A Primer on PVC, CPVC, and O Rings

Left-to-right: 3/4″ CPVC, 1″ CPVC retainer, -212 O-ring, 1″ CPVC retainer, 1″ CPVC fitting

Several of my recent design projects have required properly-fitting o-rings, and yet as I looked around the web, I was frustrated at my inability to find a good practical explanation of their use.

Which size o-rings fit with which size pipes? Which pipe sizes nest closely?

After much experimenting with various PVC fittings, copper fittings, and o-rings of all assortments, I have a few answers that I outline here my own mini-guide to the use of O-rings in PVC/CPVC projects.

Nominal vs Actual Pipe Diameter

Any discussion of O-rings needs to start with a discussion about pipe diameter.

In the United States, pipe and tubing is generally specified as a “nominal” size in imperial units, for example 1/2″ copper tubing or 3/4″ PVC pipe.

In much the same way that “two by four” lumber doesn’t actually measure 2 inches by 4 inches, a 1/2″ pipe has neither an inner nor outer diameter of 1/2″. Nominal 1/2″ copper pipe has an ACTUAL outer diameter of 0.875 inches.

Outside Diameter (OD)

Pipe sizes are specified in terms of their nominal outside diameter, also known as “OD”. The OD of 1/2″ PVC pipe from any manufacturer will always be within a fraction of a millimeter of the defined standard for PVC pipe.

Of course, since the outside diameter of a pipe mates up with the inside diameter (“ID”) of a fitting like a coupler or a tee, the ID of a 1/2″ fitting from any manufacturer will also be effectively constant.

This brings us to:

Design Rule 1 for O-rings: Any measurement that requires a close tolerance should be made relative to the OUTSIDE diameter of a pipe, or the INSIDE diameter of a fitting.

Inside Diameter (ID)

The wall thickness and inside diameter of pipes is generally NOT specified. Due to the properties of the material, wall thickness generally falls in a narrow range, but it is very common to see differences between pipe made by different manufacturers, and even across lots from the same manufacturer. Similarly, the wall thickness of a fitting like a coupler or tee is not specified, and varies widely from part to part.

This brings us to:

Design Rule 2 for O-rings: Joints that depends on the INSIDE diameter of a pipe or the OUTSIDE diameter of a coupling are not guaranteed to work (since their size is manufacturer dependent)

Across materials, 1/2″ pipe does not always match 1/2″ pipe, but 1/2″ thread always mates with 1/2″ thread

As if nominal vs actual measurements weren’t bad enough, the ACTUAL diameter of 1/2″ copper tubing is NOT the same as 1/2″ PVC. See the table below for a listing of common pipe materials and their associated sizing specs.

One saving grace: The guys who set the specs for pipe threads seemed to have their act together, so 1/2″ threads are 1/2″ threads, whether you are talking copper, PVC, CPVC, etc.

Copper Tubingreddish brownSched. 40 Copperhot and cold water plumbingComes in different grades, but all have the same outside diameter

Material Color Sizing Standard Uses Notes
PVC – Schedule 40 White Sched. 40 PVC cold water plumbing Not to be confused with DWV PVC – see below
PVC – DWV White Sched. 40 PVC drains DWV stands for drain-waste-vent. It generally has a thin wall ans is not suitable for holding pressure
CPVC – CTS Yellowish white Sched. 40 Copper Hot and cold water plumbing Very useful in o-ring applications due to its unique sizing. Not to be confused with schedule 80 CPVC which is typically dark grey and uses its own sizing standard. CPVC CTS is becoming harder to find as PEX rises in popularity
PEX Varies, commonly red, white, and blue Schedule 40 copper hot and cold water plumbing Rapidly replacing CPVC CTS in many applications

O-Ring Sizing

O-rings specs are generally sized based on “nominal” inner diameter, outer diameter, and cross-section. This “nominal” measurement should not be confused with “nominal” pipe size. A “nominal” 5/8″ ID O-ring has an ID of just about 5/8″, plus or minus the manufacturing tolerance.

Many common O-ring sizes have a sort of shorthand name, colloquially called a “dash” number. For example, a “-212” (“dash two twelve”) O-ring has an ID of 5/8″, a cross section of 1/8″, and therefore an OD of 7/8″.

Useful O-Ring sizes

Dash Number ID OD Notes Where to buy
-208 5/8 7/8 fits over 1/2″ copper, inside 3/4″ copper fitting Amazon
-212 7/8 1 1/8 fits over 3/4″ copper, inside 1″ copper fitting Amazon

Note: It supports my O-ring habit when you buy through the links above.

O-Ring Recipe for 1/2″ copper or CTS CPVC

Left-to-right, all CTS CPVC: 1/2″ pipe, 3/4″ retainer, -208 O-ring, 3/4″ retainer, 3/4″ fitting

By happy circumstance, 1/2″ CTS CPVC fits neatly inside 3/4″ CPVC, and a -208 O-ring is perfectly matches the OD of each.

Item ID OD Notes
1/2″ CPVC pipe 5/8″
Sliver of 3/4″ CPVC pipe unspecified, but larger than 5/8″ 7/8″ Glued inside the 3/4″ fitting, this serves as the outer O-ring retainer
-208 O-ring 5/8″ 7/8″
Sliver of 3/4″ CPVC pipe unspecified, but larger than 5/8″ 7/8″ Glued inside the 3/4″ fitting, this serves as the inner O-ring retainer
3/4″ CPVC fitting 7/8″ Cement the two slivers of 3/4″ CPVC inside to form a groove for the O-ring

O-Ring Recipe for 3/4″ copper or CTS CPVC

This is really just the 1/2″ recipe, with all elements bumped up by one size — 1/2″ becomes 3/4″, 3/4″ becomes 1″, -208 ring becomes -212

Left-to-right: 3/4″ CPVC, 1″ CPVC retainer, -212 O-ring, 1″ CPVC retainer, 1″ CPVC fitting

By happy circumstance, 3/4″ CTS CPVC fits neatly inside 1″ CPVC, and a -212 O-ring is perfectly matches the OD of each.

Item ID OD Notes
3/4″ CPVC pipe 7/8″
Sliver of 1″ CPVC pipe unspecified, but larger than 7/8″ 1 1/8″ Glued inside the 1″ fitting, this serves as the outer O-ring retainer
-208 O-ring 7/8″ 1 1/8″
Sliver of 1″ CPVC pipe unspecified, but larger than 7/8″ 1 1/8″ Glued inside the 1″ fitting, this serves as the inner O-ring retainer
1″ CPVC fitting 1 1-8″ Cement the two slivers of 3/4″ CPVC inside to form a groove for the O-ring

A modular, all-PVC pneumatic actuator

All-PVC pneumatic actuator. The piston is on the bottom, the pressure chamber is on the top.

Safety note: Don’t try this yourself. When overpressurized with water, PVC cracks. When overpressurized with highly energetic compressed gas, PVC shatters into potentially deadly projectiles. As a result, PVC should NEVER be used with compressed air.

For the past few years, I have built air-powered animated Halloween props. This is a prototype for a low-pressure, long-throw cylinder/actuator that might end up in a future design.

The basic idea here is that the 3/4″ piston will travel inside the 1 1/4″ pressure chamber.

I believe this design is somewhat novel in two ways:

1) The pressure chamber and piston are modular. This picture is showing a minimum-throw piston, but if you want say 18″ more throw, just extend the pressure chamber by making up an 18″ long section of 1 1/4″ pipe with a male adapter on one end and female on the other. Swap the piston with one that is 18″ longer, and you are good to go.

2) Even if you don’t care for the extensibility of the chamber, it is nice to be able to unscrew the 1 1/4″ section for maintenance of the O-ring.

A few key construction details:

1) Note the difference between CTS CPVC pipe and plain PVC pipe. CTS CPVC is yellowish, and uses copper pipe sizes. Regular PVC pipe is white uses a different sizing standard. Both are used here, and they can NOT be substituted.

2) The #212 O-ring fits perfectly between the 3/4″ CPVC pipe and the 1″ fitting, and is held in place by a couple of slivers of 1″ CPVC pipe.

3) The joint between 1-1/4 PVC pipe and 1″ CPVC coupling is very non-standard. The sizing happened to work out OK for me, but that may just be luck since neither the ID of the PVC, nor the OD of the CPVC coupling is standardized. In any event, I made this joint tight using polycaprolactone as an adhesive, using the technique outlined in my post Reversible, semi-permanent PVC glue joints and custom fittings. A construction adhesive like PL Premium might also have worked.

4) All other slip joints here are cemented with PVC glue, and all threaded connections are wrapped with teflon tape.

Power pack for compressed air rockets

I built a variant of the compressed air rocket launcher on Make Weekend Projects.

I don’t know who is having more fun with it, me or my kids. OK, I admit I am having more fun, but they love it too.

In any event, after having to repair the sprinkler valve control circuit a couple of times, I sat down to build something more reliable, and this is what I came up with.

Two 9V battery harnesses, two mono jacks from radio shack, and the box from an old ipod.

The case is upcycled from an old ipod box, and the connectors are cheap female mono audio jacks from Radio Shack. Even though valves is spec’d at 24VDC, 18VDC from the two 9V batteries is very reliable for dozens if not hundreds of activations.

The sprinkler valve and the launch release switch get the same treatment, using male audio jacks.

The switch and valve each get an 1/8″ male audio jack, again from Radio Shack

Here is the wiring diagram:

When we’re not launching air rockets, this system also works great for controlling the pneumatic-activated Halloween props I’ve built (more on that in another post).

More about Polycaprolactone


PCL pellets, before and after melting

[Edit: Updated to add PLA and ABS to the list of compatible materials]

Polycaprolactone or PCL is a white plastic that is very hard and slick at room temperature, but melts to a putty-like consistency when heated under very hot water. Make Magazine has an excellent article summarizing some of PCL’s many uses, and in fact that article is where I first learned of it.

PCL is very useful as a clay-like molding material, and as I discuss in a previous post, as a material for Reversible, semi-permanent PVC glue joints and custom fittings.

Two PVC pipes joined with Polycaprolactone

Compatibility with other plastics

Depending on your application, PCL’s tendency to stick to other plastics may or may not be desirable. I compiled this table of compatibility based on my own experiments:

Material Type Common Uses Result Notes
PLA (Polylactic Acid) 3d printing sticks very well Be careful not to overheat and distort PLA
ABS (acrylonitrile butadiene styrene) 3d printing, Lego sticks very well Custom Lego!!!
PVC (Polyvinyl Chloride) white plastic pipe sticks very well preheat pieces for best bond
CPVC (Cross-linked Poly Vinyl Chloride) yellowish plastic pipe sticks very well preheat pieces for best bond
Polycarbonate (aka Lexan) very strong glass substitute sticks very well preheat pieces for best bond
PEX flexible water pipe does not stick
PET / PETG soda bottles does not stick
Metals copper, steel, aluminum does not stick


Polycaprolactone is sold under  a number of  brand names including InstaMorph, Shapelock, Friendly Plastic, Missing Link plastic, etc.

As far as I can tell they are nearly identical products, with the exception of Missing Link — they sell a “flow formula” that is a bit less viscous when melted, which may be useful for some applications.

Where to buy

      Make Magazine’s Maker Shed sells Shapelock brand



        Amazon sells various brands and sizes for prices mostly in the $10-20 range. I found that the best way to find all your options on Amazon is to

search for polycaprolactone

        rather than any particular brand.


Buying through my Amazon links has the added benefit of modestly subsidizing my experimentation, for which I thank you.

Reversible, semi-permanent PVC glue joints and custom fittings

Two PVC pipes joined with Polycaprolactone. This joint can be undone with gentle heating.

Fans of PVC pipe know that once a PVC joint is cemented together, it is impossible to take apart without destroying the pieces. Until now. The formula is simple:

PVC + Polycaprolactone = Awesome

Poly-capro-what you say? You can read More about Polycaprolactone aka PCL here. The awesomeness of PCL is well documented by Make Magazine in their article The Many Uses of Shapelock, but I have discovered one important use that they missed!

While trying to use a bit of PVC as a mold for PCL, I was quite displeased to discover that the PCL instantly bonded to the PVC like pine sap on glass. The PCL stayed pliable, and could be scraped off with great effort, but it really, really wanted to stay put on the PVC. And once cooled and hardened, the two couldn’t be separated at all, even with pliers, screwdriver or hammer.

When PVC meets melted PCL, the result is sticky

My displeasure at the failed experiment was short lived when I realized the many implications of this happy accident, which I summarize with a simple photograph.

Two PVC pipes semi-permanently joined with a ball of low-melting point polycaprolactone plastic

As you can see, the putty like nature of the PCL combined with its adhesion to PVC allows for some very non-standard  joints.   I also found, as you can see below, that it works very well on traditional joints that need to be strong, yet not permanent.

Recipe for making reversible, semi-permanent PVC glue joints

0) Acquire Polycaprolactone. List of places to buy

1) Heat beads of PCL in very hot water until clear and putty-like, then form into an O-ring like shape

A ring of PCL ready to glue two pieces of PVC together

2) Stick the o-ring onto the male side of the joint.  Pre-warming the PVC with very hot water will ensure a good bond and keep the PCL from cooling too quickly.

PCL O-Ring

The PCL O-Ring has been attached to the male side of the joint. When pressed together, some will be forced into the gap, and the excess will squeeze out.

3) Press the fittings together and wait for the PCL to cool

Once cool, the result is nearly as strong as a cemented joint. For extra strength, you can CAREFULLY pour near-boiling water over the entire assembly after the joint is pressed together to ensure a good bond.

Finished joint is watertight, nearly as strong as cement, and can be undone by gently heating

If you ever need to take the joint apart, (gently!) heat the joint under hot water and pull the fitting apart.

Some thoughts on safety

Although this should go without saying, I’ll say it anyway:

– be careful around hot water

– heating plastic with a heat gun or hair dryer is a bad idea due to fire risk

– joints formed with this technique are neither suitable nor advisable for any plumbing application

– joints formed with this technique may unintentionally come apart if heated (duh)


Have fun and be safe!