You can make your own parts if they match the original.
14 CFR Part 21 CERTIFICATION PROCEDURES FOR PRODUCTS AND PARTS.
Subpart K--Approval of Materials, Parts, Processes, and Appliances
Sec. 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 subpart.
(b) This section does not apply to the following:
(2) Parts produced by an owner or operator for maintaining or altering his own product.
Here is an expanded version:
A replacement part may be manufactured by an owner or operator as long as it does not result in an alteration of the product from the type design. It can only be made for use on your airplane and can not be sold for use by another.
The part has to be made of the same material(s) and equal to the original part in strength, fit, form, function, etc. If it is not the same, then it is an alteration. This would "normally" just require a 337 be filled out for the major repair and a logbook entry.
The owner or operator may have someone make the part as long as he is responsible for the design or quality control of the part. But, the person returning the aircraft to service (putting name and certificate number in the logbooks) is the one responsible to make sure it is appropriate for the application and meets the airworthiness requirements of the type design.
A part made to alter the airplane would require an STC or field approval prior to the alteration being done.
________
California Dispensaries