My question is whether an MJ style hydraulic fitting, 37 degree like an AN fitting, may be used in an aviation application. These fittings are available at any hydraulics shop or good auto supplier, and are slightly less expensive than the "aviation" AN fitting.
Some research from Parker
revealed the following:
"There are several differences between "comparable" industrial 37° flare (SAE/ISO) and AN 37° flare style fittings.
AN 37° Flare: Male and female, Class 3A/3B UNJ/UNJF (radiused root threads)
SAE/ISO 37° Flare: Male and female, class 2A/2B, UN/UNF series threads
Reason: Tighter tolerances and better fatigue life for aircraft, aerospace, military applications."
"AN 37° flare and industrial 37° flare fittings function identically. In many cases they appear to be functionally interchangeable, but they are not. What this means is that while the products may look similar, you must not use an industrial 37° flare fitting design as a direct substitution.
Note: Parker Triple-Lok 37° flare fittings (or other TFD products) are not for use in Aerospace, General Aviation, Commercial Aviation or Military Aviation applications."
So the warning is clear that they should not be interchanged, although I am not sure if that only applies to Certified applications or if there is another reason.
The real question is this: How do you know that a fitting is a true AN or an MJ? Most AN fittings are not marked as such.
Anyone have any insight here?