Tuesday, February 23, 2010

Hose dash size to Inside Diameter

Such a simple title. But this is the world of "dash size" and military standards. Here are the documents that control dash size inside diameters:

Per MIL-F-5509D "Fittings, Flared Tube, Fluid Connections" referes you to the following specifications for internal dimensions:

  • MS24385
  • MS24386
  • MS33649 "Straight Thread Boss" 
  • MS33656 "Fitting End Flared Tube" 
  • MS33657 Bulkhead Fittings 
  • MS33658 Fitting End Hose connection - obsolete, replaced by AS5132 Must purchase from ASTM. Hose end fittings vary depending on hose type and if the fitting has an angle or not and the type of hose.

For External ANPT, such as AN816 flared tube to tapered pipe male consult AND10052.
For hose ID see hose specifications MIL-DTL-8794 for 111 hose. Each hose type has a different ID and angle fittings have a different ID than straight fittings for the same hose! Consult MS33658.

A common mistake is to think all hose of the same dash-size has the same inside diameter. This is often noticed when someone is replacing an older style 111 hose with a more modern Teflon-hose. The Teflon hose has a noticeably smaller inside and outside diameter than the 111. The question arises as to why the same hose dash size has different inside diameters. Shouldn't they be the same?

The dash size of a hose does not tell us what the inside diameter is. Different hose types with the same dash size will have different inside diameters.

This is not unique to hose. For example, aircraft fluid carrying tubing is similar in that the size of the tubing is the O.D. not the I.D. So 3/16 inch tube is 3/16 O.D. The inside diameter depends upon the wall thickness.

Even the hose fitting inside diameter is different. Angled fitting usually have a smaller inside diameter than a straight fitting for the same hose. A hose with a straight fitting on one end and an angle fitting on the other end will have different inside diameters.

Industrial and automotive hose is sold by the fractional size. This is the nominal (approximate) inside diameter of the hose. For example, MIL-H-6000 3/8 hose has a nominal inside diameter of 3/8 inch. Aircraft hose is sold by the dash number.

The most full-proof method of determining what size hose assembly you have is to measure the thread diameter on the fitting and compare with the chart below.

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