My Sears Craftsman Self-propelled Mower Won't Propel Itself!

My Sears Craftsman lawnmower had been losing its "oomph" as a self-propelled device during the past year.  One round with it this spring and I knew it was time for some repairs.  I started by looking into the drive mechanism located under the large plastic cover on the front of the mower.


I had expected to find a slipping belt, worn pinch-roller or other item possibly needing adjustment.  Instead, everything under the hood looked OK.  It was just that the wheels seemed to not engage with the drive mechanism.  I pulled off a front "driving wheel" and took a look.  This picture shows a new wheel on the left and my worn one on the right

 

And here is a closer look at the worn teeth: 


The gear teeth on the plastic wheel were worn down in some places on one side and totally worn off on the other.  Here is the driving gear that chops up the plastic:


O.K.  whose bright idea was it to make the driven wheels out of plastic?  I can just imagine the discussion between some mechanical engineer and a cost accountant, "Sure it will work for a while, but then ..."

Obviously the engineer lost that argument.

I drove over to the local Sears Repair and Parts location.  (Eden Prairie Center if you live in the Twin Cities area.)  With a wheel in-hand, I told the clerk that I needed two replacements.  Here's the best part:  I didn't even have to show them the part number!  "It's one of our most common replacement parts," said the other clerk as she rang them up.

Well, duh!