Model A & B

Ford Garage

'Diamond' Engine Block & Date Code Markings

Pictured above is the diamond or lozenge shaped casting mark found on post-production era Ford Model A and B engines made in the mid to late 1930's.

The raised cast diamond mark is located on the right hand side, at the front of the valve cover, and above the timing gear side cover.


Typically there is also a raised casting imprint of the date tag on the right hand side of the block, below the valve cover. It is not an actual screwed-on tag. It is found on post-production blocks made in the mid to late 1930s.

The actual date tag was screwed to the foundry pattern. The apparent 'tag' on the block is merely a casting imprint formed in the sand mold by the date tag screwed to the pattern, and then transferred to the iron casting.

Month Day Year
A January 1 1 4 1934
B February 2 2 5 1935
C March 3 3 6 1936
etc. etc. etc.
L December 31 31 1 1941

The pattern date tag number is an indication of the casting date of the block. In the photo example above, the code D305 translates to April(D) 30(30), 1935(5).


Many diamond block castings were service blocks only, not complete tested engine assemblies, and thus they did not receive an engine serial number from Ford. In those cases, the as-cast pad was left blank as shown in the photo above.

Some Model A diamond blocks do however have factory-stamped Rouge serial numbers in the format of ☆A48xxxxx☆ (or ☆AA48xxxxx☆ if produced together with a 4-speed transmisson), indicating that they were produced and tested as a complete assembled engine at the Rouge.

The first use of Rouge-numbered Model A diamond blocks has not yet been determined. Examples of factory-numbered diamond blocks indicate production dates of mid 1935 and beyond with certainty.

The last Rouge-built and numbered Model A diamond blocks were made in 1941, and the last documented serial number was ☆A4872864☆ per the original engine production records at the Benson Ford Research Center.

Engine serial numbers were only assigned by Ford to completed engine assemblies after they had successfully passed the motor-dyno run-in (run-off) tests at the Rouge.

Engines were then numbered and released for production or service use. Untested engines, sub-assemblies, and components were not serial-numbered by Ford.

The as-cast serial number pad was left un-stamped by Ford on service blocks and incomplete and untested engines. Subsequently, dealers and rebuilders may have sometimes added their own markings, or transferred the existing numbers from the old engine being replaced.


Beginning in August 1932, complete Model A service block engines were all fitted at the Rouge with Model B camshafts and push rods (tappets), according to the Engine Production Foreman's records. This feature carried forward into diamond block production.

Another unique feature of many later diamond blocks is the presence of factory installed hardened steel valve seats on the exhaust valves.


Original documentation which explains specifically why the diamond symbol was added to the block, or what it signifies has not yet come to light. If found, it may indicate the casting supplier or location.

There is a common belief in the restoration community that these diamond block engines were not cast by Ford. The diamond block casting may have been cast somewhere other than the Rouge. The main Rouge production after 1933 was V8 engines, and four cylinder engines were low volume and primarily for service.

The actual locations of the casting and/or machining operations for diamond engine blocks is not certain, but machining of castings may have been transferred to Highland Park, where Ford performed many machining operations on cast and forged components in the 1930's.

The original engine production records from the Rouge (at the Benson Ford Research Center) show that these engines were definitely final-assembled and tested at the Rouge through-out the 1930's, though the actual locations of the block casting and machining operations are not clear.


More related information on Ford Garage:

  1. For more Model A & B related information, use the Site Search box at the top or bottom of this page.
  2. Model A & B US, British, & Canadian Engine Serial Numbers (VIN Decoder)      
  3. Model A & B German Ford Köln Engine Serial Numbers  
  4. Model A Assembly Plant Body Number Decoder    
  5. 1930 Model A Ford Assembly Branch Locations  
  6. Model A, B, & Early V8 Ford Patent Plate & Patent Numbers  
  7. Model A & AA US Body Style Numbers & Body Builders Chart  
  8. 1928-40 Model A & B Ford England A & B Powered Vehicle Models Chart  

August 2003