I think we all have been fed the story once or twice that Tecumseh had no engine records and thus there is no way to accurately date a Tecumseh engine. I have been asked to give the year on a Tecumseh dozens of times. I had to answer the question again tonight and realized that I did not have the information that I do have written in a nice way. I decided to go ahead and make it easy to follow it in hopes of being able to help some folks out.
First of all, these instructions only apply to engines using a five digit serial number code. That would be a code with four numbers followed by a one letter suffix.
If it is an early engine (and I do not know when the cutoff is) it will have a long serial number, up to 8 or 9 digits. These instructions do not apply. That style would have been certainly used in the early days of the Lauson/Power Products swap to Tecumseh, up until at least the mid 1960's. There is no hidden code there. What ever we know would have to go by factory records that may or may not exist.
If it is a post 2004 engine, the instructions will be in the next post on this thread.
We are going to find out the age of an engine using a five digit serial number code. Lets take a Tecumseh engine I have sitting on a tractor out back. It is a OH160-160039B 9289E.
We know that the OH160 is the basic model family, and 160039B is the spec number that I always need to find specific parts like carburetors, crankshafts, and similar things.
The serial number is 9289E.
The first digit is the last digit of the year of manufacture. That would be 9 in my example. That could mean 1969, 1979, 1989 or (possibly depending on when Tecumseh officially dropped this style of code) 1999. To go farther, you have to have some general knowledge of the Tecumseh engine line and model years to know the decade. If you know about when the tractor was made and know that the engine is right for the tractor, you can probably narrow that down easily. On my engine, it is a replacement on a Massey Ferguson so I'm almost out of luck. But, since I can estimate fairly accurately going by the decal style and carburetor style, I am going to take a wild guess at my OH160's year as being a 1979.
The next three numbers is the calender date of the year. Since 1979 is not a leap year, date #289 is October 16th.
That means my engine was made on October 16th, 1979.
The letter suffix according to Tecumseh will tell you what line and shift the engine was made on. However, from what an old factory supervisor told me that letter was the production number for that day. The assembly line was small engine that they would never make more than 26 engines of a certain model and spec in a day. Assuming that is true, that means my OH160 was the 5th engine made on October 16th, 1979.