Hi Michael
  The easiest way to identify the tractor model you have is to locate the governor housing and look at the serial number tag. It will have the tractor model and serial number on it. 
   If the tag is missing all 18-36 tractors were made with a factory bore size of 6 3/4 inches. Oversize pistons were avaliable. 18-36 tractors were made in three series. G, H, and I. These tractors have 2 cylinder engines.
   The 12-24 tractors were made in three series. E,G, and H. The G and H series have a 5 3/4 inch bore. The E series has a 5 1/2 inch bore. All 12-24 tractors came from the factory with a two cylinder engine.
    All 12-24 and 18-36 tractors have enclosed final drive gears inside the rear wheels.
    If the tractor has a two cylinder engine with a 5 1/4 or 5 1/2 inch bore and exposed final drive gears it is a 10-20.
  22-40 and 28-50 tractors have a 4 cylinder engine.
   For more information and a serial number list go to <!-- m --><a class="postlink" href="http://www.geocities.com/hartparrtractors/">http://www.geocities.com/hartparrtractors/</a><!-- m -->
                                                                 Hoov