Lawnmower repair (09/05/13)

Our lawnmower (Tecumseh TVXL840) is an ancient machine (maybe over 25 years old) but still goes great - starts first time, every time. It's already had the skirt replaced with a piece of washing machine paneling. However, last year, the spark plug blew out, taking the threads in the cylinder head with it (the thread had worn over the years with all the removing/retightening). After a while, I decided to have a go at repairing the thread and also making another skirt since the washing machine paneling had rusted through again.

Thread repair

After lots of disassembly, we got the bare cylinder+head casting free of the rest of the engine. My original plan was to mount it on the faceplate and bore it out - however it was slightly too large. Plan B - mount it on the lathe carriage instead and use a boring tool in the chuck.

So, after a bit of swearing, I managed to get it packed up to the right height and centered with the lathe spindle. To help with alignment, I turned a couple of discs of wood on a bit of threaded rod held between centers so that the discs were a neat fit inside the cylinder bore. Then, with the casting supported by the wooden discs, I was able to find the right amount of packing to bring it into alignment. The boring bar was held in the 4-jaw chuck (I don't have a proper boring head). A few light passes were enough to clean up the bore and make it large enough for the thread repair tap. After threading, I installed the helicoil thread-repair insert, after cutting it to length, and the spark plug fit beautifully!

New skirt and final assembly

I decided to make a new skirt from some 0.9mm stainless sheet so it doesn't rust like the old one did. The plasma torch with a pivot arm made short work of cutting a disc out. For the sides, I cut three strips of sheet, welded them end-to-end, then proceeded to tack-weld the long strip to the edge of the disc, coiling it around as I went. Worked pretty well. I didn't use any shielding gas behind the weld, so it looks pretty crumbly on the inside, but it is attached.

After reassembly, the engine started first time - not bad for something that hadn't run for over a year and had just been stripped to pieces! The throttle lever was also replaced since it was getting a bit crunchy. The engine still isn't too happy about stopping (probably needs some adjustment in the travel of the throttle cable), but yanking the spark plug lead off does the trick!