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'7'
is considered to be Medium class of fit and it is used
for general applications. While for classes below '7'
are progressively having closer tolerance, (like '6'
& '5') while classes above '7' are having progressively
coarser tolerance (like '8' & '9').
For screw plug gauges i.e. NUT THREADS class 'H' is
used which is always written in capital letters.
For screw ring gauges i.e. BOLT THREADS class 'c' and
'e' are used which are always written in small letters.
Example – Screw plug gauges to have class like
'7H', '5H', '8H' etc.
Screw ring gauges to have class like '7c', '7e', '5e',
'8e' etc.
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