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I want to purchase a straight edge which I can use to measure the flatness of heads & blocks and other such nifty things. My question is, what is considered a good tolerance for a straight edge? To make this a little easier, if I wanted to buy a 24" straight edge, how much should I expect it should be straight to across the entire length?

To confine answers, I'm not looking for what you think it should be, but rather what is standard for regular machining practices. References would be helpful.

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    Check out Starrett and it ships in USA - is known worldwide and can provide certificates of conformity if you want...
    – Solar Mike
    Commented Jan 5, 2018 at 20:11
  • @SolarMike - I'm aware of Starrett (now, lol). Good stuff and thanks for the suggestion.
    – Pᴀᴜʟsᴛᴇʀ2 ♦
    Commented Jan 5, 2018 at 21:03
  • I think they use a random number generator for the prices!... No, seriously good kit costs but it is worth it...
    – Solar Mike
    Commented Jan 5, 2018 at 21:05

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what is standard

DIN 874 says less than +/- 3 microns over a 300mm "sharp edged rule"

If you purchase a "straight edge" from an engineering tool company, it will most likely meet this specification.

for a "steel rule" DIN 874 defines four grades

Flatness tolerances are

  • Grade 00: 1+(L/150)
  • Grade 0: 2+(L/100)
  • Grade 1: 4+(L/60)
  • Grade 2: 8+(L/40)

E.g. in microns 

           Grade
Length     00       0       1        2
--------------------------------------
0.5m       4        7      12       21

how much should I expect it should be straight to across the entire length?

This also depends on how you support the straight edge. There are specific procedures to be followed, some procedures involve using gauge blocks and/or other specialised test gauges in conjunction with the specially supported straight edge.

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