 |
     |
 |
|
|
Plumber's Nightmare
|
 |
|
Part 1 - Threading the Needle
|
By Wayne Scraba |
"Plumbing a drag car is easy -- you just run hose from the fuel tank or fuel cell and hook it up to the carburetor.....right?" Whoa! That's simply wishful thinking. As race car technology and performance levels have progressed, so has the need for more sophisticated, elaborate and safe plumbing arrangements. The old practice of attaching a worm gear clamp to the end of a neoprene or clear plastic hose is over. Not only is this "design" remarkably inefficient, but it's dangerous, too.
Alphabet Soup OK. Where do you begin? Start with alphabet soup. Stainless steel braided hose and the accompanying aircraft fittings are definitely the way to go. Army-Navy (A-N) hardware is definitely more expensive than the good old-fashioned rubber hose and worm gear clamp set up. But it's also more sophisticated, safer and easier to look after. Unfortunately, there are some headaches associated with A-N hose and fittings -- especially when it comes to sizing. Here's how it works: The Army-Navy sizes were first established by a fledgling aerospace industry over fifty years ago. This was a system designed to standardize plumbing and was originally instituted to correspond to a specific tube size that the hose end was to be used with. The following chart depicts the relationship between A-N numbers, metal tube outside diameters (OD's) and thread size:
A-N SIZE METAL TUBE SIZE THREAD SIZE _____________________________________________________________________________________ 2 1/8-inch 5/16-24 SAE 3 3/16-inch 3/8-24 SAE 4 1/4-inch 7/16-20 SAE 5 5/16-inch 1/2-20 SAE 6 3/8-inch 9/16-18 SAE 8 1/2-inch 3/4-16 SAE 10 5/8-inch 7/8-14 SAE 12 3/4-inch 1-1/16-12 SAE 16 1-inch 1-5/16-12 SAE 20 1-1/4-inch 1-5/8-12 SAE 24 1-1/2-inch 1-7/8-12 SAE 28 1-3/4-inch 2-1/4-12 SAE 32 2-inch 2-1/2-12 SAE
NOTE: Sharp readers will note A-N sizes 2 and 5 on the above chart. Fittings in these sizes are next to impossible to find because of this it's not a viable hose/fitting size. All hose sizes from number six through number twelve are applicable for fuel delivery systems.
Threading the Needle So far so good, but what's with the threads? Almost all automotive applications use pipe threads. Because of this, parts with pipe threads aren't compatible with the A-N fittings and adapters must be used. Rather than confusing the issue, companies such as Aeroquip, Earl's Performance, Russell and others simply name the fitting size by the A-N size rather than the SAE thread size. As a result, a No. 8 fitting is simply that -- a No. 8 (or "dash" 8) fitting. Adapters are named for their application. An example might be a straight adapter fitting, which is 3/8-inch NPT to No. 8 -- again simplifying the transition from NPT (National Pipe Thread) to A-N "dash" sizes. By the way, if you always have to scratch your head and try to translate hose size into fractions, try this: Place the A-N size over sixteen and divide. It works like this: #10 hose = 5/8-inch 16
When plumbing a race car, there's one important thing you'll have to always keep in mind: The majority of OEM components (and a large number of race car parts) don't have A-N ports. Instead, they incorporate tapered pipe thread. Pipe thread is the curse of the automotive industry. "National Tapered Pipe" thread sizes refer to the inside diameter of a piece of pipe that receives the male end. It's designed for an interference fit between the male and female threads (hence the taper which becomes larger as the two parts are tightened). Supposedly, this interference fit is engineered to stop leaks. Unfortunately, it doesn't work that way. When joining any pipe thread, you're forced to use Teflon tape or paste on the threads so that it won't leak. The following chart shows the various sizes of pipe threads along with the closest A-N fitting size:
Pipe Threads Theoretical Closest Thread Size Per Inch I.D. of Fitting A-N Fitting To Size ______________________________________________________________________________________ 1/16" 27 1/16" N/A 1/8" 27 1/8" #4 1/4" 18 1/4" #6 3/8" 18 3/8" #8 1/2" 14 1/2" #10 3/4" 14 3/4" #12 1" 11-1/2 1" #16 1-1/4" 11-1/2 1-1/4" #20 1-1/2" 11-1/2 1-1/2" #24 2" 11-1/2 2" #32 |
