In this question I asked about tolerances for straight edges. For this I have a followup question about length.

If I want to purchase a straight edge, I only want to purchase one straight edge which would cover most use cases (hopefully 90+% of all use cases). This should be able to cover deck surfaces and heads for most I4, H4, V6, H6, & V8 engines (I'm excluding I5, I6, I8, V10, exotics, & larger). With this in mind, what length should I be looking at getting which would cover these engines and why? (Please qualify and quantify your answer.)

Things to consider:

  • I don't want one which is too short, because then I'd not be able to handle the larger jobs.
  • I don't want to buy one too long, because then there would be added expense (ie: increased length causes cost to go up astronomically).
  • While a straight edge which is DIN 874 Grade 00 would be awesome, a Grade 1 or Grade 2 should work for my purposes.

NOTE: Please be mindful, I'm not asking which brand or which straight edge to purchase here. I'm looking for a way to understand what I should be looking at as far as length of the straight edge and why I should be going for it and not something longer or shorter.

  • What is the maximum head length? That should give you the length you need... Lucky you don't work on a Rolls-Royce straight 8 then :) Worked on one in a Scammell 6*6 ex-army tow truck...
    – Solar Mike
    Commented Mar 13, 2018 at 20:04
  • @SolarMike has a point - If you're going to be working on an English Electric 4SRKT (the biggest I4 I can think of at the moment!), you'll need a much longer one than if you're only going to touch 1L hatchback engines ;-)
    – Nick C
    Commented Mar 14, 2018 at 10:17
  • Another reason for not getting one too long would be access, if you're trying to check a block while it's still in the car...
    – Nick C
    Commented Mar 14, 2018 at 10:20

1 Answer 1


I would consider getting two. A straight edge flat steel square (machinist square) and a 36" straight edge should cover most standard applications.

The steel square will be great for smaller applications like small component mating surfaces. It has two shorter surfaces for easy handling. Even being able to check squareness could be helpful.

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The 36" should cover the larger areas like block and head surfaces. I found a place to get both for about $100 USD.

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