FAR 21.303, part (b)(2) authorizes owner made parts as the following :
§21.303 Replacement and modification parts.
(a) Except as provided in paragraph(b) of this section, no person may produce a modification or replacement part for sale for installation on a type certificated product unless it is produced pursuant to a Parts Manufacturer Approval issued under this sub-part.
(b) This section does not apply to the following:
(1) Parts produced under a type or production certificate.
(2) Parts produced by an owner or operator for maintaining or altering his own product.
(3) Parts produced under an FAA Technical Standard Order.
(4) Standard parts (such as bolts and nuts) conforming to established industry or U.S. specifications.
This means that you may manufacture parts for your own certificated aircraft for both the purpose of maintaining or altering it. The owner may not outsource the manufacture of these parts wholly, and may not manufacture and sell parts for aircraft other than his or her own aircraft.
It is not necessary for the part to be otherwise unavailable. If a replacement wingtip costs $800 from a PMA’d manufacturer, and you simply think you can make it for cheaper, you can!
Owner Produced Parts, LLC aims to provide design and engineering support to aircraft owners who wish to produce parts for their aircraft.
Owner Produced Parts 