Text in force at 27-51: from 27-9, effective 31 Oct 1974.
Each oil tank must be designed and installed so that--
(a) It can withstand, without failure, each vibration, inertia, fluid, and structural load expected in operation;
[(b) [Reserved.]
(c) Where used with a reciprocating engine, it has an expansion space of not less than the greater of 10 percent of the tank capacity or 0.5 gallon, and where used with a turbine engine, it has an expansion space of not less than 10 percent of the tank capacity.]
(d) It is impossible to fill the tank expansion space inadvertently with the rotorcraft in the normal ground attitude;
(e) Adequate venting is provided; and
(f) There are means in the filler opening to prevent oil overflow from entering the oil tank compartment.
39 FR 35461 · Source