In February of 1944 the Jacobs Aircraft Engine Co. of Pottstown, PA issued Service Bulletin #30 (see attached), which described a fuel pump adapter that was designed to be used on all their engines using the four stud, 2 inch, crankshaft driven fuel pump. This adapter moved the fuel pump back toward the firewall about two inches. The reasons given in the Service Bulletin for this modification were twofold: 1) using the adapter makes the pump much easier to service and inspect, and 2) the adapter introduces a “universal joint” action between the pump and the crankshaft so that any mismatch between the crankshaft drive hole and the fuel pump shaft is compensated for. Jacobs recommended that the fuel pump adapter be installed on all their engines for which it was designed, which would now include the R755-9 (L4MB), R755A2, R755B2, R755S, and L6MB.
Another issue that Jacobs did not mention in the Service Bulletin is that of fuel pump drive shaft engagement. Without the fuel pump adapter, he fuel pump drive shaft on the Pesco 2PR400BLYA fuel pump engages directly into a square hole in the aft end of the crankshaft. The square crankshaft drive is broached through to a depth of .520”, but the square fuel pump drive reaches into that hole only to a depth of .220”. Since the fuel pump drive shaft is only about half engaged in the crank, after a time of engine operation both the fuel pump drive shaft and the crankshaft drive hole begin to wear so that the corners of both become rounded. In a few extreme cases, one or both of the drives have rounded completely out and the engine driven pump has stopped turning.
The photos in Figure 1 show enlarged views of the square fuel pump drive in two Jacobs crankshafts. The crankshaft on the left has the normal ¼” square fuel pump hole into which the fuel pump will mate. The fuel pump drive on the right has rounded out and failed completely (note the displaced metal in the hole). The fuel pump driven by this engine was no longer turning and the engine was running strictly on gravity feed. Though all aircraft fuel systems are designed with a redundant source of fuel pressure (gravity, wobble pump, boost pump, etc) the goal is to not get into a situation where the second pump is essential to keep the engine running. Unlike the Pesco engine driven pump (with only .220” of drive shaft engagement) the square portion of the drive shaft on the fuel pump adapter extends .695” (through the full depth of the crankshaft drive hole). This distributes the load of the fuel pump drive over an area 137% greater than that of the pump without the adapter. Further, with the adapter the fuel pump drive shaft extends into the adapter coupling to a greater depth than it does into the crankshaft when used without the adapter. Rather than being only the “convenience modification” that Jacobs envisioned in 1944, the fuel pump drive adapter truly adds a margin of safety to the operation of the engine driven pump.
Unfortunately, the adapter was not produced in great quantities during WWII and is fairly scarce today. Figure 2 shows the fuel pump and the fuel pump drive adapter mounted on a Jacobs engine. Radial Engines, Ltd. has a limited quantity of NOS adapters in stock. The unit comes complete with the housing, coupling, bearing, & circlips assembled; and will include the drive shaft and both gaskets needed for installation.