Are There JIS and ANSI / ASME Screws? Why Different Thread Systems Cannot Be Mixed
- 翰君 陳
- Apr 1
- 4 min read
In practice, many people have encountered this situation:
A screw looks about the same size, but it just won’t go in — or it gets stuck halfway.
Most people first suspect size or tolerance issues.However, in most cases, the real problem is: the thread system is different.
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1. What Are JIS and ANSI / ASME Thread Systems?
In screw design, the whole world does not use the same standard.
The two most common systems today come from different countries:
JIS (Japanese Industrial Standards) — Japan
ANSI / ASME (American National Standards Institute / American Society of Mechanical Engineers) — United States
These two systems developed independently and formed today’s two major systems: metric and inch-based threads.
After 1968, JIS has largely aligned with ISO metric standards, so most modern metric threads are interchangeable.However, in some older equipment, different pitches may still exist.
For example:
Current M10 coarse pitch: 1.5 mm
Older JIS version: 1.25 mm
Therefore, when dealing with older Japanese equipment, even if it is marked “M”, it is still recommended to measure the pitch to confirm.
On the other hand, ANSI / ASME is a U.S. standard system, but it uses inch units.In engineering classification, thread systems are defined by unit (metric vs inch), not by country.Therefore, ANSI / ASME threads are categorized as the inch-based system.
System Comparison
Item | JIS / ISO (Metric) | ANSI / ASME (Inch) |
Unit | mm | inch |
Marking | M6, M8, M10 | 1/4-20 UNC, 5/16-18 UNC |
Thread Type | Metric Thread | Unified Thread |
Regions | Taiwan, Japan, Europe | USA, export equipment |
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2. What Are the Differences?
Simply put, the difference is not just the unit — it is the entire thread design system.
Key Differences
Item | Metric (JIS / ISO) | Inch (ANSI / ASME) |
Size Representation | mm | inch |
Pitch Representation | Pitch | TPI |
Thread Angle | 60° | 60° (UNC / UNF) |
Design Logic | Metric system | Inch system |
A common misconception is:Since both metric and UNC/UNF threads use a 60° angle, people assume they can be interchangeable.
But in reality:
Thread height is different
Pitch calculation is different
Contact surfaces are not fully matched
When threads do not fully match, localized stress concentration occurs during tightening, which eventually leads to thread stripping or damage.
Another older system, Whitworth (BSW), uses 55°, making it completely incompatible.
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3. Even Within the Same System, There Are Different Thread Types
Many people think once you distinguish between metric and inch, that’s enough.However, even within the same system, there are different thread types.
Even if the system is correct, the screw may still fail if the thread type is wrong.
Why Are There Coarse and Fine Threads?
Threads are divided based on different application needs such as strength, assembly speed, and resistance to loosening.
In simple terms:
Coarse threads → more forgiving, easier to assemble
Fine threads → more contact area, tighter and more stable
Metric Thread Types
Type | Description |
Coarse | Most common, general-purpose |
Fine | Smaller pitch, tighter fastening |
Special | Custom applications |
Example (M10):
Type | Pitch |
Coarse | 1.5 |
Fine | 1.25 |
Inch Thread Types
Type | Description |
UNC | General-purpose, coarse |
UNF | Higher strength, fine |
Example (1/2"):
Type | TPI |
UNC | 13 |
UNF | 20 |
The most common mistake on-site is M12 vs 1/2".
Because the diameters are almost identical, the screw may go in for the first few turns.However, due to different pitches (1.75 vs ~1.95), it will eventually jam.
If forced, it will usually damage the thread completely.
Common Size Comparison
Metric | UNC | Approx. Diameter |
M6 | 1/4-20 UNC | ~6.35 mm |
M8 | 5/16-18 UNC | ~7.94 mm |
M10 | 3/8-16 UNC | ~9.53 mm |
M12 (1.75) | 1/2-13 UNC | ~12.7 mm |
Metric vs UNC / UNF Thread Pitch Comparison Table
From the table, it can be clearly seen that even if the diameters are the same, as long as the pitch or thread count is different, the threads are completely non-interchangeable.
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4. Why Can’t They Be Mixed?
Many people try to substitute metric and inch threads.
At first, it may seem usable — but in reality, the threads are not properly matched.
What Happens When Mixed?
Situation | Result |
Stops halfway | Thread geometry mismatch |
Forced tightening | Stress concentration → stripping |
Long-term use | Wear or failure |
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5. Common Mistakes On-Site
Mistake | Problem |
Assuming similar size is acceptable | Wrong system |
Forcing when it doesn’t fit | Thread damage |
Not checking equipment specs | Wrong selection |
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6. How to Identify Quickly?
Method | Clue |
Marking | M = metric / fraction = inch |
Pitch | decimal = metric / integer = inch |
Equipment origin | Japan/EU = metric / USA = inch |
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7. How Is This Different from Retaining Rings?
Item | Retaining Rings | Screws |
Difference Type | Size difference | System difference |
Interchangeability | Sometimes possible | Almost impossible |
Result of Error | Loose fit | Thread damage |
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8. What Problems Occur If the Thread System Is Wrong?
Problem | Description |
Thread seizure | Cannot install/remove |
Thread stripping | Damage |
Loosening | Failure |
Equipment damage | Requires replacement |
These issues usually do not occur immediately,but appear gradually after a period of use.
By the time they are discovered, it often requires:
Disassembling equipment
Re-machining
Or replacing parts
Which significantly increases cost.
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9. Why Choose Sheng Fong Precision?
In practice, screw problems are rarely about size —they are about incorrect system selection.
Sheng Fong Precision helps you:
Identify correct thread systems
Match equipment origin and application
Avoid thread damage and rework
📩 LINE: @s9000
#Screw #MachineThread #MetricThread #UNC #UNF #ANSI #ASME #JIS#Fasteners #IndustrialParts #ThreadSystem #EquipmentMaintenance#ShengFongPrecision #SFP
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