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Unified National Thread: ANSI vs. BS vs. ISO vs. JIS

USA Standard: ANSI B1.2
British Standard: BS 919-1
ISO Standard: NONE
JIS Standard: NONE

The Unified National Screw Thread Standard B1.1 was established in 1949 to obtain screw thread interchangeability between Canada, United Kingdom (UK) and the United States of America (US). The product is now interchangeable, but there are some differences in the gauging practices between the UK and US. These differences are specifically related to the NOGO gage and detailed in the standards. What follows is a summary of the differences.

UN STANDARD GAGES

GO PLUG GAGE:
BS and US measure maximum material condition which is the minimum product size.
Major Diameter Tolerance: BS and US = plus
Pitch Diameter Tolerance: BS and US = plus

NOGO PLUG GAGE:
BS and US measure minimum material condition which is the maximum product size.
Major Diameter Tolerance: BS and US = minus
Pitch Diameter Tolerance: BS = plus; US = minus

GO RING GAGE:
BS and US measure maximum material condition which is the maximum product size.
Major Diameter Tolerance: BS and US = minus
Pitch Diameter Tolerance: BS and US = minus

NOGO RING GAGE:
BS and US measure minimum material condition which is the minimum product size.
Major Diameter Tolerance: BS and US = plus
Pitch Diameter Tolerance: BS = minus; US = plus

Some general guidelines in determining which gage system to use:
1. The US system will not pass bad parts and may reject some good parts.
2. The BS system GO will not pass bad parts and may reject some good parts.
3. The BS system NOGO will accept some marginally bad parts.
4. All parts accepted by the US system will be accepted by the UK system.
5. All parts accepted by the UK system may not be accepted by the US system.
6. When in doubt, obtain direction from your customer.

Comments about the above statements can be sent to: gageguy@gagecrib.com

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