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Grease Zerk Thread Size: How to Tell Which You Have

Short answer

Most grease zerks use one of a few threads. The grease zerk thread size on US-built equipment is usually 1/4-28 (a taper "SAE-LT" or a straight UNF) or 1/8 in pipe thread (1/8-27). Metric machines mostly use M6 x 1, M8 x 1 or M10 x 1, and M6 x 0.75, M8 x 1.25 and M10 x 1.5 also exist. To tell them apart, measure the thread diameter and count the pitch.

Grease zerk thread sizes

ThreadStraight or taperMeasures across threadsPitchNotes
1/4-28 SAE-LTTaper0.250 in (6.35 mm)28 TPI (0.91 mm)Made to wedge into a straight 1/4-28 UNF hole
1/4-28 UNFStraight0.250 in (6.35 mm)28 TPI (0.91 mm)Same size, no taper
1/8 in pipe (1/8-27 PTF / NPTF)Taper0.405 in (10.3 mm) at the large end27 TPI (0.94 mm)Larger thread; the fitting body is bigger too
M6 x 1.0Check the part6.0 mm (0.236 in)1.0 mm (25.4 TPI)
M6 x 0.75Check the part6.0 mm (0.236 in)0.75 mm (33.9 TPI)Fine pitch
M8 x 1.0Check the part8.0 mm (0.315 in)1.0 mm (25.4 TPI)
M8 x 1.25Check the part8.0 mm (0.315 in)1.25 mm (20.3 TPI)
M10 x 1.0Check the part10.0 mm (0.394 in)1.0 mm (25.4 TPI)
M10 x 1.5Check the part10.0 mm (0.394 in)1.5 mm (16.9 TPI)

John Deere's parts catalog and Kimball Midwest both list these threads; Deere also lists a few less common ones (5/16-32 and 1/4 in pipe, for example). Our sources don't say whether each metric fitting is straight or tapered, so check the part listing before you buy.

How to measure a grease fitting thread

  1. Remove the fitting with a correctly sized wrench, turning counter-clockwise. Clean the threads.
  2. Measure the thread diameter with calipers:
    • about 0.25 in (6.35 mm) → 1/4-28
    • about 6.0 mm → M6
    • about 8.0 mm → M8
    • about 10.0 mm → M10
    • about 0.40 in (10.3 mm) and tapered → 1/8 in pipe
  3. Check the pitch with a thread gauge, or count threads over 1/2 in: 14 means 28 TPI, about 12.7 means 1.0 mm, and 13.5 means 27 TPI.
  4. Check for taper. If the thread is wider at the base than at the tip, it's a taper fitting.
  5. Still unsure? A grease-fitting thread checker (John Deere sells one, part TY27327) tests SAE and metric fittings in seconds.

Look-alikes that cause trouble

You might think it's…But it could be…How to tell
1/4-28M6 x 1.06.35 mm vs 6.0 mm; 28 TPI vs 25.4 TPI
1/8 in pipeM10 x 1.0Pipe is tapered and slightly larger (10.3 mm); M10 is straight, 10.0 mm
M6 x 1.0M6 x 0.75Same 6 mm diameter; only a thread gauge tells 1.0 mm from 0.75 mm pitch

How to replace a grease fitting

  1. Wipe the area clean so dirt doesn't fall into the hole.
  2. Unscrew the old fitting. If it snaps off, the stub needs an extractor before you can fit a new one.
  3. Check the thread (above) and buy the same thread, length and angle (straight, 45° or 90°).
  4. Thread the new fitting in by hand, then snug it with a wrench. Taper fittings seal as they tighten. Don't crank them, or you can crack the fitting or stretch the hole.
  5. Grease the joint the way the machine's manual says, then wipe off the excess.

Common mistakes

  • Forcing a fitting that doesn't thread in by hand. It's almost always the wrong thread, usually 1/4-28 vs M6, or 1/8 pipe vs M10.
  • Replacing a straight fitting with a taper one (or the other way) without checking the hole. A taper SAE-LT fitting is meant for a straight 1/4-28 UNF hole. Ask your supplier what the hole is.
  • Using the wrong angle. If the old fitting was 45° or 90°, a straight one may not leave room for the grease gun coupler.

Not sure what you have? Snap it

Snap the fitting next to a ruler and Mechy helps you narrow down the thread. Thread pitch is hard to read from a photo, so confirm it with a gauge or a thread checker before you order. Identify a fitting from a photo.

Questions

What is the most common grease zerk thread size?

There's no single size. On US-built machines and vehicles you'll mostly meet 1/4-28 (taper SAE-LT or straight UNF) and 1/8 in pipe thread. Metric machines usually use M6 x 1, M8 x 1 or M10 x 1.

Is a 1/4-28 grease fitting the same as M6?

No. 1/4-28 measures about 0.250 in (6.35 mm) with a 0.91 mm pitch; M6 x 1 measures 6.0 mm with a 1.0 mm pitch. Forcing one into the other strips the hole.

What size is a 1/8 grease fitting thread?

1/8 in pipe thread, 27 threads per inch, about 0.405 in (10.3 mm) across at the large end. It's tapered, so it seals as it tightens.

Are grease fittings tapered?

Some are. The 1/4-28 SAE-LT and 1/8 in pipe (PTF/NPTF) fittings are tapered. Straight 1/4-28 UNF fittings are not. For metric fittings, check the part listing before you buy.

What is a 1/4-28 SAE-LT grease fitting?

A fitting with a slightly tapered 1/4-28 thread made to screw into a straight 1/4-28 UNF hole and wedge tight.

Sources

  1. Grease Fittings and Tools – John Deere Parts
  2. Grease Fittings – Kimball Midwest
  3. Grease Fittings, 1/4"-28 thread size – McMaster-Carr
  4. National pipe thread (1/8 in OD and TPI) – Wikipedia
  5. ISO metric screw thread – Wikipedia

Updated

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