Although the Thunderjet went through many upgrades, both at the factory and in the field, during its use, the fuselage extension made starting with the E-model seems to be the most confusing, so far as I am able to determine from available documentation on the subject. Therefore, I have put this page first, because I believe understanding the fuselage extension details helps in understanding both some of the other upgrades made to the Thunderjet, and how those upgrades affected the external appearance.
The Thunderjet fuselage was extended as part of the upgrades to the aircraft. This extension was implemented in the E-model. Authorities differ about the length of this extension. F-84 in Detail, page 8, and F-84 in Action, page 11, both state it was 12 inches; other sources say 10 inches or 13 inches. The Republic archives indicate the extension was 15 inches. I have confirmed that the extension was 15 inches by measuring the forward fuselages on both an F-84C and an F-84E.
My opinion is that the discrepancy in lengths mentioned originates from three probable sources. One is that published figures for aircraft lengths differ, partly it seems, from "general" vs. "exact" dimensions shown on Republic's own documents. Another is that the tailpipe lengths were changed, both as engineers' attempts to deal with dissipating tailpipe heat, and also as changes to engines as furnished from the engine manufacturer. Consequently, because of these changes, the actual lengths of models changed as tailpipes and engines were modified, so the early as compared to late models did not necessarily vary by exactly 15 inches. Also, the canopy frame was lengthened by 12 inches rather than 15 inches, the other 3 inches being filled by a "splice" panel aft of the canopy frame. So, because of the variety of changes, the general statement that the fuselage was lengthened, followed by a measurement of both early and late models, may show a difference in length of only 10 or 12 or 13 inches. In any case, the correct dimension for the fuselage extension itself is 15 inches.
The extension was added at station 157½. As the graphic from the Republic archives below shows, the stations aft of the extension were not renumbered. Rather, the stations on the E- and G-models were referenced as 157½ Forward, and 157½ Aft. These two stations are indicated by the double ended arrow (<-->) just below the leading edge of the wing. Part of the 15 inch extension was added to the length of the canopy (8 inches according to my measurements) and part added to the fairing behind the canopy (4 inches according to my measurements). Also, note the "fuselage splice" indicated on the drawing at stations 217 to 220 where the 3-inch "splice" was added due to the fuselage extension. The photos on the Spine Splice comparison page show the difference in the early and late models before and after the extension.
Graphic from the Republic archives courtesy of E. J. Boss
There were many other changes to the F-84 which accompanied the fuselage extension. Some of these changes are documented below or on pages as linked.
Although photographs do not readily reveal the difference, generally, there were two basic canopies fitted to the Thunderjet. This discussion does not address the variations of those two basic canopies which is documented on the Canopies page. Because of the fuselage extension, the E- and G-models were fitted with a canopy which is (according to my measurements) 8 inches longer than that fitted to the D- and earlier models. The other 7 inches difference is accounted for as 4 inches at the aft end of the canopy fairing and 3 inches at stations 217 to 220. This difference in length can be observed in photographs by careful inspection if the canopy is upgraded with the fiberglass tape bracing. Generally, fore to aft, including the front of the canopy, there were three narrow transverse straps, then one wide transverse strap which was at about the midpoint of the canopy. For sake of reference, I refer to these as 1, 2, 3, and 4. On the D- and earlier models, the distance between 2, and 3 is noticeably less than on the E- and G-models. The photos and illustrations on the Canopy Comparison Page visually show the difference in the two canopies.
Significant changes were made to the fuselage panels forward of and around the wings. The leading edge wing root fairing on the XP to D-models was rectangular, while on the E- and G-models, it was angled at the top. The two illustrations on the Wing Root Fairing Comparison Page show this difference, along with other panel variations forward of the wing leading edges. Note that the auxiliary intakes were fitted only to the G-model starting with the G-21 models on the production line, and were retrofitted to all G-models.
One of the reasons mentioned for the lengthening of the fuselage was stated as, "An additional 12 inches was added to the fuselage length in the cockpit area for the pilots comfort." F-84 Thunderjet in Action, page 11. Considering the total extension was 15 inches, I believe (from my own measurements) that the cockpit was only lengthened by eight inches, that another four inches is accounted for in the rear deck under the canopy, and the final three inches in the fuselage splice. I do not have enough information at this time to confirm the "pilots comfort" as a reason for the extension, nor do I have enough detailed measurements to confirm my "belief" about how much the cockpit was lengthened.