1972 convertible w/ date code of M4 on block vin 1z67k2s523283

[Gary A.]

I have a original 1972 convertible with a block casting 3970010 that has a M4 date code with one round head and one flat head screw attaching the mark. I have been told that this is rare but I am not sure. The communication has revolved around engineering testing a new process in production of blocks. Evidently they would have 25-50 blocks built with the letter M cast into the block where the casting date would go. This evidently designated the block as "special" which was being tested for "process improvement" or sampling. The number to the right of the M was to have signified what was done to the block for improvement. I can not find what the number represented. Discussions have revolved around "resin" testing. I have read that after 25-50 blocks were produced the Chemistry and Quality personnel would take half of them and cut them in half and even smaller pieces to verify the quality and consistency of the core dip and thickness of the core. Evidently The remaining were sent to the Flint Engine to be used on the line. The screws evidently represent shift work - if tag had 2 flat heads that meant 1st shift; one flat and one round meant 2nd shift; 2 round head screws meant 3rd shift.

i wanted to post this to see if anyone can confirm the authenticity of the "M" code and if it really is rare to find an original numbers matching car with the "M" code still around today.

thank you,
Gary Allen (61387)

[Michael W.]

Re: 1972 convertible w/ date code of M4 on block vin 1z67k2s523283

Gary,

The M code blocks have been very well researched and documented. They are not that rare. There was an excellent article in the Restorer some years back with the full story.

[Leonard M.]

Re: 1972 convertible w/ date code of M4 on block vin 1z67k2s523283

eng date.jpgeng cast.jpgeng stamp.jpgDSCN3107.jpg

[Gary A.]

Re: 1972 convertible w/ date code of M4 on block vin 1z67k2s523283

Thank you for your quick response I will try and locate that article

[Leonard M.]

Re: 1972 convertible w/ date code of M4 on block vin 1z67k2s523283

Hi Gary, Reading all the articles show the reason for the M code blocks, but no idea on how many were made, were not cut up or were sent to assembly and now, how many still exist. I don't think the numbers mean that there were consecutive numbers starting from 1 onward; as in our case. Meaning, that there Has to be pairs of 1-2, 6-7, 8-9, 12-3 etc. made for the 1972 model year. Or, Consider that your car is 23283 with a M4 and I have car 10569 with a M11. My block was probably cast in 1971 so was the M10-11 pair the last 1971 castings and your car made in May, 1972 (engines cast in 1972) had M4 making it toward the end of 1972's tests??? Or, were some blocks laying around the foundry and sent for assembly long after being cast?
Funny how the M code carried a lot of importance at one time and now it doesn't matter. I guess the most important thing for us is to have a reasonable explanation for a buyer if we were selling the car. I would say M blocks are rare, but more of an oddity than a value related item. I probably would rather have a L cast date code for my Jan. car rather than trying to explain what a M code is. Thanks, Lenny