Producing Twin Navions for Jack Riley wasn't enough and in the spring of 1953, TEMCO secured production rights to the D-16, with Riley Aircraft remaining involved with marketing and sales (their strong point). Dan Hearn remembered going to Ft. Lauderdale, FL in the early 1950s, looking at the X-16 prototype and Riley Aircraft operation. "As I recall, the first prototype Twin Navion was done by a guy in Torrance, CA and his name was Daubenberger. I was not at all impressed with the Riley operation. He had taken the engines off of Super Cubs and mounted them on the nacelles. The props were not feathering and all the metal work was done by hand. I mean the nacelles and the nose baggage. The nacelles were not as wide as they should have been but Jack used the Cub engines which exhausted the cooling air along the sides." These first Twin Navions were certified as a note to the original single Navion's Type Certificate, which eased the requirements imposed by the Civil Airworthiness Authority.

"We used the Lycoming O-320 150 hp engine on the first 30 or 40 birds. We called them D-16s. It was a reliable engine, but when the Piper twin (Apache) came out we thought we needed more horse power to get better single engine performance. I took our D-16A prototype to Lycoming, they had one O-340 (170 hp) so they installed it on the right hand side (I think), then Bodie Bouknight and I flew it over to Hartzell, near Dayton, OH and left it to get the vibration testing done. This engine was a quick fix; the Lycoming O-360 was still in design as it required new cylinders. I had a chance to meet the Chief Engineer at Piper on this trip and we swapped rides. He agreed that the Twin Navion was a better bird than the Apache, but he said, "Don't tell my boss."

"I thought the T-35 Buckaroo (tandem made from a Swift) had a good looking cowl so we designed the cowl of the D-16s that way. We had a large spinner and an elliptical intake. We wanted to exhaust the cooling air downward so we had to make "rocker box covers" in the cowl doors so they would seal on the sides.

Other changes occurred between the D-16 and new D-16A aircraft. To gain access into the rear baggage compartment a small door was added to the left hand side of the fuselage, just behind the wing. The fuel system was expanded dramatically over the single Navion and D-16s too. "The Navion had 40 gals of fuel in the main wing tank and some folks would put a 20 gal aux tank in the baggage area. Since we were getting a new STC the FAA would not let us put fuel in the cabin which was not a good idea anyway. I don't recall which came first but we ended up with good looking tip talks of 20 gal each. Bodie Bouknight was my Electrical Engineer but he did the best tip tank job I have ever seen. We also installed bladder tanks in the nacelles thinking that the 32 gal x 2 = 64 + 40 would be fine. The problem was Jack Riley would not let us take off the tip tanks, so we ended up with 144 gallons. Jack thought that was great and he routinely flew nonstop from Greenville to Ft Lauderdale out over the Gulf taking 9 hours to get there. He said you had to take a wide mouth pickle jar along for company." Finally, the gross weight was increased to 3,600 lbs. Dan Hearn and Jack Riley came to this weight through testing. "We had done the design and testing for gross weights as high as 3,900 lbs as I recall. I told Jack that we ought to be able to hold 5,000 ft (on one engine), the D-16 would not. We loaded it up to 3,900 lbs then we kept taking weight out until it would hold 5,000 ft on one engine."

Besides Jack Riley having his own D-16A, TEMCO operated a Twin Navion for themselves. With the company's plants in Grand Prairie, Greenville and Garland, Chief Pilot Lynn Meyers would carry employees or documents in an almost regular schedule.

Along with the D-16A, which was marketed as the 'Riley 55,' came for the first time, a standard paint scheme. Prototype N108N was sent to Ft. Worth, TX to be professionally painted in an eye-catching cream and bright red pattern. Inside the cockpit, the twin had real, red leather seats. All of the D-16s that had been built were painted in a hodge-podge collection of colours and patterns, designed either by the customer, or by Jack Riley himself. One was even painted to match the owner's blue Ford Thunderbird.

To meet the tough requirements of the Twin Navion redesign, TEMCO assembled a skilled Engineering Department with a broad range of skills. This group helped the company win contracts of ever increasing technical complexity not only from the military but also from airlines and many foreign countries. One such program was the conversion of three new Convair 340 aircraft into portable throne rooms for the King of Saudi Arabia. Years later, this work lead TEMCO to another contract, converting a Boeing 747SP into another flying throne room for Saudi Arabia.