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Special Pi Tape® Tapes: Precision Vernier Periphery Tapes for Direct Diameter Readings

special pi tapes

What is a Precision Diameter Measuring Tape?

It is a measuring tool to measure the diameter of a cylindrical object.

It simply takes a precision standard measurement and divides it by π ( pi )



Easy Read Tapes

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  • Designed for use in poor ambient light conditions where reading the graduations on standard tapes becomes difficult.
  • The easy-read version is manufactured on a 'blued' steel with the acid etched graduations and numerals being filled with a white pigment.
  • The blue background with white graduations and numerals makes reading the tape much easier, hence an easy-read tape.

Stainless Steel Tapes

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  • Manufactured using 4210 ground stainless steel strip.
  • The manufacturing process is the same as for standard tapes.
  • Stainless steel tapes are recommended for use in any corrosive atmosphere where rusting could be a problem
  • They are not recommended for diameters above 108 inch / 2700 mm due to the camber which is inherent in long stainless steel strip.

Internal Diameter Tapes ( ID Tapes)

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  • Available in either the standard, easy read or stainless steel versions.
  • The tapes are manufactured and graduated allowing for two factors.
  • Firstly that the thickness of the tape will be on the inside of the circumference, secondly that the tape will not be under tension.
  • An Internal Diameter tape thus reads the true internal diameter directly.
  • A Standard Outside Diameter Tape can be used to measure internal diameters by adding twice the tape thickness to the reading taken with an outside diameter tape, although the accuracy is slightly impaired.
  • It is therefore recommended that Internal Diameter Tapes are used for measuring internal diameters accurately.
  • When measuring large internal diameters, pieces of masking tape can be used to hold the tape in a proper parallel position.

Inch Inside Diameter Tape

Metric Inside Diameter Tape

Narrow/Wider Width Tapes
  • As an alternative to the standard main body width of 1/2 inch (12.70 mm) from which all tapes are manufactured, there are two narrower and two wider widths available.
  • Remember the standard tape is reduced to 1/4 inch (6.53mm) at either end to ensure that the tape is parallel to the component for measurement.
  • When taking a reading the vernier section 1/4 inch wide (6.35mm) and the gauge member 1/4 inch wide (6.35 mm) lie along side each other so the overall width becomes 1/2 inch (12.70 mm).
  • The alternative narrower widths available for both metric & imperial size tapes are ; 3/8 inch (9.53 mm) main body width, with 3/16 inch (4.77 mm) vernier / gauge members and for imperial size tapes only:
    1/4 inch (6.35 mm) main body width, with 1/8 inch (3.18 mm) vernier gauge members.
  • Narrow width tapes are most useful when measuring the diameters of grooves, when the width of the groove will not accept a standard width tape.
  • Also for measuring the diameters of small lands or steps where the overall width is limited.
  • The alternative wider widths available for both metric & imperial size tapes are:
    3/4 inch (19.05 mm) main body width, with 3/8 inch (9.53 mm) vernier / gauge members or, 1 inch (25.40 mm) main body width, with 1/2 inch (12.70mm) vernier / gauge members.
  • Wider width tapes are used on rubber covered rollers, or other soft/thin walled parts, where the pressure imparted by a standard width tape would distort the component and lead to an error in the diameter being measured.
pi tape dimensions  
Special Thickness Tapes
  • Used as an alternative to the standard thickness of 0.010 inch (0.25 mm), tapes can be manufactured in either 0.007 inch (0.18 mm) or 0.005 inch (0.13 mm).
  • Thinner tapes are more vulnerable to damage and so should only be specified when absolutely necessary.
  • They are not available in the easy-read versions.
  • Their main use is to reduce the pressure and distortion on soft / thin walled parts, where it is not -practical to use a wider width tape
Go/NoGo Inspection Tapes
  • Used in production or inspection areas, this tape differs from the standard tapes in that no actual measurement is obtained.
  • There is no vernier scale only a 'Zero' line, and on the gauge member a set of tolerance lines.
  • The tolerances lines are for the minimum and maximum diameters, and if required the nominal diameter, for a particular component.
  • The tape is used to separate the parts into three categories, oversize, undersize and within limits.
  • A tape can have just one set of tolerance lines or a number of sets dependent upon the type of component being inspected.
  • The minimum diameter distance between any two tolerance lines is 0.004 inch (0.10 mm).
  • It is not possible to etch lines closer together than this on a tape.

go-no go pi tape

May have one or more sets of ABC dimensions on one tape. Anytime "o" line falls between A B C lines part is within tolerance. The minimum diameter distance between lines is .004" (.12mm).

Extra Capacity Tapes
  • For practical engineering reasons tapes are manufactured to cover a nominal range of diameter measurement of 12 inches or 300 mm.
  • Tapes having too large a capacity are very vulnerable to damage during use - remember, when measuring the smallest diameter there is some 39 inches /1 metre of extra tape, hanging loose, not being used.
  • If damage occurs to this section the tape will probably have to be scrapped.
  • The larger the extra capacity the more vulnerable the tape becomes to getting damaged.
  • Since the major cost in any tape is not the steel but the cost of etching the graduations and numerals there is no significant cost saving in requesting an extra capacity tape.
  • If 2 or 3 standard length tapes are purchased to cover a total range, they are not only less vulnerable to damage, but it is also cheaper to replace one of them rather than having to replace an extra capacity tape.
Intermediate Range Tapes
  • Tapes can be manufactured to cover any range required to suit a particular component.
  • They are not a stock item and are made to order.
  • On the Standard Size diameters up to 144 inch /3600 mm, they are slightly more expensive due to the one of cost of the set-up.
  • However on the Large Size diameters since all these sizes are made to order, there is no additional cost, and the price is pro-rata.

link to Precision Circumference Tapes

for precision direct reading of circumferences


Trademarks are the properties of their respective owner(s).

December 2022, March 2021, September 2020, March 2017,June 2012 CSS, November 2006, December 2001

The descriptions, illustrations, specifications, dimensions, and part numbers are subject to change without notice.

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