The 4-ton large truck Model KC, which had been continuously produced both during and after the war, was improved to a 4-ton Model BM with production commencing in March 1947.

Production of the B engine installed in large trucks had commenced when the Koromo Plant began operation in November 1938. The maximum output of this engine when installed in a 2-ton Model GB truck was 75 hp at 3,000 rpm. Subsequently, the engine was improved to 78 hp at 3,000 rpm in January 1940. The 78-hp B engine was also installed in the 4-ton Model KB truck developed in 1942 and the simplified Model KC (developed in 1943 while the war was going on). The output of the B engine installed in the Model BM truck launched in 1947 was 82 hp at 3,000 rpm.

As explained, based on Japan National Order No. 30 issued on September 13, 1940, Toyota Motor Co., Ltd. had continued to research and produce prototype engines as well as prototype vehicles using these engines even during the war. The order instructed Toyota Motor Co., Ltd. to develop an engine with 85 hp or larger output by improving the B engine, and led the company to create the D engine with 100-hp output by enlarging the bore from 84.1 millimeters to 92 millimeters in early 1944.

After the war, Toyota Motor Co., Ltd. continued development efforts based on this research and in December 1948 completed a prototype of the F engine by enlarging the bore of the B engine to 90 millimeters. The F engine was initially installed only in the Model BM truck to be exported to Brazil and other countries, but a Japanese model was added in 1950. Table 1-33 compares the specifications of the F and B engines when they were first developed.

The F engine incorporated the results of R&D conducted during the war, such as lightweight alloy pistons and an improved lubrication method, achieving efficient output increase. Subsequently, both the B and F engines continued to undergo design improvements whenever possible to improve performance.