What a 5/16-24 UNF Fitting Is
A 5/16-24 UNF grease fitting is a fine machine thread component used to replace lubrication ports on existing machinery. The 5/16 major diameter is paired with a fine 24 TPI pitch, a combination chosen by OEMs where a coarse thread would remove too much material from a thin or shallow port wall. This is a replacement thread, not a general-purpose size. Most new equipment arrives with 1/8-27 NPT or metric ports, so buyers searching this size are usually repairing a stripped or damaged port on a machine already in service. The fine pitch holds engagement in shallow bosses and resists loosening where vibration is present. If your port measures 5/16 across but the pitch is coarse, you need a different fitting.
Technical Specifications
Key dimensions for the 5/16-24 UNF grease fitting. Confirm your port pitch before ordering, since same-diameter coarse and fine threads do not interchange.
| Thread | 5/16-24 UNF fine machine thread |
| Major diameter | 0.3125 inch (7.938 mm) |
| Pitch | 24 TPI |
| Thread angle | 60 degree |
| Overall length | 15, 19 or 25 mm by type |
| Hex | 7/16 inch (11.11 mm) |
| Body diameter | 8.5 mm |
| Thread length | 8, 10 or 12 mm by size |
| Minimum thread engagement | 5 mm |
| Ball check | yes, holds grease and keeps dirt out |
5/16-24 UNF vs 5/16-18 UNC and 1/4-28 UNF
Two mix-ups cause most stripped ports. The first is same-diameter coarse versus fine: 5/16-24 UNF and 5/16-18 UNC share the 5/16 major diameter but differ in pitch, so a coarse fitting will start into a fine hole and then cut away the threads. The second is the smaller fine thread: 1/4-28 UNF is close in pitch but smaller in diameter, so it will not seat or seal in a 5/16 port. Neither substitutes for the other. If your machine was built for a fine machine thread, match the pitch exactly. For other port styles, see our 1/16 NPT grease fitting and M10 metric grease nipples.
Installation and Thread Engagement
- Clean the port and remove old sealant or debris.
- Confirm the pitch before threading anything in.
- Engage a minimum of 5 mm of thread.
- Tighten by size, roughly 10-15 Nm where a figure is needed.
- Verify where the head points before final seating.
- Seat the coupler square on the head.
Head Styles and Length Options
This thread is available in straight and angled heads. Straight heads suit open access where the coupler approaches in line with the port. Angled heads route the coupler away from nearby castings, belts or guards. Choose overall length so the coupler clears surrounding parts: a 15 mm body works in tight recesses, while 19 or 25 mm bodies bring the head clear of a deep boss. Thread length varies by size, so check that the engaged portion reaches the minimum. Browse our angled grease nipples or view the full hydraulic-type grease nipple range.
What Is Supplied
Fittings are supplied in the thread, head style and length you specify, with the ball check fitted. Tell us your port measurements and required head orientation, then request a quote.
Frequently Asked Questions
Is 5/16-24 UNF the same as 5/16-18 UNC?
No. Both share the 5/16 major diameter, but 24 TPI is fine and 18 TPI is coarse. They do not interchange.
What does 24 TPI mean on this fitting?
It means 24 threads per inch along the fitting body. That fine pitch suits shallow OEM ports.
Can a 1/4-28 UNF fitting substitute for 5/16-24?
No. The 1/4-28 is a smaller fine thread and will not seat in a 5/16 port.
Is 5/16-24 a tapered or parallel thread?
It is a fine machine thread. Confirm the port form with us when ordering rather than assuming either shape.
How deep must the thread engage?
Engage a minimum of 5 mm of thread in the port for a stable fit.
Which overall lengths are available?
Bodies come in 15, 19 or 25 mm overall length, with thread lengths of 8, 10 or 12 mm by size.
Why is this size called an OEM replacement thread?
It appears on existing machinery rather than new builds, so buyers usually need it to replace a damaged port.
Do standard couplers fit the head?
Yes. The conical hydraulic-type head mates with common four-jaw couplers used across workshop grease guns.
Which material should I choose?
Zinc-plated steel suits general duty. Choose stainless where the fitting sits in wet or washdown areas.
Can IsoHiTech confirm the thread before ordering?
Yes. IsoHiTech Lubrication Systems, founded in 2023, engineering since 2006, supplies this thread size in straight and angled heads. Send your port measurements to inquiry@isohitech.com or via the quote page for confirmation.
