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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.

Additional Observation: Notice the significant mismatch of concentricity between the cast tappet boss, and the actual machined tappet bore!



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 found on post-production blocks made in the mid through late 1930s, until 1941.

It is not an actual screwed-on tag. The actual date tag was screwed to the foundry mold pattern. The apparent 'tag' appearing on the block is merely a casting imprint formed in the sand mold by the date tag screwed to the pattern, and then formed in the subsequent 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 earlier example photo above, the code D305 translates to April 30, 1935.

That is to say, (D)=April, (30)=30, and (5)=1935.

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 heavier truck clutch). The serial number was assigned after production and testing 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 to the pad, or transferred the existing numbers from a previous 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 such evidence is found, it may indicate the casting location, facility, or supplier of the raw un-machined casting.

The main Rouge foundry production after 1933 was for V8 engines, and four cylinder engines were low volume and primarily for service.

There is a common belief in the Model A and B restoration community that these diamond block castings were not cast by Ford, at least not at the Rouge. It is plausible that the raw castings were outsourced, and that the diamond block casting may have been cast elsewhere, or by and outside supplier.

The actual suppliers and locations of the casting and/or various machining operations for Model A and B diamond engine blocks is not certain, but machining of castings may have been transferred from the Rouge to Highland Park. Ford utilized the former Model T factory at Highland Park to perform many machining operations on cast and forged components throughout the 1930's.

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

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