3 inch <> 4 inch duct adapter (75mm <> 100mm)

Version 2 is out for every use case!

Thank you for all the feedback from the community! I didn't realize there is so much need for a high quality hose/duct adapter. Based on your comments, I realize that there are so many use cases and situations, so I made a bunch of different permutations to match whatever you need!

Why I made this

This is designed to bring two ducts of 4 inch (100mm) and 3 inch (75mm) together. I've made several different versions:

• Male - nominal outer diameter + variance
• Female - nominal outer diameter - variance
• OD - nominal outer diameter without variance

And all the permutations from these three diameters. Remember to de-select what you don't need.

So many options. Where do I start?

1. Measure what you're connecting to. Get both outer and inner diameter
2. Choose whether you want the printed adapter to fit OVER or fit UNDER
3. Choose the right adapter based on the table below, with the closest fit for inner or outer diameter 
   1. Note: if your hose outer diameter is larger than the inner of the male option, then you must choose to fit UNDER
   2. Note: if your hose is inner diameter is smaller than the outer of the female option, then you must choose to fit OVER

I'm still confused. Got examples?

Yes!

Example 1:

• Say you measure your hose. It's exactly 3" or 76mm outer diameter, and 2.9" on the inside. Great! You've got a couple choices.
• You can either:
  • Choose one of the male 3 (M3) options because the inner diameter is 3" (76mm) so it fits OVER your existing hose
  • Choose one of the female 3 (F3) options because the outer diameters is 2.88" (73mm) so it fits UNDER your hose. But I use a clamp because it's 0.02" (6mm) too small / loose.
• You cannot use any of the OD3 options because it is the same size as what I'm connecting to.

Example 2:

• You measure your duct and it's 3.2" (81mm) outer diameter, and inner diameter is 3.00" or 76.20mm 
• You would choose an OD3 adapter because the outer diameter is 3" (76.2mm) and the printed adapter fits UNDER the duct.
• It may not even need a clamp because it's a tight tolerance.

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Version 1 write up: 

Designed to bring two ducts of 4 inch (100mm) and 3 inch (75mm) together. The ducts need to go OVER the printed adapter. I also recommend getting a duct clamp depending on your application and how much strain is on the duct.

An interesting detail is I wanted to make sure air flow within the duct is smooth so there are no sharp edges inside. Many of the cheap Amazon ducts are simply stacked cylinders where there is an abrupt sharp corner where the two cylinders meet. This causes airflow friction and slows down your cooling, heating or whatever you are using this for.

You could buy one at a big box home store or you can print one for a fraction of the cost. Enjoy!

Looking for a larger adapter? Check out my latest 4 to 6 inch adapter (100 to 150mm) here.