Buying guide · Thread gages
Thread Plug Gage 2B vs 3B: How GO/NOGO Works and How to Check a Used Plug
Used thread plug gages show up by the drawer: shop cleanouts, auctions, a box of mixed taperlocks with half the NOT GO ends missing. Before you buy or use one, you need to know what the 2B or 3B on the handle actually means, how a go/no-go check is supposed to work, and what you can and can't tell about wear on a bench.
1. What 2B and 3B mean
Unified inch screw threads (ASME B1.1) come in classes 1, 2 and 3. An A means an external thread (a bolt or stud) and a B means an internal thread (a tapped hole or nut). The class number sets how much pitch diameter tolerance the thread gets.
2B
The general-purpose class for tapped holes and nuts. It's what most prints call out, and it's what a standard tap is usually made to cut.
3B
A tighter pitch diameter tolerance for closer fits, often called out on aerospace or high-strength fastener work. It is a different thread spec, not a "better" gage.
On metric (ISO) threads the common internal fit is 6H, and metric gages are marked that way instead.
2. Why the GO is often the same and the NOT GO isn't
- Internal threads in classes 1B, 2B and 3B have no allowance, so the smallest allowed pitch diameter is the basic size for all of them.
- The GO member checks that minimum (and the full thread form), so for the same size and pitch it is usually built the same for 2B and 3B. Many makers mark GO members for both.
- The NOT GO (sometimes marked HI) checks the largest allowed pitch diameter. 3B allows less, so a 3B NOT GO is smaller than a 2B NOT GO.
- That means the NOT GO end is the one that decides which class you're actually checking. A plug with a GO and no NOT GO can't tell you the class.
3. How to use a go/no-go thread plug
- Clean chips, coolant and burrs out of the hole first, and wipe the gage.
- GO: should thread in by hand, without forcing it, for the full length of the thread. If it binds or stops short, the thread is undersize, damaged or not deep enough.
- NOT GO: may start, but under common ASME B1.2 practice it should not go in more than about three complete turns. If it keeps going, the thread is oversize.
- Never wrench a gage in. Forcing it wears the gage and can hide a bad thread.
- A plug checks pitch diameter and form. It doesn't check thread depth, position or perpendicularity, so those need their own check if the print calls for them.
4. Reading the markings
- Size and pitch, such as 1/4-20 UNC or 3/8-24 UNF, then the class (2B or 3B), and GO or NOGO / HI on each end.
- Many NOT GO members have a shorter threaded length than the GO end, and some makers add a red band or a groove on the NOT GO side.
- A gagemaker's tolerance class such as W or X tells you how closely the gage itself was made. W is tighter than X. It is separate from 2B or 3B.
- Special threads (modified pitch diameters, plated-thread allowances, STI for helical inserts) carry their own markings. Don't use one as a standard 2B or 3B gage.
5. Checking a used thread plug before you buy
- Both members there: a GO-only plug is half a gage. Confirm the NOT GO end matches the GO size, pitch and class.
- Under good light and a loupe, look at the first few threads of the GO end. Flattened crests, bright worn flanks, nicks, burrs, galling or rust pitting are signs of wear or abuse. The GO end wears first because it goes into every good part.
- Members tight in the handle, with no wobble. On reversible (trilock-style) designs, check whether the member has already been flipped to its second end.
- Run it gently into a known-good tapped hole or a matching thread ring if you have one. That's a sanity check, not a measurement.
- Pitch diameter wear can't be judged by eye or with a regular micrometer. It takes thread measuring wires and a proper setup, which in practice is a calibration lab job. If the plug will be used to accept parts, get a lab check before you rely on it, and get that quote before you agree on a price.
- Old certs are history. A cert shows the gage measured in tolerance on that date, not today.
Walk away from: a missing or mismatched NOT GO, a GO end with chewed or flattened lead threads, rust pitting on the flanks, a loose member, or a "set" priced as complete when the sizes you need aren't in it.
6. Thread gage sets and the photos to ask for
- Count the sizes against the case or listing. Mixed drawers often have GO ends from one set and NOT GO ends from another.
- Rings for external threads are a separate check. Adjustable rings should still have their lock screws and sealant intact; broken seals mean someone adjusted them. More in the gage buying checklist.
- Photos to ask for: each handle's markings, both ends of each plug close up (especially the GO lead threads), the case with every slot, and any cert with a serial that matches the gage.
Thread plug gage questions
What is the difference between a 2B and 3B thread plug gage?
2B and 3B are classes of internal Unified inch threads. 2B is the general-purpose class most prints call out. 3B has a tighter pitch diameter tolerance for closer fits. Because neither internal class has an allowance, the GO member is usually built to the same minimum pitch diameter for both, and many makers mark it for both. The NOT GO member is what changes: a 3B NOT GO is smaller than a 2B NOT GO of the same size and pitch.
Can I use a 2B gage on a 3B thread?
No. A 2B NOT GO is larger than a 3B NOT GO, so it can pass 3B threads that are oversize. Going the other way, a 3B NOT GO used on a 2B callout can reject threads that are actually good. Match the gage class to the class on the print.
How do you use a go/no-go thread plug gage?
Clean the part and the gage. The GO member should thread in by hand, without force, for the full length of the thread. The NOT GO member may start, but under ASME B1.2 practice it should not enter more than about three complete turns. If the GO will not go in, or the NOT GO keeps going, the thread is out.
How can I tell if a used thread plug gage is worn?
Look at the first few threads of the GO member under light and magnification for flattened crests, bright wear, nicks, burrs, galling or rust. Check that the member is tight in the handle. Wear on pitch diameter cannot be judged by eye or with a regular micrometer; it takes thread wires and a measuring setup, which is usually a calibration lab job.
What do W and X mean on a thread gage?
They are gagemaker's tolerance classes for how closely the gage itself was made. W is tighter than X. X is common for everyday product gages. This is separate from the 2B or 3B class of the thread being checked.