Quick answer: A left-hand (LH) thread tightens counter-clockwise, so it must be checked with gauges that are themselves left-hand. A right-hand gauge will not even start on an LH thread. Order LH thread plug gauges for internal threads, LH ring gauges for external threads, and LH setting plugs if the rings need to be set or calibrated.
Need gauges for this? GageTools manufactures GO/NO-GO thread plug gauges and thread ring gauges in Dongguan, China. Standard sizes ship from stock within 72 hours, MOQ is 1 piece, and every RFQ gets a reply within 24 hours — request a quote.
How LH Threads Appear on Drawings
| Thread system | Example callout |
|---|---|
| ISO metric | M20x1.5-LH-6H |
| Unified inch | 1/2-13 UNC-2B-LH |
| Trapezoidal | Tr40x7-LH-7H |
| Pipe threads | G1/2-LH or 1/2-14 NPT-LH |
If “LH” is missing, the thread is right-hand by default. When a drawing shows both RH and LH threads on one part (for example, a turnbuckle), list each thread separately in your RFQ.
Where Left-Hand Threads Are Used
- Gas cylinder valves and regulators, where LH threads help prevent mixing up gas connections
- Turnbuckles and adjusters with one RH and one LH end
- Rotating parts that would loosen under a right-hand thread, such as some pedals, spindles and blade nuts
- Hydraulic and pneumatic fittings that must not be swapped with RH versions
What to Send for an LH Thread Gauge Quote
- Full thread callout including “LH” and the tolerance class
- Plug (internal thread) or ring (external thread), GO / NO-GO or both
- Quantity, and whether setting plugs are needed for the rings
- Coating or plating condition of the parts
- Calibration certificate requirement, if any (available on request)
Common Left-Hand Gauging Mistakes
- Ordering from a catalogue part number that is RH by default.
- Storing LH and RH gauges of the same size together without clear marking — they look identical at a glance.
- Forgetting that ring gauges need LH setting plugs for calibration.
- Checking only the RH side of a double-ended part such as a turnbuckle body.
Frequently Asked Questions
Are LH thread gauges more expensive than RH gauges?
LH gauges are less common, so they are usually made to order rather than taken from stock. Price depends on size and class, like any custom thread gauge.
What is the lead time for LH thread gauges?
LH gauges are typically produced as custom gauges: 10–15 working days after the specification is confirmed.
Can I use a RH gauge upside-down on an LH thread?
No. Turning the gauge over does not change the hand of the helix; it will not engage an LH thread.
Related Buyer Guides
- What Is a Thread Plug Gauge? Sizes, Standards and How to Order
- What Is a Thread Ring Gauge? Sizes, Standards and How to Order
- Do Thread Ring Gauges Need Setting Plugs?
- How to Order Custom Thread Gauges: 12 Details for Your RFQ
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Order Left-Hand Thread Plug and Ring Gauges
Send your thread callout, tolerance class and quantity — or just your drawing. Standard sizes ship from stock within 72 hours, custom gauges in 10–15 working days, MOQ 1 piece. Reach us on WhatsApp at +86 173 1689 5054 or [email protected].