"The Lawn Mower Repair Man's Front Page"
Why are they important? They tell the story of your engine. When you ask a question or want to get parts, they tell the person you need assistance from what you have.
Every manufacturer uses some sort of identifying number/letter system and they are all different while doing the same thing. Briggs being the most common I'll use their system here.
Briggs has a three set, number/letter system.
This will be a 5 or 6 digit sequence, depending on engine cid. If the engine is 9 cid or less it will be 5 digit, 10 cid or more and it will be 6 digit. The cid will be the first number(s) in the model. I'll use a random set of numbers throughout. We'll use 92902 1234-01 90012201. Since this is only a five digit model number, only the first number is used to determine the cid. This is a 9 cubic inch engine.
The next number/letter is the basic design series. Usd to always be a number only but letters are now being used, particularly with some of the OHV engines. Briggs doesn't list the particular meaning of this design series but in general it relates to changes over the years. So loosely speaking, the "2" in this model means it has had a 2nd major design change from the original incarnation.
The next digit, a 9 in this case, says this is a vertical crankshaft engine with a pulsa-jet carb. This means the engine is probably made for a rotary lawn mower and has a diaphragm carb. You can't swap this engine to a go kart and you don't want carb parts for an engine with a float carb.
The next digit is "0". This indicates a basic engine, no gear reduction, no auxillary pto, and the crank rides on plain bushing bearings.
The last digit is "2". This indicates a rewind, recoil starter. Other numbers here would indicate 12 volt electric start, with or without onboard alternater, wind up rope start or 120 volt electric start and others.
This is a 4 digit, dash, and 2 digit. This tells what equipment manfacturer the engine was made for and the options that manufacturer had built into the engine that is not indicated in the model number. This can be as useless as the color of paint or as important as crankshaft sizing for the application. This model engine probably has in excess of 20 crankshafts that could be installed in it and without the type number, it's a time consuming task of figuring it out.
The code tells when and where the engine was made. 9001201. The 90 is the year, 1990. Third and fourth digit, 01, is the month=January. Fifth and sixth number, 22=day of the month. Last two digits, 01, is the assembly line.
This number allows two things. If a production problem developes in an abnormal number of engines, Briggs can trace back to the exact assembly line and the time frame that problem happened. This helps them to prevent a re-occurance and enables them to issue service bulletins for or if needed, recalls for engines within the problem period.
The second thing it does is to identify parts changes that didn't justify a major change designation in the before mentioned model number design change number. In the early 80's point ignitions were upgraded to electronic. Early to mid 90's, pistons and rings were changed to improve (reduce) oil consumption. Lots of minor changes over the years. Some changes are OK on older engines, some are not. The date code makes sure the right part is used.
Well, that about covers the basics. Tecumseh only uses a 2 number set but in general does the same thing. They use just a model and serial number with the long serial number covering what Briggs use it's "Type and Code" numbers to do. Newer Tecumsehs also use a D.O.M. (date of manufacture) number to free up the serial number for identifying purposes. If your Tecumseh is one of the newer units be sure to use this D.O.M. number as well.
What I hope to have done here is to show the importance of these numbers when you have a problem and ask a question here, at the shop or when buying parts. Numbers do mean things.