How to Measure ORFS Fittings The Right Way, Every Time

Ordering the wrong size ORFS fitting is one of the most common and avoidable mistakes in hydraulics. Thread sizes can look identical to the naked eye but a 1mm difference in thread pitch means the fitting won't seal, the job gets delayed, and you're paying for shipping twice. Here's how to measure correctly the first time.

What You'll Need

•       Caliper (digital or vernier)  accurate to 0.1mm

•       Thread pitch gauge  to confirm UN threads

•       SAE/ORFS thread ID reference chart

•       Clean rag clean threads before measuring

First — Understand What You're Measuring

ORFS (O-Ring Face Seal) fittings are sized by three things: the thread outer diameter, the thread pitch, and the tube or hose size they're designed to connect. The "dash size" you'll see on catalogs like -4, -6, -8, -12 — refers to the tube OD in sixteenths of an inch. A -8 fitting connects to 8/16" = 1/2" OD tube.

ORFS threads are straight (parallel) UN/UNF threads  not tapered like NPT. That's important because you cannot use a tapered thread gauge to identify them correctly.

Step-by-Step: How to Measure an ORFS Fitting

Step 1 — Measure the Thread Outer Diameter (OD)

Place your caliper jaws across the outer edge of the male thread — or measure the inner diameter of the female thread. Take the reading in inches.

Example: A reading of ~0.750" (3/4") points to a -12 ORFS fitting with 1-1/16-12 UN threads.

Step 2 — Check the Thread Pitch

Use a thread pitch gauge to count the threads per inch (TPI). ORFS fittings use UNF (Unified National Fine) threads. Lay the gauge blade against the thread and find the one that sits flush with no gaps.

Tip: Don't confuse thread pitch with tube size. Two fittings can have the same OD but different pitches — they won't thread together.

Step 3 — Look at the Face & Confirm the O-Ring Groove

On a male ORFS fitting, the flat end face will have a machined circular groove cut into it  that's where the O-ring sits. If you don't see this groove, you're likely looking at a JIC or another fitting type entirely. Don't guess confirm it visually before measuring further.

Step 4 — Cross-Reference With the ORFS Size Chart

Take your thread OD and TPI together and match them against the SAE ORFS sizing chart. That gives you the dash size and confirms the exact part number you need.

 

ORFS Fitting Size Reference Chart

 

Dash Size	Tube OD	Thread Size	Thread OD (in)	TPI
-4	1/4"	9/16-18	0.5625	18
-6	3/8"	11/16-16	0.6875	16
-8	1/2"	13/16-16	0.8125	16
-10	5/8"	1-14	1.0000	14
-12	3/4"	1-3/16-12	1.1875	12
-16	1"	1-7/16-12	1.4375	12
-20	1-1/4"	1-11/16-12	1.6875	12

 

ORFS vs JIC — Quick Comparison

 

	ORFS	JIC
Seal Method	O-ring face seal	37° metal flare
Leak Resistance	Excellent	Good (torque-sensitive)
Vibration Tolerance	Very high	Moderate
Over-torque Risk	Low	High — damages flare seat
Standard	SAE J1453	SAE J514
Common Industries	Construction, agriculture, mining, OEM	Aerospace, fuel, older hydraulic lines
Maintenance	Replace O-ring if worn	Inspect seat on reassembly

 

Mistakes People Make When Measuring

Measuring the hex body instead of the thread

The hex (wrench) size has nothing to do with the thread size. Always measure the threaded section specifically.

Confusing ORFS with BSP or NPT

ORFS threads are straight. NPT tapers. BSP also has different pitch standards. If you're working on imported equipment, always confirm the thread form before ordering.

Measuring a worn or damaged thread

If threads are corroded or galled, your caliper reading will be off. Clean the fitting first and inspect for damage a worn fitting may need replacement rather than a new mate.

Skipping the O-ring size check

If you're resealing an existing ORFS fitting, also measure the O-ring groove diameter and width. Using the wrong O-ring even on the right fitting will cause a leak at pressure.

 

GEAR HOUSE HYDRAULICS Still not sure about your fitting size? Bring it in or send us a photo — our team identifies ORFS sizes every day and can get you the exact part number and stock availability in minutes. We carry the full ORFS range in steel and stainless, with same-day service on most sizes. Shop ORFS Fittings: gearhousehydraulics.com/pages/categories/orfs-fittings 24/7 Emergency Service: +1 (209) 629-7113

 

The Short Version

Measure the thread OD with a caliper. Check the pitch with a thread gauge. Confirm the flat face and O-ring groove. Cross-reference the dash size chart. That's it. Done right the first time, you order once, the fitting seals perfectly, and you move on. Done wrong, you're back at the parts counter.