Can't seem to find my paper copy reference materials and internet searches are coming back lacking. Probably my wording on the search I'm guessing. So I pose this question to the collective gurus,what's the 6 after the M4 code signify? Slightly different size and font than the EUN and code stamping. M4 is readily identifiable but I'm assuming the 6 is either a trans upgrade or added option designation. Or it just a case of being added or mistake for unknown reasons? Should be the last block related question I'll have for a while. Next in line will be crankshaft questions (missing digits and D proceeding partial part # as one example) but I'm trying to pace myself.
the 6 digit number is the design drawing that the casting was made to. I recon the 6 is an inspection stamp, independent of the M4, which is the specific rating the engine was assembled to. M4 is a 1960 or 61 389 tri-power with 3-speed manual trans rated at 348HP. What are the date codes near the distributor hole? More info here: Engine Codes – Pontiac Power Net
Thanks for the info. The date on the block is D 28 0. I had looked at Pontiac Power and Wallace Racing and found the M4 code but didn't see anything refencing what looked like the hand stamped digit following it. I had originally thought it was an A4 with 6 until I cleaned it up a bit. That seemed odd as that denotes a lower hp engine and this block is a 4 bolt. As shallow as these stamps are I'm a bit leery to do anything else to abrasively remove the rust and crud.
What you have is a very rare block. It's a 4 bolt 389 tri power engine also known as a 425A. The 6 is factory done and indicates it was a 4 speed manual transmission application. Only 722 cars were built in 1960 with the manual 4 speed, I have one of these blocks myself.
RT468 do you have a pic of any numbers like these on the front of the block? They’ll be below the ones you posted a pic of.
The Jalopy Journal
