Overview
The 1/8″ Tube x 1/4-28 UNF straight compression fitting joins 1/8″ OD tubing to components with a 1/4-28 UNF straight-thread port. A single metal ferrule bites the tube wall for a metal-to-metal seal, and the nut pulls up on a 5/16″ hex.
Four neighboring pages cover the rest: the metric LL, L and S compression fitting family for metric tube; the long 1/4-28 UNF grease fittings for a port taking a nipple, not a tube; the push-up lubrication fitting for thread-free connections; and the ferrule component alone, sold separately.
How the Compression Joint Works
The fitting body carries the 1/4-28 UNF straight thread into the equipment port. On the tube side, the nut drives a ferrule into a tapered seat; its edge bites into the outside of the 1/8″ tube. The seal is metal-to-metal, with no soft material in the joint.
Specifications
| Item | Specification |
|---|---|
| Tube size | 1/8″ (3.175 mm) OD |
| Port thread | 1/4-28 UNF straight |
| Configuration | Straight (180°) |
| Materials | Brass or stainless steel (SS316) |
| Pressure rating | Brass 1,000 psig; SS316 1,500 psig |
| Temperature range | -40°F to 250°F (-40°C to +121°C) |
| Sealing method | Metal-to-metal with ferrule |
| Hex size | 5/16″ (7.94 mm) |
Both materials share the same tube end and thread; the choice is driven by pressure and environment.
Brass or Stainless Steel
The brass version, rated 1,000 psig, suits indoor lines such as machine tools and centralized grease distribution. SS316 raises the rating to 1,500 psig for outdoor, washdown or chemically aggressive environments where brass would pit. Both cover -40°F to 250°F and take the same 5/16″ wrench.
Installing the Fitting
Cut the tube square and deburr it. Start the nut by hand so the ferrule seats squarely, then wrench the 5/16″ hex until it bites firmly into the tube wall. Over-tightening can oval the tube or drive the ferrule too deep; under-tightening lets the joint weep once the line is pressurized. Check each new joint at working pressure.
Where This Fitting Is Used
Typical placements: supply lines feeding distribution blocks, grease delivery to bearing points, oil feeds on chain and conveyor lubrication, and small-bore runs on automated equipment. The fitting is dimensionally compatible with common lubrication equipment ports; confirm the port thread when ordering.
What Is Supplied
Each fitting ships with its ferrule in place, ready for a clean, square-cut 1/8″ tube end. Replacement ferrules are sold separately as brass compression sleeves for tube fittings.
Honest Limitations
- This fitting is for 1/8″ OD tube only; metric tube lines need the metric compression family.
- 1/4-28 UNF is a straight thread; a tapered port needs a different joint. Confirm the thread form and sealing arrangement when ordering.
- Soft plastic tube is not recommended without an internal support sleeve, which the ferrule page covers.
- A ferrule already compressed onto a tube is not recommended for reuse; fit a new one.
Unsure about the port thread or tube? Tell us your tube OD and port thread and we will confirm.
Frequently Asked Questions
Is this the right fitting for my lubrication line?
With 1/8″ OD tube and a 1/4-28 UNF straight port, yes. Metric tube points to the metric compression family, a nipple-type port to the long 1/4-28 grease fittings, and a thread-free need to the push-up fitting.
Is the tube size measured by OD or ID?
By outside diameter. This fitting takes 1/8″ OD tube, 3.175 mm across the outside; the bore does not set the size. Match it to the documented or measured tube OD.
How do I confirm the port is 1/4-28 UNF?
Check the equipment manual or gauge the port. It is a straight, fine thread sealing at the ferrule joint, not in the port threads; confirm the thread form and sealing arrangement when ordering.
Can it carry both oil and grease?
Yes. The metal-to-metal ferrule joint does not depend on the lubricant, so oil and grease lines both work. Stiff grease at low temperatures flows slower; that is a fluid property.
Is the ferrule included, and can I buy spares?
The ferrule is included and ships with the fitting. Spare and replacement ferrules are sold separately on the brass compression sleeves page.
Can I disconnect and reuse the fitting?
The body and nut can be reused. A ferrule already compressed onto a tube is shaped to it and is not recommended for reuse; fit a new ferrule when remaking the joint.
How tight should I tighten it?
Start by hand, then wrench the 5/16″ hex until the ferrule bites firmly and the nut seats. Over-tightening can oval the tube; under-tightening lets the joint weep at working pressure. Verify at line pressure.
Should I choose brass or stainless steel?
Brass at 1,000 psig covers most indoor lines. SS316 at 1,500 psig suits outdoor, washdown or chemically aggressive environments. Both share the same tube end, thread and hex.
Can I use it with plastic tube?
Soft plastic tube needs an internal support sleeve, otherwise the ferrule can crush or slip; the brass compression sleeves page covers that. For help choosing, send us your tube details.
Who makes this fitting?
IsoHiTech Lubrication Systems, founded in 2023, engineering since 2006, builds lubrication fittings and accessories for grease and oil systems. For quotes or configuration help, write to inquiry@isohitech.com.
