AC HD21 dozers

There were 7 different series of Allis-Chalmers HD-21 crawlers from 1954 thru 1975. The 1954-56 HD-21AC had a 6 cyl supercharged engine w/ 204 fwhp; the same model for 1956-58 had a 6 cyl turbocharged engine w/ 225 fwhp as did the HD-21A from 1958 to 1961. In 1963-66 there was the HD-21A/P with either a torque converter or powershift (all the previous ones I mentioned had only the torque converter configuration). The HD-21A/P had a 6 cyl turbocharged engine w/ rated fwhp of 256 w/ torque converter or 274 fwhp w/ power shift. The 1967 HD-21P had a 6 cyl turbocharged engine w/ powershift only and it had 268 fwhp. The last one in the line was the 1969-75 HD-21B with a 6 cyl turbocharged engine, powershift, and 273 fwhp. The Fiat-Allis 21-C followed this in 1976-81 with a 6 cyl turbo engine, powershift and 300 fwhp. The track gauge of all the above tractors was 84", the overall length was 106.6" for the early HD-21AC, and all the others exc. the HD-21B (119") were 120.1"; The shipping weights went from 44,000 (1954) to 57,100 (1969). Hope this info will be of some help.