American Association for Laboratory Accreditation
G111 – Guidance on Pitch Diameter and
Simple Pitch Diameter
Document Revised:
April 8, 2015
Page 1 of 2
In the appendices on measurement in ANSI/ASME B1.2, the National Standard for
unified threads, we find this:
This Section presents specifications and techniques for the measurement of screw
thread plug gages and setting plugs by measuring over accurate cylinders or
wires inserted in the tread grooves. The purpose is to make available a standard
United States method for making such measurements. The practices described
measure groove diameter, which is equal to pitch diameter only on a thread with
perfect pitch spacing. 1
Further, the Euramet cg-10 Version 2.1 (12/2012) “Determination of Pitch Diameter of Parallel
Thread Gauges by Mechanical Probing” says:
4.2.1 Measurement of diameter only (1a): The simple pitch diameter is calculated
from this measured diameter, corrected for the rake and the measuring force, and
from assumed nominal values for the pitch1 and thread angle.
The term “groove diameter” is seldom used, and it is used as a synonym of “Simple Pitch
Diameter”.
There are very few labs that measure the true pitch diameter because it requires the pitch
be measured so that corrections can be made. This is done at NIST, but labs typically do
not follow this practice except perhaps manufacturers of American Petroleum Institute
gages (API).
Finally, note that in a proficiency test (PT) it is unlikely that the protocol requires pitch
measurements, and thus all of the labs are actually reporting Simple Pitch Diameter. In
this case, the pitch has no effect on the measurand. If a lab did measure the pitch and
make a correction, the lab would probably be an outlier and fail the PT.
As long as the lab properly puts “Simple Pitch Diameter” in the scope, the uncertainty
will be rather low. It should probably be between 60 and 80 microinches.
Contributions by: Dr. Ted Doiron
 2015 by A2LA
All rights reserved. No part of this document may be reproduced in any form or by any means
without the prior written permission of A2LA.
1
Highlight added by the author of this guidance document.
L:\Guidance\G111 – Guidance on Pitch Diameter and Simple Pitch Diameter
American Association for Laboratory Accreditation
G111 – Guidance on Pitch Diameter and
Simple Pitch Diameter
Document Revised:
April 8, 2015
Page 2 of 2
Document Revision History
Date
May 5, 2011
Description
Original Document Issue
November 19, 2012
•
•
Updated header
Updated EA-10/10 to Euramet cg-10 Version 2.0 (3/2011)
April 8, 2015
•
•
Updated Euramet cg-10 Version 2.0 to verstion 2.1 (12/2012)
Updated ASME B1.2 to ASTM/ASME B1.2
L:\Guidance\G111 – Guidance on Pitch Diameter and Simple Pitch Diameter