SMALL BLOCK I.D. BOSS
Decoding the I.D. boss on small blocks is difficult simply because not all engine production facilities used the same format when stamping the information. The types of information found on the pad includes engine displacement, a year code, a plant code, a production date, special notes about the engine, and a production sequence number. Unfortunately, not all the assembly plants included every bit of this information on the pad, and those that did inevitably changed the order in which it was listed. Confused yet? Don't worry, by the end of this you will be...
One piece of information that always appears is the engine displacement. This can only be one of four possible numbers: 273 (or 27), 318, 340, 360. Once the engine displacement is found, we can then try to decode the information around it.
To the left of the engine displacement there may be one or two characters. This (or these) characters can denote the production year only (in which case it would be a single alpha or numeric character), the assembly plant only or both, with the year usually preceding the plant code (which would be one digit and one alpha character, or two alpha characters).
If the character to the left of the displacement is a single digit, it represents the last number of the model year. If the single character to the left of the displacement is a letter, then a little more detective work is required since the letter could be year code or an assembly plant code. To determine which it is, check the casting date on the side of the block to determine what year it was cast, and compare this date to the Year Codes chart. If the year code letter matches the letter to the left of the displacement, then it's a year code. If it doesn't match, it's an assembly plant code.
Say, for example, you have a block cast in 1968 according to the casting date. Looking up 1968 in the Year Code chart you find "D" as the code. But the letter to the left of the displacement on the I.D. pad is an "M". This tells you it's not a year code, so it must be an assembly plant code, and "M" decodes as the Mound Road assembly plant. An example of two characters would be "4W", which decodes as a 1974 assembly date built at the Windsor, Ontario plant. As a side note, Chrysler never used the letter "I" when coding years, if a character that resembles this is before the engine displacement it would be a "one" making it a 1971 engine. See the list below for the assembly plants and their codes, these are the typical listings, but because of production inconsistencies some other codes may have been used.
Okay, if you thought the left side of the displacement number was tough, you ain't seen nothin' yet. To the right of the displacement number, there may be a letter or a symbol. This is "secondary" or "additional" assembly information. For example, if you see an "R" to the immediate right of the displacement number, as in "318R" it means this engine runs on regular fuel. If no letter is present to the immediate right, it means the assembly plant either failed to stamp it, no special notes need to be taken into consideration, or this information is placed elsewhere on the engine. For example, undersize crank information may be stamped on the #8 crank counterweight, or an oversize lifter bore may be stamped on the tappet boss. Like we said at the beginning of this section, there is very little in the way of formal guidelines on small blocks. See the chart under the big-block I.D. boss explanation for a guide to symbols, letters and their meaning.
Further to the right there may be one or two more sets of numbers. These could be the build-date of the engine, or the build-sequence number (which is simply what number this particular engine is in a "build cycle" for a particular plant), or both. The build-date can appear in two fashions, as a "10,000 day" date (ex: 2637) or as a standard month/day sequence (ex: 10 16 or 10-16). A "10,000-day calender" is comprised of, you guessed it, that many days, giving each day a unique number (which is in about a 28-year span). As a point of reference, January 1st, 1967 was the 1,983rd day on Chrysler's 10,000-day calender. With a calculator and a calender you should be able to convert your code to a standard date. Don't forget leap years ('64, '68, '72).
If you do find two sets of numbers, keep in mind that, usually, the build-date will appear before the build-sequence -- unless a second line of stampings is used (which isn't uncommon). If there is a second line, the date is almost always on it, which would make the number on the upper line the build sequence number. (All this makes that stuff to the left of the displacement number seem simple, right?) But if only one number is present, and there is no second line, then the number could be either, and you have to put your detective hat on again. Compare the casting dates to the numbers you've found to determine what the number is you're looking at.
Probably all we've done to this point is thoroughly confuse you. If so, don't feel bad, we're not sure about any of this either! We do recommend Galen Govier's book (part # GGB1), which lists all the assembly plants, codes, symbols, hieroglyphics, etc. Good luck and good hunting.
