Glossary of Automotive Plumbing Terms

Whether you’re a seasoned mechanic or a new enthusiast, there are some common terms in automotive plumbing you might be wondering the definition or origin of. Read on for the ABC’s of plumbing to see how many abbreviations you already know, some you might not, and others you probably can’t pronounce. 


AN fittings, also known as Army/Navy (or sometimes Air Force/Navy), were originally developed as a military standard that dates back as early as World War II. AN fittings use a 37° flare as the sealing surface. DO NOT use AN fittings with different degree flare-type fittings. This will cause leaks and damage to the fittings. Each AN size refers to the outer diameter (O.D.) of the hose or tube that it’s used with. The numbers assigned equal the tubing O.D. in 1/16 inch increments. Each AN size also has its own standard thread size. For a printable guide explaining thread sizes, torque specs, and more, click here.


Anodizing is a method of increasing the corrosion resistance of a metal part by forming a layer of oxide on its surface. The metal part to be treated (generally aluminum) is first submerged in an electrolytic solution bath along with a cathode. When a current is passed through the acid solution, hydrogen is released from the cathode and oxygen forms on the surface of an anode. This results in a metal oxide film growing on the surface of the part being treated. Anodized aluminum parts are more corrosion and wear resistant than non-treated parts. It also helps protect against galling.

Bend Radius

The radius of a bent section of hose, measured to the innermost surface of the curved portion.


Used on AN fittings. The common description of the nut that is used to connect fluid lines and hoses.


British Standard Pipe. They are similar to the NPT but have angle cross threads of 55° and have rounded peaks and valleys. They are not interchangeable with NPT fittings. They have the designations BSPT for tapered threads and BSPP for parallel threads. BSPT fittings require sealants whereas the BSPP use banded seal rings to seal fittings in between the shoulder on the male and the face of the female and do not require sealants.


Chlorinated polyethylene. A type of synthetic rubber commonly used as an inner liner for braided hose. Less resistant to fuel additives than FKM or PTFE.


Fluoropolymer elastomer. Common name for the brand Viton® which is the registered trade mark of DuPont Performance Elastomers. Fluoropolymer resins have exceptional resistance to high temperatures, chemical reaction, corrosion, and stress-cracking. Used as an inner liner for hoses offering better resistance to fuel additives. More resistant than CPE, less resistant than PTFE, however offers better flexibility than PTFE.


Inside diameter

IVF / Inverted Flare

Sealing of the fitting is made by means of an inverted 37º flare.


Joint Industries Council (JIC). Standardized the specifications on the AN type of fitting by creating the “JIC” fitting standard, a 37° fitting with a slightly lower class of thread quality than the military AN version.


There are two systems for measuring threads – metric and imperial (same as SAE or American). Metric thread is measured in millimeters. Imperial thread diameter is measured in fractions of an inch and imperial pitch is measured in threads per inch (TPI).


The average size of the openings between pieces of filter media are represented in microns. A micron rating for a fluid filter is a generalized way of indicating the ability of the filter to remove contaminants by the size of the particles. The smaller the micron number, the smaller the particle it can filter. However smaller micron filters can result in flow restriction.


National Pipe Thread is a tapered machine thread for threaded pipes and fittings . NPT threads can be referred to as “Male pipe thread” or “Female pipe thread”.  NPT are a fitting-to-fitting connection used for attaching a fitting to a hole or another fitting. NPT threads are tapered to create an interference fit without the use of crush washers, O-rings or gaskets. NPT fitting sizing does not refer to the physical dimension of the threads. For a printable guide explaining thread sizes, torque specs, and more, click here.


Outside diameter


O-Ring Boss. The male fitting has a straight thread and O-ring. The female port has a straight thread, a machined surface (minimum spotface) and a chamfer to accept the O-ring. The seal take place by compressing the O-ring in the chamfer. The threads hold the connection mechanically.


Pounds per square inch. Pressure a gas or liquid exerts on the walls of its container. Commonly used in hose pressure ratings.


Polytetrafluoroethylene. Common name for the brand Teflon™ which is the registered trade mark of DuPont. PTFE is an engineered fluoropolymer. Outstanding resistance to chemicals and a broad temperature range is some of its primary attributes. An extremely low coefficient of friction, (0.05 to 0.20), provides a non-stick surface. PTFE is chemically inert, it will not break down or deteriorate in service and it has an unlimited shelf life because properties do not change with age or exposure to weather. As an inner liner for hoses, offers the best resistance to fuel additives over CPE or FKM.


Push-Lok hose barbs are a means to attach a hose-end to a rubber hose. Several sharp barbs lock into the inside diameter of the hose and create a seal.  Most hose manufacturers offer a hose type specifically made for Push-Lok fittings.


Society of Automotive Engineers. A U.S.-based, globally active professional association and standards developing organization for engineering professionals in various industries.


In a smooth-bore hose core, the tube’s inner wall is smooth. With smooth bore, precise flow control is possible because no irregularities in the wall can cause interruptions. Smooth bore also promotes drain-ability. The primary disadvantage is kinking, particularly in larger diameters. Reinforcement layers help solve kinking.