What Fittings Go Inside PVC Pipe?
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Time to read 5 min
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Most PVC fittings go outside pipe. This makes sense because, regardless of schedule, all PVC pipe of the same nominal size have the same outer diameter. That means any outer fittings can go on any pipe that is the same size. But sometimes you need to extend an already-existing pipeline. Other times you need to couple pipe but there's no room for a regular outer coupling. In both of these instances, you will need fittings that go inside pipe. With interior fittings, the schedule of pipe is important, as this affects the inner diameter of the pipe.
Pipe Extenders
Pipe extenders fit inside schedule 40 PVC pipe and extend it another couple inches, which can be helpful in many different situations. They consist of a small end that fits inside the pipe and a larger end that matches the outer diameter (OD) of the pipe it's connecting to. This means that, from the outside, the surface of the pipe will be the same size the whole way down. An extended pipe can be treated the same as a regular pipe. Fittings can be placed on the pipe extender the same as they would be placed on regular PVC pipe.
PVC pipe extenders are typically used in repair scenarios when an extra piece of pipe is needed to connect a fitting, valve, or other piece of hardware. They only work for schedule 40 pipe because the inner diameter (ID) is different for other kinds of pipe. For example, a 2" PVC schedule 40 pipe extender will only fit with a 2" nominal size schedule 40 PVC pipe. Fittings that fit on the outside of pipe are interchangeable, but interior fittings can only be used with their designated schedule of PVC pipe.
Inside Connectors
This part does exactly what it sounds like: connect from the inside. It consists of two pipe ID spigot ends that fit inside of pipe with the same nominal size and schedule. These do the same job as a regular PVC coupling, but do the work from the inside instead of from the outside. The reason you would choose an inside connector instead of a coupling is because you lack the space around the pipe to have a fitting around it. Another possible reason to want an inside connector is that it makes the surface of the pipe continuous; a coupling causes a bump along the surface of the pipe.
The problem with these inside connectors is that they can reduce flow through a pipeline, as they lessen the space inside of the pipe. This could cause issues in some applications where maintaining a certain rate of flow is important. To measure flow and see how much an inside connector affects it, try using a flow meter! Just like with the pipe extenders, the schedule of inside connectors and PVC pipe have to match or else they will be incompatible.
Installing Interior Fittings
The installation of interior fittings is mostly common sense, but there are a few important things to remember. First, the smaller part of the fitting (usually referred to as a pipe ID spigot) goes inside the pipe and should fit snugly. You may need to use a mallet to get them completely into position. Second, to handle any liquid or gas transportation, interior fittings will need to be installed with PVC cement or some other sealant to assure there are no leaks. Below are a few illustrations to show how these fittings will look once installed. Clear PVC pipe has been used so you can see how the fitting sits inside the pipe.
In the world of PVC, fittings that go inside pipe are rare, so it can be easy to buy the wrong part. Make sure you do your research before purchasing a pipe extender or inside connector!
Purpose of Internal Fittings: Unlike standard external fittings, pipe extenders and inside connectors slide inside the PVC pipe. They are used when exterior space is limited or when extending a previously cut pipeline during repairs.
Pipe Extenders: These connect into the inside of a pipe on one end while matching the outside diameter on the other, allowing you to add extra length without changing the outer profile of your pipeline.
Inside Connectors: These function similarly to standard couplings but connect two pipes entirely from the inside, eliminating outer surface bumps and allowing for smooth, flush alignments in tight spaces.
Critical Design Trade-offs: Because these components sit inside the pipe, they slightly restrict internal space and reduce fluid flow rate. Furthermore, since they fit internal boundaries, they must perfectly match the pipe's specific schedule (e.g., Schedule 40) and nominal size to ensure compatibility.
FAQs
What is the difference between a pipe extender and an inside connector?
While both parts slide inside a PVC pipe, they serve different purposes. A pipe extender is a repair fitting used to lengthen a pipe by a few inches, featuring a small end that fits inside the pipe and a larger end that mimics the pipe’s standard outer diameter (OD). An inside connector functions like an internal coupling, joining two separate pipes from the inside when there isn't enough external clearances or workspace around the outside of the pipe.
Can I use these internal fittings with Schedule 80 PVC pipe?
No. These interior fittings are specifically engineered to fit the unique internal diameter (ID) of Schedule 40 PVC pipe. Because Schedule 80 pipe has thicker walls, its inner diameter is smaller, meaning Schedule 40 internal fittings will not fit inside it.
Do inside connectors affect the water flow rate?
Yes. Because inside connectors sit entirely within the pipe, they naturally reduce the internal opening size and restrict the waterway. This can cause a minor drop in pressure or flow rate, which is an important consideration if your application relies on precise fluid dynamics.
Do I need to glue or weld internal fittings?
Yes, standard PVC installation rules apply. To ensure a completely leak-proof, airtight seal for liquid or gas transport, you must apply PVC primer and solvent cement to both the inside of the pipe and the outside spigot of the internal fitting before sliding them together. Because they can fit quite snugly, a rubber mallet may also be helpful to tap them fully into position.
Mark Ligon
Mark Ligon is the marketing manager at PVC Fittings Online, a supplier of commercial PVC and CPVC pipe, fittings and valves. Ligon enjoys educating businesses on the parts of piping systems so managers can make informed decisions. He can be reached at webmaster@commercial-industrial-supply.com.
