Bulkhead Hole Size

AN837 Bulkhead Fitting

Bulkhead Hole Size Chart for AN Fittings

Here is how bulkhead-hole-size is determined. Use the thread size as the outside diameter of the fitting. For example, a -4 bulkhead fitting has a 7/16-20 screw thread. The hole needs to be slightly bigger than 7/16 inch. A AN960 washer designed for a 7/16 thread has an internal hole size of .453 inch. So we make the bulkhead hole size the same size as the washer internal hole size.

MS21344 is used as a guide for installing bulkhead-fittings. The instructions to the left are from TO 00-25-223 INTEGRATED PRESSURE SYSTEMS AND COMPONENTS (PORTABLE AND INSTALLED). Slight differences between the two documents, specifically in the type of washer used. MS21344 indicates AN960 washer while TO 00-25-223 shows AN901 washer.

The discussion below concerns using bulkhead fittings in hydraulic systems. Note that this is not a good practice and generally prohibited by the military. "Universal fittings conforming with MS33515 and MS33657 shall not be used in boss applications in hydraulic systems..." MIL-H-5440G. Nevertheless, it is used and the bulletin below details problems with this practice.

Special-Airworthiness-Information-Bulletin-SAIB CE07-46

The FAA received reports of leaking hydraulic fluid due to improper installation of bulkhead universal fittings when installed in a hydraulic pump pressure port. The bulkhead universal fittings were turned in too far or not far enough causing the o-ring to contact the fitting threads resulting in o-ring damage and failure. While superseding standards and specifications exist, this SAIB refers to the installation of standard parts that resulted in the reports of leaking hydraulic fluid. The standard design installation of a bulkhead universal fitting into a port includes specific procedures to assure that the fitting is positioned so that the o-ring is located between, rather than on either of the two threaded portions of the universal fitting. These installation procedures are applicable, unless superseded by the Instructions for Continued Airworthiness for a specific airplane.

Standard designs for installation of a bulkhead-universal-fitting (flared, flareless, and straight threaded connectors) into a port utilize an AN6289 nut with a recess for a back up retainer for the o-ring. The use of an AN924 nut should no longer be proposed as a standard design for a new or modified installation of a bulkhead universal fitting into a port. The use of an AN924 nut instead of an AN6289 nut with a backup ring was initially included within the standard design per AND10064 (for flared tube and straight threaded connectors) for fuel and engine oil applications only. The use of an AN924 nut on a bulkhead universal fitting installed in a port became inactive for design in 1955 via AND10064. Refer to the attached excerpt from AND10064. The use of an AN924 nut instead of an AN6289 nut with a backup ring was initially included within the standard design per MS33566 (for flareless tube and straight threaded connectors) with nominal use identified for aircraft engine fluid connections. The use of an AN924 nut on a bulkhead universal fitting installed in a port became inactive for design in 1975 via MS33566. Refer to the attached excerpt from MS33566.

Previously approved installations using an AN924 nut that have acceptable in-service performance remain approved and remain acceptable. While acceptable performance of the AN924-nut on a bulkhead universal fitting installed in a port in low pressure hydraulic systems is known to have been achieved, un-acceptable performance in medium or high pressure systems is expected. Refer to ARP-4752 Aerospace – Design-and-Installation-of-Commercial-Transport-Aircraft-Hydraulic-Systems and AS4716 (R) Gland Design, O-Ring and Other Elastomeric Seals for additional seal information and general rule information that o-rings operating above 1500 psi should utilize backup rings. Installations of an AN924 nut on a bulkhead universal fitting installed in a port without acceptable in-service performance warrants review and consideration for a design change.

MS21344 installations and MS33566 installations (with AN6289 nut with MS28773 backup retainer) of a bulkhead universal fitting in a port is accepted by the FAA as a standard design for fluid pressures up to 3000 psi. Fitting design evolution continues. AS33566 retains the use of bulkhead universal fittings with an AN6289 nut and MS28773 retainer into a port and consequently is an accepted standard design. The FAA has also received reports that AS5440 includes information for the design authority to preempt the use of bulkhead universal fittings due to their problematic service history. Refer to AS5440.