Compression Fittings

About Compression Fittings

Compression fittings are most commonly used in combination with copper, polyethylene (i.e. LLDPE) or PEX tubing to create a watertight connection. They do not require any special tools for installation and allow for the connection to be easily disassembled if needed.

Compression fittings anatomy:
A compression fitting is comprised of (3) main components: fitting body, sleeve (ring) and nut. The amount of nuts and sleeves always corresponds to the number of compression outlets.

How it works:
After the pipe is cut and reamed, compression nut is slid over it, followed by the compression ring (sleeve). Next, pipe is inserted into the fitting and the nut is tightened over the compression fittings’ threads.
When the nut is tightened, the sleeve compresses over the pipe, preventing it from popping out under pressure and at the same time creates a watertight seal over the fittings’ ends.
The fitting can be disassembled when needed for maintenance or repair.
When installing compression fittings with plastic tubing, pipe stiffening inserts (brass or poly) and Delrin sleeves (O-rings) must be used.

Understanding compression fitting sizes:

All compression fittings are in OD sizes, meaning that they are intended for use with same OD-sized tubing (copper or plastic). MIP or FIP connections indicate male threaded or female threaded outlets respectively. All threaded connections are US standard NPT and are compatible with regular threaded pipe and fittings available from PEX Universe and elsewhere.

OD – Outside Diameter (ALWAYS used for compression fitting size identification. SOMETIMES used for pipe size identification).
CTS – Copper Tubing Size (OFTEN used to denote standard pipe sizes, especially 1/4", 3/8", 1/2", 5/8" and 3/4". CTS is always 1/8" smaller than the actual OD of the pipe/tubing).
ID – Inside Diameter (NEVER used for pipe or fitting identification).

Note: there’s a common misconception that pipe “Nominal size” and pipe “ID size” are the same. They are not. Nominal = CTS, and these terms are often used interchangeably.
Example: 1/2" CTS size K, L or M copper pipe has the same CTS size (1/2") and same OD size (5/8"). However, their ID varies due to different wall thickness.

Compatibility with different types or copper tubing:
Compression fittings are compatible with all common copper pipes, including K, L, M and ACR types. They are intended for relatively low-pressure, vibration-free or low-vibration applications, including hot/cold water plumbing or heating, hydraulic, instrumentation and pneumatic applications.
Actual pressure ratings vary depending on the fitting size. Soft-tempered copper tubing is best suited for compression fittings, although hard-tempered is also compatible.

Frequently Asked Questions:

Q: Can I use compression fittings with PEX/poly pipe?
A: In general, we do not recommend using compression fittings on larger plastic pipe sizes (3/8" CTS or 1/2" OD and over) and instead would suggest using either brass (or plastic) push fittings, or brass/poly PEX fittings (for PEX pipe) where possible. PEX fittings cost considerably less than compression in larger sizes, while push fittings cost about the same with a fraction of labor required to install.
Compression fittings can be used with smaller-size PEX and other non-rigid polyethylene tubing (i.e. LLDPE), provided that:
1. Only plastic sleeves/rings (such as Delrin/POM) are used. Brass sleeves are designed for brass/copper pipe only and will cut into PEX, potentially causing leaks (while they may hold pressure initially, they may and most likely will fail in the future).
2. Pipe inserts (stiffeners) must always be used to stiffen the soft plastic pipe and prevent it from deformation, which is the main cause of leakage.

Q: Are plastic pipe inserts included with the fittings?
A: No. Inserts (stiffeners) are sold separately.

Q: Do I need to use any tape, grease or lubricant on compression fittings?
A: The short answer is No. Teflon (PTFE) pipe tape is designed for use with regular threaded connections and is not intended for use on compression fittings. A small amount of food-safe, non-hardening lubricant, oil or plumbers’ grease may be applied to threads (not the sleeve) to ease the assembly process.

Q: How far should the nut be tightened?
A: When the nut can no longer be screwed by hand, use a wrench or pliers to tighten it. In general, half (1/2) turn is sufficient for most connections and sometimes another quarter (0.25) turn is needed. Hardness of the pipe (i.e. hard-tempered or soft-tempered) and pipe wall thickness will determine how far the nut could be tightened.

Q: Are these approved for potable water use?
A: Yes, compression fittings we sell are made from lead-free brass and can be used for potable water. There are a few exceptions which are clearly marked as “not-LF”. Delrin sleeves are made from POM polymer, which is also approved for potable water.

Q: Are compression fittings compatible with flexible water connectors?
A: Yes, these fittings can be used with flexible (poly or stainless steel) braided faucet, toilet, dishwasher, icemaker and similar connectors from BrassCraft, Watts, Matco-Norca and other manufacturers. Since most flexi-connectors have 1/4" or 3/8" compression connection, corresponding compression fitting sizes would be 1/4" OD and 3/8" OD compression.

Q: Can I re-use a compression fitting?
A: Yes, however, if a new connection is made with copper tubing, the brass sleeve (O-ring) is not re-usable, since it permanently compresses over the pipe and cannot be removed.

Q: Which tools do I need to make a connection?
A: No special tools are needed except for a pair of wrenches or pliers and a cutting/reaming tool for the tubing (such as a regular copper pipe cutter).

Q: What’s the difference between the OD and Nominal size?
A: OD size corresponds to the Outside Diameter of the pipe (either copper or plastic). Nominal or CTS (Copper Tubing Size) size of the same pipe is always 1/8" smaller. Below is the OD-CTS reference list for common copper tubing sizes (ASTM B88 standard):
3/8" OD - 1/4" CTS
1/2" OD – 3/8" CTS
5/8" OD – 1/2" CTS
3/4" OD – 5/8" CTS
7/8" OD – 3/4" CTS

Q: Can brass compression fittings be used with stainless steel tubing?
A: No.

Q: What is the difference between MIP and MPT threads?
A: There’s no difference. MIP, MPT, MNPT – all refer to US standard Male threaded connection. Similarly, FIP, FPT, FNPT refer to US standard Female threaded connection.

Q: Sleeves included?
A: All compression fittings we sell come with sleeves and nuts included. In addition, both nuts and sleeves are available for purchase separately if needed.

Q: What’s the difference between BrassCraft and Everhot compression fittings?
A: First, a large portion of BrassCraft fittings are manufactured in the USA. Second, BrassCraft threads are “DrySeal pipe threads” (ANSI B1.20.3), meaning they are precision-made to exceed the industry requirements. Both brands meet industry standards & requirements.