The context of this question is the Form, Fit and Function (FFF) rule defined in ASME. It states that form is used to define any product with reference to its size, mass, shape, dimensions, and other visually identifiable features. For example "SCREW, PAN HEAD, M3 x 0.5, 2mm Lg, 316 SS". I am guessing this is Lag length based on some research i.e. the length from the head where the thread starts.

Q) What does Lg actually mean? Please quote resources for more reading

  • $\begingroup$ The non machined section between the head and the thread is known as the shank afaik. But have you read ASME for this? $\endgroup$
    – Solar Mike
    Commented May 30, 2020 at 6:53
  • $\begingroup$ @SolarMike No, I can't get a hold of the ASME mainly due to cost issues. This is the first time I am hearing of Shank afaik. $\endgroup$ Commented May 30, 2020 at 7:09

1 Answer 1


enter image description here

*Figure 1. For McMaster-Carr 'Lg' is the length of the screw."

Note that they specify the threaded length separately so Lg will be the distance from the bottom of the head to the tip of the screw.

  • $\begingroup$ Think of it as Lg means length of the grooved (threaded) portion. $\endgroup$
    – Fred
    Commented May 30, 2020 at 11:26
  • $\begingroup$ I imagine that for counter-sink or flat-head screws that Lg may be the overall length. $\endgroup$
    – Transistor
    Commented May 30, 2020 at 12:48

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