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+86 13143444311
Email：sst@szsst88.com
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Address：Building 9, First Industrial Zone, Tantou Industrial City, Songgang Street, Baoan District, Shenzhen 518105, Guangdong, China
Type:Reverse Thread Screws, 4 Inch Long, Low Head
Materials:Steel
Surface treatment:White Zinc Plated
Head Type:Ultra Low Head
Drive:Per drawing; plain in standard form
Thread:Left-hand, half thread
Point:Blunt
Forming:Single Die Heading
Size:M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M14
Number:257
We make 4 inch long screws with an ultra low head, a half thread and a reverse left-hand thread, in white zinc plated steel, quoted from M1 to M14. Thread direction, lengths, head and marking come from your drawing, and the first article is fitted and turned the correct way.
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ISO 9001:2015 · Manufacturing since 1998 · 8,678+ customers · 89 countries and regions
Made to order – there is no stock catalogue. Each part is produced to your drawing, and production starts after drawing review and first-article approval.
These are long screws with a low head, a half thread and a left-hand thread — reverse thread — so they tighten anticlockwise. This one is white zinc plated steel with an ultra low head and no drive recess in the standard form, quoted from M1 to M14 with Number 257. A four inch long screw is about 100 mm, one of the common lengths in this family, and the reverse thread is what the part is bought for.
Reverse thread. A left-hand thread that tightens when turned anticlockwise and loosens when turned clockwise.
Ultra low head. A shallow head that stands only a fraction above the part, with a plain face in the standard version.
Half thread. The plain shank runs through the joint and the thread engages only at the far end.
White zinc. A bright sacrificial zinc deposit over carbon steel, the general indoor finish.
A four inch long screw with a reverse thread is a service part more often than a design part: it replaces a fixing on a rotating assembly where the original thread direction is what keeps it in place, and getting the direction wrong is the usual reason a repair fails.
A reverse thread exists to defeat rotation. On a joint that turns — a shaft, a spindle, a wheel — a right-hand screw can work loose because the rotation of the assembly acts on the thread in the loosening direction. A left-hand thread reverses that: the same rotation now pushes the screw towards tight, and the joint stays closed.
On a reverse thread the cost is that everything about assembly and service is reversed. The screw is driven anticlockwise, the nut that fits it is a left-hand nut, and the driver has to be turned the other way from the habit of the trade. A fitter who does not know will either find the screw will not start — because it is being turned the wrong way into the first thread — or, worse, will turn it clockwise and drive a half-threaded screw further into the far part instead of out.
That makes marking part of the specification. A reverse thread is usually identified by a groove, a notch or a mark on the head, or by a thread that is visibly opposite to the standard in the bin. On a long screw with a plain low head there is very little room for a mark, which is one reason these parts are specified with a drawing rather than picked from a catalogue.
The low head adds a second constraint. A head that is only a fraction of a millimetre high has no room for a deep recess, so the drive — where one is specified — is shallow, and the torque the head will take is limited. On a long screw the shank takes part of the torque as twist anyway, so the drive and the length together set how much the joint can be tightened.
The half thread is the third element and the most ordinary. The plain shank passes through the front parts and the reverse thread engages at the far end, so the joint is clamped by a left-hand thread acting on a smoothly sliding shank. That combination is what a rotating assembly usually needs: a smooth passage and a thread that holds against the rotation.
The recorded range is M1 to M14, with lengths made to the drawing; four inches, about 100 mm, is one of the common long sizes. Custom lengths up to 2,000 mm are subject to DFM review after drawing submission. For long screws with reverse thread the quotation carries thread direction, thread length, plain length, head diameter and height, and the drive specified.
| Item | How it is defined | What we control |
|---|---|---|
| Thread direction | Left-hand, reverse | Handedness confirmed on the first article |
| Thread | Machine thread, half thread | Pitch, length and gauging |
| Shank | Plain through the joint | Diameter and length |
| Head | Ultra low, plain or with a shallow drive | Diameter, height and recess depth |
| Material | Carbon steel | Grade and heat number on the certificate |
| Finish | White zinc plated | Thickness and gauging after plating |
| Marking | Identification of reverse thread | Groove or mark position |
Thread direction is stated on the drawing and confirmed on the first article, never inferred. A batch made to the wrong hand is a batch that will not assemble, and it often will not be noticed until the parts are at the machine.
On a reverse thread the marking is part of the drawing too. Where the head is too low for a groove, the marking may be a dot or a line, or the reverse thread itself may be the only indication; a customer who services the joint needs to know which it is.
Three features define the part: the left-hand thread, the ultra low head and the long shank. The first holds against rotation, the second keeps the fixing out of the way, and the third gives the reach.
| Feature | What it gives | What it costs |
|---|---|---|
| Reverse thread | Holds against a rotating joint | Everything about assembly is reversed |
| Ultra low head | Almost flush on the surface | Shallow or no recess; low torque |
| Long shank | Reaches through the assembly | Twists under driving torque |
| Half thread | Front part stays unthreaded | Both lengths must be specified |
| White zinc | Bright sacrificial protection | Thin; wears at the shank |
The reverse thread is the reason the part exists. Anywhere a screw sits on a rotating assembly and has to stay put without a locking compound, a left-hand thread is the mechanical answer, and it does not depend on a washer or a patch.
The low head is the reason it fits. A recess or a flange on a rotating assembly can foul a cover or a moving part; an ultra low head leaves almost nothing above the surface, which is why this family is used where clearance is measured in tenths of a millimetre.
The recorded finish is white zinc plating over carbon steel. The thread is gauged after plating, which matters more here than usual: a left-hand thread is made with a left-hand tool, and its fit is checked with a left-hand gauge, so the coating has to be allowed for in the same way as a normal thread but with tooling that is not on every bench.
| Option | How it protects | Effect on thread and head | Typical use |
|---|---|---|---|
| White zinc plated | Sacrificial zinc, bright | Thin build; gauged after plating | Indoor rotating assemblies |
| Black zinc plated | Sacrificial zinc, dark | Thin build; wear visible | Dark housings |
| Black oxide | Conversion film, oiled | No measurable build | Internal mechanisms |
| Dacromet | Inorganic zinc-aluminium layer | Thick; thread allowance needed | Wet outdoor assemblies |
| Stainless alternative | Passive film, no build | Thread as made | Wet and food service |
Where a reverse thread works in a wet joint, the material should change rather than the coating. A reverse thread that corrodes is a screw that cannot be removed at all, because the usual instinct is to turn it the wrong way and tighten it further into the corrosion.
Where the assembly is rotated at speed, the thread is also the surface that frets. A thin coating wears at the contact and leaves a bright band; a stainless body avoids the coating question but brings its own galling risk, which is managed with a lubricant at assembly.
Three assemblies where a reverse thread long screw is the fixing:
| Where it sits | The screw holds a component on a shaft where the rotation of the assembly would loosen a right-hand thread. |
| When it goes wrong | A fitter drives it clockwise and tightens it into the far part instead of releasing it. |
| Why it happens | The joint rotates, the thread is left-hand, and the direction is not obvious. |
| What we control | Direction confirmed on the first article; marking specified on the drawing. |
| Where it sits | The screw fixes a knob or a wheel on a control shaft where the operating rotation is always the same way. |
| When it goes wrong | The screw works loose with every operation, and the control slips. |
| Why it happens | The joint turns one way in service and is not locked with compound. |
| What we control | Thread direction and torque checked; first article operated. |
| Where it sits | The screw holds a cover at a stand-off where the head must be almost flush and the joint is inside a moving assembly. |
| When it goes wrong | A standard head fouls the moving part, and a long shank twists before the joint seats. |
| Why it happens | The clearance is tight, the head is low, and the span is long. |
| What we control | Head height and shank checked; first article fitted in the assembly. |
Four reasons that matter on a reverse thread part, each tied to what the drawing has to prove:
|
Thread direction confirmed
The left-hand thread, thread length, plain length and head are agreed and confirmed on the first article, because a wrong hand is a batch that will not assemble. Basis: Direction confirmed on the first article. |
Headed, threaded with left-hand tooling
The low head is formed and the reverse thread rolled or cut with left-hand tooling within one route, so pitch and hand match the drawing. Basis: Left-hand tooling in one route. |
|
Thread gauged by hand
The thread is gauged with a left-hand gauge and, where the drive allows, fitted to the mating part, because a normal gauge will not prove the hand. Basis: Thread gauged for hand. |
Hand recorded
The hand and the marking are recorded from the first article, so a repeat order cannot arrive with the wrong thread. Basis: Hand and marking recorded. |
Shi Shi Tong forms these long screws on single-die headers that form the ultra low head and cut the drive where the drawing calls for one, then draws the long shank and threads it left-hand to the specified length before white zinc plating. The first part of every run is checked for hand before anything else, then measured on head, lengths and thread.
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A left-hand thread needs left-hand tooling, and that is the practical difficulty of the family. The thread rolls, the gauges and the nuts are all opposite to the standard set, so a shop that runs them has to keep them separate; a part made on the standard equipment is a part that looks right and screws the wrong way.
What decides whether these screws pass is the hand, the thread, the head and the length. The inspection agreement covers those, and the criteria come from the drawing.
Head and thread on the projector |
Thread gauged for hand |
Hardness checked on the wire |
Coating proved by salt spray |
On a reverse thread the hand is checked first, on the first part and per batch, with a left-hand gauge. The head is measured for diameter and height, the shank and thread for length, and the plating for thickness. The marking, where specified, is checked for position.
For a reverse thread the first article is fitted to the mating part and turned the correct way. That is the only test that shows both the hand and the fit at once, and on a reverse thread it is the one check no dimension can replace.
Every shipment of these long screws is traceable to the wire it was made from and to the plating batch. The grade and heat number are recorded on the material certificate supplied with the parts, and the inspection records follow the criteria agreed on the drawing.
ISO 9001:2015 |
RoHS test report |
Registered trademark |
Patent: security screw and driver |
With each batch we supply the material certificate, the inspection record for the criteria on the drawing and, where it is listed, the third-party report for the coating. The documents carry the drawing revision and the batch number, and the hand is recorded with them so a repeat order cannot be made the other way.
More than 8,678 customers in 89 countries and regions have ordered from Shi Shi Tong; 87% of orders come from repeat customers and 56% through referrals. Most projects follow the same route: drawing review, first-article approval, then batches checked against that sample. On a reverse thread part the first article is fitted and turned in the correct direction before anything else is approved. Buyers visit during the process to review samples.
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8,678+
customers served
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87%
of orders from repeat customers
|
56%
of orders through referrals
|
89
countries and regions shipped to
|
Buyers and quality engineers visit for sample review, process discussion and supplier audits; the visit is arranged around production so the thread rolling and the plating line can be seen on the day the parts run.
Customer group visit – sample review |
Customer group visit – process discussion |
Customer group visit – factory tour |
Customer visit – project kick-off |
Danish customer – drawing review |
Industry exhibition – meeting customers |
Anticlockwise. A left-hand thread tightens when turned the opposite way to a normal screw.
On a rotating joint the rotation acts on a right-hand thread in the loosening direction. A left-hand thread reverses that and holds.
By a groove, a mark or a dot on the head where the drawing specifies one, or by the thread itself. On a very low head the marking is limited.
No. It needs a left-hand gauge and, ideally, a trial fit to the mating part.
It is a common long length, about 100 mm. The thread sizes in this family are metric, from M1 to M14.
Thread direction, thread length, plain length, head diameter and height, drive, marking and the finish.
How a reverse thread long screw project runs from drawing to repeat delivery:
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1
Send the drawing with the thread direction
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→ |
2
Hand, lengths and head agreed
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→ |
3
First article fitted and turned the right way
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→ |
4
Batch production and repeat orders
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Each step is confirmed with you before the next one starts; the drawing revision from the first article is what repeat orders are checked against.
For a quotation, send the drawing with the thread direction, thread length, plain length, head diameter and height, the drive and marking, the grade and the finish.
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Thread direction
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Thread length
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Plain length
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Head height
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Drive
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Marking
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Sending a drawing? The form below does not take attachments. Email the PDF, DXF or STEP file to sst@szsst88.com with the part name and quantity in the subject line – the drawing is reviewed and a quotation follows.
+86 13143444311
sst@szsst88.com
