When it comes to the inception of a custom concept, the big ideas are the easiest to envision. However, once you see how a vehicle's mechanics were manipulated for the right custom aesthetic-without eliminating its function-you'll gain respect for the time people spend in design and ingenuity.
Getting the fine details to succumb to the vehicle's conceived theme requires a great sense of design and knowledge. Other than wiring, plumbing is the primary element that can cause an aesthetic subtraction from a custom engine compartment's cleanliness.
When choosing a type of line, there are a number of things to consider. When adding something to your stock system, such as adding an inline fuel filter, cutting out a section of your stock line can be done in many different ways. Part of your homework in accomplishing the task of adding something to your plumbing, or a complete replumbing, is learning the hydrodynamic principles of that system, and what it needs in relation to its dynamic.
These committees approve all fittings and couplings used to flow liquids and/or gasses. For glossary terms' sake, these industry bodies and their acronyms are:
National Pipe Thread (NPT)
British Standard Pipe Thread (BSP)
Joint Industry Council (JIC)
O-Ring Face Seal (ORFS)
Society of Automotive Engineers (SAE)
LINE TYPES
There are three different types of line used in automotive. These are: | 
Hard line: This line type is sold in straight lengths of steel, aluminum, or stainless tubing. The rigid body tubing requires the use of a tubing bender to manipulate it. | 
Flexible line: This is similar to the hard line, but it is significantly more flexible. The flexible line is sold in rolls and doesn't require a bender to manipulate the material. For nice, smooth lines, we'd suggest a bender. |
Soft line: This form of line can be found in rubber, plastic, or rubber-insulated plastic. These hose-type lines are also sold in rolls; obviously tubing bender isn't required to manipulate it. | 
There are hundreds of different hose types. Here, a nylon line was wrapped in rubber and breaded; the two outer layers provide a protective thermal barrier, where the nylon is equipped for high-pressure fuel injection that can push gasoline to an excess of 100 psi. | 
PLUMBING FITTINGS |
Getting lines, valves, compressors, pumps, or any other system components to function requires fitting them to one another to form a complete system. Initially the fitting used to form a seal was a tapered thread, which by screwing it in with enough force would create a seal. This was not always completely sealed, and it usually leaked. This type of seal was developed for water and air pressure, and functioned well under a hydraulic system subject to 800 psi to as much as 200 degrees Fahrenheit. | 
This progressed the hydraulic fitting to a full-contact fittings that required flareing the line to form a taper. A nut, or (as shown) a nut with a tapered collar equipped with the same taper, slips over the line and supports and seals the flared line by wedging the line against the flare on the fitting with the torqued nut pressure. There are two different flares used in the industry: a 45- and 37-degree sealing surface. The 37-degree fits into smaller fitting bodies, but neither one provides a better seal over the other. | 
Light-weight aluminum fittings are excellent for marine or alcohol fuel. They are anodized or coated to prevent contaminating conductivity. Aluminum is prone to corrosion and oxidation breakdown when exposed to Mother Nature's elements, if not properly coated. |
This two-piece ferrule fitting is a product of Swagelok. Swagelok makes stainless fittings designed to hold 10,000 pounds of psi, in a range of -28 to 1,000 degrees Fahrenheit. They are also available in polished stainless steel. | 
This push-lock fitting is a soft-line fitting that uses a spring-loaded crimping mechanism. The higher the line pressure, the more force is used to clench the line. The only problem is the seal is extremely prone to failure when subjected to vibration and line movement. | |