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The CPI 1-1/2" Reducer Fitting for 3" Hub Outlet (Part #320) is the exact OEM-grade replacement fitting designed for RV gray water holding tank systems. If your 3" hub outlet needs a reliable reducer to accept 1-1/2" pipe, this is the correct part — engineered to fit inside the hub for a secure, leak-free connection. Compatible with 3-3/4" hole size installations.
Features & Specifications
- Outlet Size: 1-1/2" pipe fits inside reducer
- Hub Outlet Size: 3" hub outlet
- Required Hole Size: 3-3/4"
- Application: RV gray water holding tank systems
- Fitting Type: Reducer (pipe-fits-inside design)
- Manufacturer Part #: 320 (also listed as 500159)
- Brand: CPI / Custom Plastics Inc.
Why Choose the CPI Part #320 Reducer Fitting?
When it comes to RV holding tank plumbing, using the correct OEM-grade fitting matters. The CPI 1-1/2" RV tank reducer fitting is purpose-built for gray water applications, ensuring a proper seal and long-term durability. Unlike generic fittings, Part #320 is engineered to the exact tolerances required for 3" hub outlet installations — no guesswork, no leaks.
This reducer fitting is the go-to solution for technicians and DIY RV owners who need a precise, reliable replacement for their holding tank outlet. With the pipe-fits-inside design, installation is straightforward and secure.
Compare RV Tank Fittings
Not sure which fitting is right for your application? Compare the CPI Part #320 with other popular RV tank fittings available at Elkhart RV Parts:
| Model / Part # | Fitting Size | Hole Size | Fitting Type | Application | View Details |
|---|---|---|---|---|---|
| CPI Part #320 (500159) | 1-1/2" Reducer for 3" Hub | 3-3/4" | Reducer (pipe-fits-inside) | Gray Water Holding Tank | Current Product |
| CPI Part #57M (500496) | 1-1/2" FPT Flush Spinweld | 2-1/4" | Flush Spinweld w/ Membrane | Fresh Water & Holding Tanks | View Details |
| CPI Part #54 (500151) | 1-1/4" FPT Threaded Raised | 1-3/4" | Threaded Raised Spinweld | Fresh Water & Holding Tanks | View Details |
| Dometic Part #290369727 | 1" MPT x 0.5" FPT Bush | N/A (threaded) | PVC Bush Reducer | Hot & Cold Water Systems | View Details |
Tip: Always match your hole size and hub outlet diameter before ordering. The Part #320 is specifically designed for 3" hub outlets with a 3-3/4" hole — not interchangeable with spinweld or threaded fittings.
Technical Specifications
- Pipe Size: 1-1/2"
- Hub Outlet: 3"
- Required Hole Size: 3-3/4"
- Fitting Style: Reducer — pipe fits inside hub outlet
- Material: High-density plastic (OEM grade)
- Application: Gray water / black water holding tank outlet
- CPI Part Number: 320
- Cross-Reference Part Number: 500159
- Weight: 1 lb
What's Included
- 1x CPI 1-1/2" Reducer Fitting (Part #320)
- OEM-grade quality — no additional hardware required for standard installations
Compatible Models & Related Products
This fitting is commonly used with RV holding tanks featuring 3" hub outlets. Browse related products:
- Shop All RV Tank Fittings
- 1-1/2" Flush Spinweld Fitting w/ Membrane - Part #57M
- 1-1/4" FPT Threaded Raised Spinweld Fitting - Part #54
Frequently Asked Questions
What hole size do I need for the CPI Part #320 reducer fitting?
You need a 3-3/4" hole in your RV holding tank for this fitting to install correctly.
Is this fitting compatible with gray water and black water tanks?
Yes — the Part #320 is designed for gray water applications and is also suitable for black water holding tank outlets with a 3" hub.
How does the pipe-fits-inside reducer design work?
The 1-1/2" pipe slides inside the 3" hub outlet reducer, creating a secure mechanical connection without threading or welding.
What is the cross-reference part number for CPI 320?
The CPI Part #320 is also listed under part number 500159 — both numbers refer to the same fitting.
Can I use this fitting for fresh water tanks?
This fitting is designed for gray/black water holding tank applications. For fresh water tanks, consider a spinweld fitting with membrane such as Part #57M.
Why Buy from Elkhart RV Parts?
Elkhart RV Parts is your authorized source for OEM-grade CPI (Custom Plastics) fittings. Every part is verified authentic — no knockoffs, no substitutions. We ship fast from our Elkhart, Indiana warehouse, the RV capital of the world. Backed by our satisfaction guarantee and expert support team.
Our AI Live Chat is available 24/7 to help confirm compatibility and get you the right part fast.
| Material | Plastic |
|---|---|
| Hole size (inches) | 3.75 |
| Reducer size (inches) | 1 1/2 |
| Hub outlet size (inches) | 3 |
- What is the size of the reducer fitting? The reducer fitting is 1-1/2 inches for a 3 inch hub outlet.
- What type of tanks is this reducer fitting compatible with? This reducer fitting is designed for use with RV holding tanks.
- What is the hole size required for this fitting? The fitting requires a hole size of 3 3/4 inches.
- Does this fitting work with gray water tanks? Yes, the pipe fits inside and the reducer is intended for gray water.
- Who is the vendor for this product? The vendor for this product is CPI.
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