Am I required to have a data plate on my aircraft and engine?
You are required to have a data plate on your amateur-built aircraft, but you are not required by regulation to have a data plate (or any identification) on your engine. The regulatory requirements can be found in 14 CFR Part 45, Identification and Registration Marking. Paragraph 45.1, Applicability, states:
This part prescribes the requirements for -
(a) Identification of aircraft, and identification of aircraft engines and propellers that are manufactured under the terms of a type or production certificate:
(b) Identification of certain replacement and modified parts produced for installation on type certificated products; and
(c) Nationality and registration marking of U.S. registered aircraft.
Initial reading of this paragraph would seem to indicate that, because your aircraft is not type certificated, this regulation does not apply. However, further guidance is found in Part 45.11, the pertinent part of which states:
(a) Aircraft and aircraft engines. Aircraft covered under § 21.182 of this chapter must be identified, and each person who manufacturers an aircraft engine under a type or production certificate shall identify that engine, by means of a fireproof plate that has the information specified in § 45.13 of this part marked on it by etching, stamping, engraving, or other approved method of fireproof marking.
This paragraph states that aircraft covered under Part 21.182 must be identified. Part 21.182 refers to applicants for airworthiness certificates, and amateur-built aircraft are covered under that part, which in turn requires them to be identified as per Part 45. However, you'll note that 45.11 states that only engines that are manufactured under a type or production certificate are required to be identified, which means that you are not required by regulation to have a data plate on your experimental engine. However, more info on engine identification is included later in this article.
We now know that the aircraft must be identified by means of a fireproof plate containing the information specified in 45.13, the pertinent part of which states:
45.13 Identification data.
(a) The identification required by § 45.11(a) and (b) shall include the following information:
(1) Builder's name.
(2) Model designation.
(3) Builder's serial number.
(4) Type certificate number, if any.
(5) Production certificate number, if any
You'll note that items number 4 and 5 would not apply to amateur-built aircraft, so items 1 through 3 are the only info required on an amateur-built aircraft data plate.
Placement of the aircraft data plate is also covered in Part 45.11, the pertinent part of which states:
[T]he aircraft identification plate must be secured to the aircraft fuselage exterior so that it is legible to a person on the ground, and must be either adjacent to and aft of the rearmost entrance door or on the fuselage surface near the tail surfaces.
Note that the regulation specifically requires the data plate to be on the exterior of the aircraft. It cannot be mounted on an interior surface, even if it is visible from the outside. The regulation also stipulates that the date plate should not be attached in such a manner that it will not likely be defaced or removed during normal service, or lost or destroyed in an accident. Thus, the data plate should not be placed on an inspection cover or access door, or on any part that is commonly removed for maintenance or inspection. It should be attached to the fuselage itself.
Now, back to engine identification. While you are not required by regulation to have any identifying info on your experimental engine, we recommend that you do identify it in some fashion. There may come such a time that the engine might be separated from your airframe, and having it properly identified will make this separation easier to document. Also, keeping track of maintenance, repair, and overhaul info is made easier if the engine is specifically identified. As you are not required by regulation to identify the engine in any particular manner, you are free to decide how you might wish to accomplish this, but we feel that it is a good idea to identify the engine in some manner.