizes can vary slightly, which leads me to assume that many early manufacturers made their own taps and dies and didn’t always adhere to national standards. In addition to thread standards being “loose,” I have come across nuts or bolt heads with unusual hex sizes, sometimes made to the nearest 1/32 inch.

A frustrating thread-related problem recently occurred restoring a 4 hp Excelsior engine, where I found that all the tapered pipe threads used on the fuel tank, fuel pump, mixer and return circuit were oversized. The threads on a modern fitting bottomed out before they mated up. Clearly, Excelsior made their own taps and chose to make them about 1/32 inch oversized!

To overcome this, I purchased a taper turning attachment for my Hardinge lathe in order to custom make the oversized tapered pipe fittings. This attachment has since come in very handy for producing British Standard Pipe threads. Anyone owning or contemplating owning an English engine will find the common use of BSP threads. In every case the Whitworth thread angle is 55 degrees and in almost every case there is a different number of threads per inch. The basic pipe size and taper are the same, which means you cannot visually tell the difference between NPT and BSP. Don’t make the mistake of thinking a few extra turns of Teflon tape will fix the problem!

NPT and BSP are the only two tapered pipe thread standards in the world. Modern machinery uses one of the two, and you might be surprised to know that machinery (CNC lathes, milling machines, etc.) made in Taiwan or Japan is quite likely to have BSP threads used on the various hydraulic components.

I have experienced some real disasters that occurred at the port of entry where accessories are added before shipment to the end customer. Unfortunately, there is no color coding or distinguishable marks to alert technicians to the lack of interchangeability, and the inevitable happens. I never cease to be surprised at the number of “experienced” engineers and technicians who believe there is a metric pipe thread system or that BSP is metric: neither is true.

Perhaps the most shocking aspect of lack of standardization occurs in the metric thread system. While metric threads don’t show up often in engine restoration, they are out there. I came across two instances of standardization lapses recently. The first showed up when I tapped a metric M-10 x 1.0 hole in a model engine to accept an NGK CM-6 spark plug. The plug was so loose I dared not get it more than finger tight. When I measured the pitch diameter of the threads I discovered that they were 0.006-0.008 inch (0.2mm) undersized.

More recently, I made an adapter to install a modern NGK glow plug into an early Ruston & Hornsby open-crank diesel engine. The published size of the glow plug was M-10 x 1.25 and I ran into the almost identical problem after tapping with an ISO standard tap; the plug fit so loose I couldn’t risk using it. Again, the threads checked out at 0.008 inch (0.2mm) undersized. I sent an email to NGK outlining this problem, but have not received a response.

Having been an engineer and a journeyman machinist for over 60 years (I trained at Rolls Royce’s aircraft engine pision) I have certainly seen my share of peculiar thread problems. Many younger engineers are convinced the metric system is the ultimate in standardization, but as an “old timer” I know that the Whitworth thread standards (no longer used) and the American Unified screw thread systems are the two most standardized in the world, and this results in absolute interchangeability within their respective systems, something that does not happen with metric threads.

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