"M" Code Engine Block
 

I've been visiting Corvette Forum for a while now, but this is my first time posting. I recently purchased a 1970 LT1 convertible advertized as "numbers matching", and have been attempting to decode the various engine components.

Everything checks out so far for both numbers and dates (carburator, intake and exhaust manifolds, cylinder heads, alternator, distributor, radiator, transmission, rear, etc.), but when I got to the engine block I got confused.

The VIN is 194670S402856 reflecting a February 1970 build date, the pad reads (on the left) 70S402856 and (on the right) V0I20CTU and the door certification label shows a 2-70 build date, but the block casting date shows "M 108".

All of the reference materials that I have state that the first letter of the casting date is a reference to the casting month (A=January...L=December), the second and third numbers are the day of the month, and the last number is the year. Nowhere is an M listed as the first letter of the casting date. After further research I found a camero site that referenced 3970010 "M" code engine blocks. It said,

""M"-Coded Blocks: There is one exception to the normal block casting date convention that has been observed: some blocks have been seen with "M" cast codes. Research by the Saginaw Metal Casting Operations of GM Powertrain Division has found that these "M" codes are not date codes, but actually a foundry code used to indentify parts that were made to evaluate a casting process change. After evaluating the parts, the blocks would either be destroyed or released into production. There is no way to determine the date an "M"-coded block was poured."

Does anyone have any information concerning the above? Any help would be appreciated.

Thanks

This is a correct explanation ... that is also the highest "M"number I've heard of ... see more here:
http://www.ncrs.org/forums/showthrea...ginaw&uid=4876

(if you're a ncrs member, you can see the attachments ...)

Also, there is a story in the ncrs corvette restorer by Gene Gamache, volume 27, #4, Spring 2001 titled "M was for mystery".

If you can't access the ncrs attachments or find the restorer article, I can mail you a copy if you'll PM your home address. You will need all this information should you ever decide to have your car judged. Also, if you have pictures of your Mcode and the front pad, can you post them here??

Frank, thanks for the additional information. I had planned to join NCRS weeks ago but never got around to it. I joined this morning but won't have my membership number for about a week. Once I receive it I will check out the article and the picture attachments to the post that you referred to.

I tried numerous ways to post the pictures, without success. I setup an album on this forum and added the pictures. I think you can access the pictures if you click on my public profile. Also, although it doesn't show in the picture of the casting date, there is a "1" about an inch to the left of the "M" in the date. Lastly, if you look closely at the casting date, there appears to be a trace of another set of letters and numbers under the "M 108".

Thanks.

For some parts the letter I isn't used, which would put M at December in this case. I don't have my books handy, so I can't verify this at the moment...but this might explain the M. :cheers:

Thanks for your interest, Frogday. I did check the NCRS Technical Manual, as well as other sources both in book form and on the internet, and all state that the Letter "I" was used in block casting date codes.
The letters used were from "A" for January to "L" for December.

Thanks

The "1" refers to the conveyer this block was on. As for other numbers/letters under the M108, I think, once you clean off that paint and goo, you'll see a tag that looks like this ...
http://i83.photobucket.com/albums/j3...ngineM39-1.jpg

PS ... the round head and flat head screws indicate "2nd shift"

Thanks again, Frank, and yeah it is pretty grimy down there. And that's the area of the engine that I tried to clean so that I could read the date. Over time I will be restoring the car so, hopefully, I'll eventually be able to get some good pictures of the tag.

Earlier in the thread you mentioned certification. Do you forsee any problems with Flight certification due to the "M" code block casting date? And are "M" code engine blocks unusual in corvettes?

Thanks

Yes, from my experience, I would say the "Mcode" is unusual in any car, let alone a corvette. Your "3970010 M108" is the only 3970010 M108 in existence.

As for certification, that would depend on the wisdom and knowledge of the judge(s) on the field that day ...

I also have an "M" engine. In fact according to the book called "Chevrolet By The Numbers" 1970-75, by Alan L. Colvin, page 73 right hand column towards the top of the page. It states " ....Some #3970010 blocks have been verified with a December "M" month letter code. It is unknown why this was done."

There are many anomolies where the M is used instead of the traditional "L" for December in various GM parts date stamping. It's not limited to just engine blocks.

NCRS does not certify cars, just so that you are aware.

If you do go through Flight Judging, have a copy of Gene Gamache's article with you in case the judges have not read it.

I owned an October built L46 coupe with an "M" date code engine as well. I was told - as also confirmed by the NCRS literature, that the "M" coded castings were coded as such to designate them as experimental batches where the foundry was trying different casting techniques, sands, etc. If the castings came out good, they sent them onto the line. If they had problems, they recycled them. Your "M" code was one that made it thru QC. :thumbs:

Great Post. I have a 010 block with M84. With the M being described above, what does the 84 stand for?

Thanks

Per GM explanation of the Mcode ...
"The M tag was used for denoting samples being made for anything from:
*running a new set of cope & drag patterns for customer approval.
*running a new core box for approval parts.
*running parts with different sand chemistry, mold pressures, core coatings, iron temperatures, etc.
*engineering changes."

There were 2 tags per sample run, so your block would have been paired with M83. Unfortunately, there is no information available to describe just what "exactly" was intended in each run. Whatever it was though, it was successful, as your block would have been scrapped if it were not.

I'm glad it was successful, its in my car. lol :cheers:

Is there any other way to date the casting? My car is a 72.

The NCSR link mentioned going to a X method. When was this done?

according to GM ...
"in later years, the foundry turned to the "X" tag, with 3 digits denoting day of month and year."

I understand "later years" to mean sometime around the mid 1970's, but I don't know that as a fact.

As for dating your block, nothing definitive. Most likely, it's a date within the 30 days prior to the engine build date on the pad.

Speacial M code block
 

I think this is the same type 350 that was in my 73 when I purchased. The motor Runs pretty good unfortunatly the M cast on the block was followed by the letters exico ? further cleaning revealed made in M exico? THanks I thought this might shed some light on the mysterious M code?

Two different M's. The "M" code discussed in the previous posts is in the casting date code location on the passenger side of the rear apron on Saginaw-cast blocks.

The "M" you noted (part of "Hecho en Mexico") is on service replacement blocks (and Goodwrench and Targetmaster engines and crate engines) cast in Toluca, Mexico. Toluca is the only remaining GM engine plant that has foundry, machining, and assembly tooling for Gen I small-blocks. :thumbs: