I am building Rudy K's Sterling powered tractor. Rudy specifies the rear axle to be 6-1/4" long. Actually should be 7-1/4" long.
I made the axle 6-1/4" at first and found that it was too short when installing it . I then added up each part width and came up with 7-1/4"
Wheels are 1-1/8" wide. bull gears are .438 wide, spacers are .562 wide. 2 wheels two gears and two spacers come to 7.24"
This is the only error I have found after building several of his models.
mike