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I've never searched for an old GMC engine number before. It looks like the first three digits designate the engine as a 270. I believe the second three digits are a serial number. I believe I 4 I would be where a date code is found, and that the format should follow the same format as the casting date codes. If so, that the second I should be a number.

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Lastly, here is another helpful bit of information on reading date codes from Andy Siechen, our GMC Engine Technical Advisor: Several people have asked here and through my email about how to tell what year an engine really is. Here goes: I attached a picture of what is used to determine the casting date of the GMC engine blocks and heads.

The first digit is always a letter of the alphabet A-L. A=January, B=Febuary, C=March etc, etc all the way to L=December. The next digit is a number 1-31 which is the day of the month. The last digit is always a single number and represents the year of the casting.

So unless the I is a 1, which would indicate a '51, we have a genuine mystery.

My suggestion is to look for the casting dates on the block and head using the following reference:

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The date code casting for the block is usually on the same side if the block as the casting number. The cylinder head location is usually outside the valve cover near the center. Some of the later 270 and 302 heads had all the casting numbers and date castings under the valve cover down near where the pushrods come through. The raised area with the date almost looks like a small plate attached with two screws and they are often mistaken for something that can be removed. They actually are part of the casting.

Good luck in the search.