The definitive solution for 3-inch to 4-inch duct transitions.
Finding a clean, structurally sound 3" to 4" (75mm to 100mm) transition is notoriously difficult. Most off-the-shelf options are thin-walled, flimsy, and prone to collapsing under clamp pressure. The DT3D 75→100mm adapter is engineered to solve this.
Whether you are transitioning a 3-inch branch into a 4-inch trunk, stepping down a 100mm inline fan, or connecting a diesel heater to a larger main run, this adapter provides the rigid, professional-grade connection your build deserves.
 
✅ Engineering Excellence
Reinforced 4mm Wall Thickness: Unlike standard thin-walled fittings, we maintain a consistent 4mm thickness throughout the entire transition cone. This prevents the mid-zone flexing and deformation commonly seen under heavy clamp loads.
Integrated Dual Hose Clamp Grooves: Features precision-molded grooves at both the 75mm and 100mm ends. This provides a positive mechanical seat that prevents hose slippage and resists rotation caused by vibration or thermal expansion/contraction.
Optimized Aerodynamics: The 25mm diameter differential is managed via a gradual, smooth taper. This ensures a clean internal bore that minimizes turbulence and prevents pressure drops within your airflow system.
Universal Utility: Designed for bidirectional use—use it as a 75mm to 100mm increaser or a 100mm to 75mm reducer.
Print-Optimized Geometry: Engineered for a support-free print when oriented large-end down.
🔧 Versatile Applications
Diesel Heaters & Van Life: Perfect for transitioning 75mm/3" heater outlets to 100mm/4" flexible trunk lines in campervans and RVs.
HVAC Systems: Ideal for residential or commercial branch-to-trunk transitions.
Ventilation & Extraction: Seamlessly connect 100mm inline fans to 75mm ducting, or adapt workshop dust extraction and grow tent ports.
Marine & Industrial: Reliable use in marine cabin ventilation and workshop air management.
 
📦 Product Details
1× STL File: High-fidelity, print-ready model. Pre-oriented for optimal layer adhesion and structural integrity through the transition cone.
 
🖨️ Recommended Print Settings
To ensure the structural integrity required for clamp-loaded fittings, please follow these specifications:
Material Selection:
PETG: Recommended for general-purpose, high-performance use.
ASA: Essential for outdoor, marine, or exhaust-adjacent environments (UV and moisture resistant).
PLA+: For indoor, ambient-temperature use only. Do not use near diesel heater heat sources.
Slicer Configuration:
Layer Height: 0.2mm.
Wall Loops: 6–8 walls (Crucial: The walls carry the structural load; high wall counts ensure the part handles clamp pressure without delamination).
Top/Bottom Layers: 5–6 layers.
Infill: 0–15%. (Note: High infill in expanding geometries can lead to inefficient, direction-reversing paths. Thick walls are more structurally sound and efficient than high infill).
Orientation: Print large-end down. This eliminates the need for supports and ensures that layer lines are perpendicular to the clamp stress, significantly increasing resistance to splitting.