The vast majority of airframes and powerplants use U.S. standard/SAE hardware. Which manufacturers of engines and airframes use metric hardware?

  • $\begingroup$ I believe there was a hardware spec used in Europe called ASN that was metric and was used in some European GA airplanes. $\endgroup$
    – John K
    Oct 9, 2018 at 20:54
  • $\begingroup$ @fooot thanks fooot. I’m aware of this post and have read the whole thing. While some good points are made, it doesn’t really answer my question... only a few examples are given. $\endgroup$
    – Pugz
    Oct 12, 2018 at 21:26
  • $\begingroup$ You mean imperial measurements vs metric? RR use both, depends on the engine. $\endgroup$
    – Notts90
    Oct 13, 2018 at 9:51
  • $\begingroup$ @Notts90 some use US standard, some use imperial, but I am specifically interested in who uses metric. $\endgroup$
    – Pugz
    Oct 13, 2018 at 12:54

1 Answer 1


All use metric dimensions, only the numbers look sometimes rather odd.

This statement is more serious than it might look, in some places you order parts in metric dimensions and they come as 3.175 mm or 12.7 mm.

All Russian (and, by extension, all former East Bloc country) engines use metric dimensions. You only see fewer of them.

I am less sure about French engines like the CFM-56 which uses a GE-designed hot section, but the fully French-designed engines use metric, too.

In the GA world the modern Diesel engines are all metric.

  • $\begingroup$ Can you please elaborate on what you mean by “all use metric dimensions”? I’ve been working on several power plants and airframes - Cessna, Piper, Grumman, Beechcraft (King Airs), P&W PT-6 turbines, Lycoming, Continental, and all seem to use SAE only dimensions and fasteners/hardware. $\endgroup$
    – Pugz
    Feb 1, 2019 at 17:44
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    $\begingroup$ @Pugz: OK, you didn't get it. Use a metric caliper. Measure some dimensions. You will get metric dimensions, right? Even from a US engine. $\endgroup$ Feb 1, 2019 at 23:23
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    $\begingroup$ After all, 1 inch is, by definition, exactly 25.4 mm... $\endgroup$
    – xxavier
    Feb 2, 2019 at 8:06
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    $\begingroup$ @xxavier: Yes, you are right: The imperial units are defined by the corresponding SI units. Therefore, the inch is now defined as 25.4 mm. Want more precision? OK: 1 inch = 25.400000000000000000 mm. Therefore, all dimensions are metric but with odd numbers. This joke has more depth than appears on first reading. $\endgroup$ Feb 2, 2019 at 8:31
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    $\begingroup$ @PeterKämpf Well, yeah :)) - Then again it's rather shallow, not really working so simple, as metric bolts are made to ISO pitch/lead while 'imperial' bolts are defined by the ANSI maintained "Unified Inch Screw Threads Standard".So while a 3/8 bolt does measure about 9.5 mm, a 9.5 mm nut won't run along. Diameter is just one of 6 major (and mord than a dotzend secondary) values to describe such a simple piece :)) And yes, it's relevant, as the OP asked for SAE specifications. Standards include way more than just the way length is counted. $\endgroup$
    – Raffzahn
    Feb 2, 2019 at 16:15

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