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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:Hi Low Thread Screws
Materials:Carbon Steel
Surface treatment:Nickel Plated, White Zinc Plated
Head Type:Hex Washer Head
Drive:External hexagon on the washer head, wrench or socket driven
Thread:Two-start hi low thread, rolled: one high helix and one low helix between it
Point:Sharp tapered point; no drill point, no flat end
Forming:Single-die heading, two-start thread rolling, plating, sorting
Size:M1 M2 M3 M4 M5 M6 M7 M8 M9 M10 M14
Number:497
We manufacture hi low thread screws with a two-start thread — one high helix that holds and one low helix that leaves material in place — under a hex washer head, in carbon steel with nickel or white zinc. Both thread diameters, pitch, head, point and length are taken from your drawing.
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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.
Hi low thread screws are self-tapping screws with a two-start thread: one high thread that does the holding and one low thread that runs between it. The recorded item on this page is carbon steel with a hex washer head, finished in nickel or white zinc, quoted from M1 to M14 with Number 497.
Two threads, two jobs. The high thread has the larger diameter and is what cuts or forms the load-bearing thread in the hole. The low thread is smaller and sits between it, taking up space without cutting as deep — which is what makes the screw easier to drive and gentler on the material.
Made for plastics. A hi low thread screw is the standard answer for plastic bosses and thin mouldings. A single coarse thread removes too much material and builds stress that cracks the boss; the low thread leaves material in place between the high turns.
Hex washer head. A hexagon with an integral washer flange. The flange spreads the clamp load on a plastic face, and the hexagon gives a wrench drive that does not need a recess.
Sharp point. A tapered point that starts in a pilot hole. On plastic the hole is moulded or drilled; on sheet it is drilled or punched.
Nickel or white zinc. Two finishes are recorded. Both are thin and decorative, and both are chosen so the thread geometry — which is the whole function of this screw — is not altered by a coating.
The hi low thread is rolled as a two-start thread: two helices wound together, one at the major diameter and one at a smaller diameter, with the same pitch. That is what gives the screw its characteristic look and its behaviour — the high turn bears, the low turn clears.
The pitch of the two starts is the same, so the screw advances at twice the rate of a single-start thread of that pitch. On a plastic boss that matters: fewer turns to seat, less friction heat, and less chance of the material relaxing before the head bears.
The hex washer head is formed with the flange in the same stroke, so the flange is integral and the head sits flat. The flange is what carries the load on a plastic face, and on a thin moulding it is the difference between a fixing that holds and one that dishes the boss.
The point is formed after rolling, so it has a clean taper and a lead-in. On a moulded boss the point has to find the hole without cutting the entry; a blunt point that catches will burr the moulding before the thread engages.
The finish is applied after rolling and is deliberately thin. A heavy coating on a hi low thread fills the gap between the high and low turns, and that gap is the feature.
The family is quoted from M1 to M14, with M1, M2, M3, M4, M5, M6, M7, M8, M9, M10 and M14 in the recorded range. Custom lengths up to 2,000 mm are subject to DFM review after drawing submission; hi low thread screws are normally short, because the thread works in a boss or a panel rather than through a stack.
| Item | How it is defined | What we control |
|---|---|---|
| Thread | Two-start hi low, one major and one minor helix | Pitch and both diameters checked on the section |
| Head | Hex washer head, integral flange | Across flats and flange diameter on the projector |
| Pilot hole | Set by the high thread and the material | Confirmed in a sample of the customer’s moulding |
| Point | Sharp tapered point | Checked on the first article |
| Length | From under the flange to the point | Measured on the shop plate |
| Finish | Nickel or white zinc, thin | Thickness and tone checked per batch |
The pilot hole is the number the customer owns, and on a hi low thread screw it is less forgiving than on a single-thread screw. Too small and the high thread tears the boss; too large and the high thread has too little material to bite. The hole size belongs on the drawing, with the material it is moulded or drilled in.
Length is measured from under the flange. On a plastic boss the depth of the hole and the length of the screw have to leave a defined clearance at the bottom, because a screw that bottoms in a blind boss stops before the head bears and the joint looks tight while it is not.
The second thread is the whole idea, and it is worth being precise about what it does — because a hi low thread screw is often specified for the wrong reason.
| Feature | What it gives | What it costs |
|---|---|---|
| High thread | The load-bearing thread in the material | Cuts a deep groove; needs the right hole |
| Low thread | Leaves material between turns; reduces stress | Adds nothing to pull-out strength |
| Two starts | Half as many turns to seat | Different feel on the driver; torque rises faster |
| Hex washer head | Load spread on a plastic face | Needs clearance for a wrench |
| Thin finish | Thread geometry preserved | Little corrosion protection for outdoor work |
The low thread does not make the joint stronger; it makes the material survive. In a plastic boss, the failure is usually a crack that starts at the thread root and propagates out through the wall, and a single coarse thread removes enough material to make that likely. Leaving a turn of material between the high threads is what prevents it.
The two starts change how the screw drives. It advances twice as fast for the same rotation, so it seats in fewer turns and generates less friction heat in the plastic — which matters on a moulded boss that softens as it warms. What the operator feels is a torque that rises more quickly and then stops, rather than a long slow climb.
The head is the third part of the answer. A hex washer head spreads the clamp load over the flange, so the plastic under it is not crushed by a narrow bearing ring. That is why the recorded part pairs a hi low thread with a washer head rather than with a pan head.
Two finishes are recorded: nickel and white zinc. Both are thin electroplates, and on a hi low thread screw the thinness is the point — the gap between the high and low turns is the functional feature, and a heavy coating fills it.
| Finish | How it protects | Effect on the thread | Typical use |
|---|---|---|---|
| Nickel | Hard bright deposit | A few microns; both crests checked | Visible plastic assemblies |
| White zinc | Sacrificial zinc, blue-white passivation | Thin; thread gauged after | Indoor general work |
| Black zinc | Zinc with black passivation | A few microns; recess re-checked | Dark plastic housings |
| Zinc nickel | Alloy layer, higher corrosion resistance | Thicker; thread re-checked | Damp or outdoor equipment |
| Stainless (equivalent part) | Alloy provides protection | No coating | Wash-down and coastal work |
The high thread is the surface that has to stay sharp. A deposit on its crest blunts the cutting edge, and a hi low thread screw that does not cut cleanly tears the boss instead of forming a thread in it. That is why the finish is kept thin and the thread is checked after plating rather than before.
Where the joint is outdoors, neither recorded finish is enough. Hi low thread screws are usually a plastic-assembly part, and a plastic assembly that lives outside needs a stainless version or a coated one that is specified for the environment — not simply a brighter finish.
Three assemblies where the material being joined is the limiting factor:
| Where it sits | The screw drives into a moulded boss inside a plastic enclosure, where the boss is thin and cracking it writes off the whole moulding. |
| When it goes wrong | A single coarse thread removes too much material and the boss splits along the thread root, and a screw that is too long bottoms out and splits the end. |
| Why it happens | The material is a moulding, the boss is thin, and the assembly is made once. |
| What we control | Pilot hole and point checked in a sample of the moulding, thread section checked on the first article. |
| Where it sits | The screw fixes a board, a bracket or a cover into a plastic chassis where the fixing is driven by machine and the boss is small. |
| When it goes wrong | A screw that needs many turns slows the line, and a thread that generates heat softens the boss before it seats. |
| Why it happens | The volume is high, the bosses are small, and the drive is automated. |
| What we control | Thread gauge and head checked, torque set from the first article in the moulding. |
| Where it sits | The screw fixes a thin panel where a coarse thread would remove too much material and a fine one would not hold. |
| When it goes wrong | A single thread strips in thin sheet, and a head without a flange dishes the panel around the hole. |
| Why it happens | The panel is thin, the fixing is seen, and the joint is serviced occasionally. |
| What we control | Flange diameter and point checked, thread gauged per batch. |
Four reasons that matter on a two-start thread, each tied to what the drawing has to prove:
|
Thread form and hole reviewed together
Both thread diameters, the pitch and the pilot hole are reviewed as one set against the material, so a screw that would tear a boss is raised first. Basis: Thread form and hole reviewed as one set. |
Head, thread and point in one route
The hex washer head is formed in one stroke, and the hi low thread is rolled over the full length to a sharp point. Basis: One route from wire to plated screw. |
|
Both threads checked on the section
The high and low diameters and the pitch are checked on a section of the first article, and the thread is gauged per batch. Basis: First article checked on the thread section. |
First article to repeat production
A repeat order is measured against the first article, where drawing revision, wire heat, die set and thread roll were fixed. Basis: First article signed off before the batch is released. |
Shi Shi Tong forms these hi low thread screws on a single-die header that upsets the hex washer head in one stroke, then rolls the two-start thread over the full length and forms the point. The parts are nickel or white zinc plated as the drawing specifies. The first part of every run is measured on the head, checked on a thread section and driven into a sample of the customer’s material, before the batch continues.
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The thread roll is the tool that defines this part. A two-start roll has to hold both helices in phase all the way along the screw, and a worn roll lets the low thread drift until it starts carrying load — at which point the screw behaves like a single coarse thread and the boss cracks. The thread is checked on a section, not by eye.
What decides whether hi low thread screws pass is the two thread diameters, the pitch, the head, the point and the plating. The inspection agreement covers those, and the criteria come from the drawing.
Thread section on the projector |
Thread gauged per batch |
Hardness testing against the property class |
Salt-spray testing of the plated part |
The two diameters are checked on a section of the first article, because the difference between the high and low crests is what the screw is for and it cannot be seen from the outside. A batch that measures correct on the high thread and has drifted on the low one behaves as a different part.
The driving test is done in the customer’s material. A hi low thread screw that tears a boss has failed even if every dimension is right, and the only way to catch it is to drive one into a sample of the moulding and look at the thread it left.
Every shipment of hi low thread 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 material or the plating. The documents carry the drawing revision and the batch number, so the paperwork can be matched to the parts.
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 hi low thread screw the first article is driven into the customer’s moulding, because the thread only proves itself in the material it was designed for. Buyers visit during the process to review samples and to see how the threads are rolled.
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8,678+
customers served
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87%
of orders from repeat customers
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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 header 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 |
A two-start thread: one high helix at the major diameter and one low helix between it. The high thread holds; the low thread leaves material in place.
A single coarse thread removes too much material and builds stress at the thread root, which is how a boss cracks. The low thread reduces that stress without taking away the grip.
No. Pull-out comes from the high thread. The low thread is there to keep the material intact and to make the screw easier to drive.
Two starts advance twice as fast, so it seats in fewer turns and generates less heat in the plastic. Torque also rises more quickly.
One sized to the high thread and the material. It is less forgiving than a single-thread screw and belongs on the drawing.
Both thread diameters and the pitch, head across flats and flange diameter, point form, length and datum, material, finish and the pilot hole.
How a hi low thread screw project runs from drawing to repeat delivery:
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1
Send the drawing and a material sample
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→ |
2
Thread form and hole review
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→ |
3
First article driven in the moulding
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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 both thread diameters and the pitch, head across flats and flange diameter, point form, length and datum, material, finish and the pilot hole.
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Thread diameters and pitch
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Head across flats
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Flange diameter
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Point form
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Length and datum
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Pilot hole
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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
