Technician reported errors in measurements after a firewall was fabricated and replaced on a C172 aircraft. A Design Engineering Representative was engaged to approve the fabrication and replacement.

Narrative

While replacing the firewall assembly on a Cessna 172N Aircraft X I measured the thickness of the lower firewall that I had removed at .020'. I later learned that the material is .016'; I remeasured the old firewall section and it was in fact .016'. I contacted a Design Engineering Representative for approval for the increase in thickness. He issued me a repair authorization approving the repair. In my log book entry; I failed to use the word 'Fabricated' when referring to the lower firewall section; I used 'replaced' because I had replaced the upper firewall section with a purchased part from Cessna. I installed the firewall sections per the Cessna drawing. The drawing that I had did not specify the thickness of either section. With the Design Engineering Representative approval the aircraft is airworthy and the repair is legal. I feel the factors that caused this error where me miss reading the caliper and the length of time it took from start of the project to the finalization of paperwork (3 years). Also in my mind I was sure that the metal thickness was correct because in my experience .020 is more widely used and .016 is an odd thickness for powered aircraft. I know that this firewall replacement is done correctly and at no time was this aircraft unsafe.