The engine that inspired this project is the Yanmar L40AE-D seen below. It is a 57 pound 4 HP single cylinder air cooled diesel engine which was made in Japan. It is a precise piece of machinery that cost a pretty penny new (I mean a LOT of money compared to a gasoline engine). In fact, these are hard to find in good condition and I am lucky to own one. These engines can last well over 5,000 hours if taken care of !!!
       The Yanmar has quite a bit less power than the Tecumseh although I won’t be spinning a 38” mower deck so a 10 horse isn’t needed.

       The LA40 series engine is a 199cc displacement engine and is one of the  smallest of the “L” series engines that Yanmar made (there was an L35 which was a 3.5 HP model FYI). I believe this one was made in the mid 80’s and was used very little during it’s use as a pump engine. Yanmar has a unique history and was the first engine manufacturer to master the small diesel engine. You can read up on their history here. It’s a very interesting read !!!

       To clear way for the Yanmar I needed to cut the fuel tank support from the tractor’s chassis/frame. This will make room for the engine and the starter generator that I will add later in the build.

       Sometimes a “Sawzall” is the best tool for the job !

       The engine fits nicely in it’s new home.

       The stock exhaust will not fit with the starter generator so I needed to figure out an alternative. I started out by making the flange that mounts to the engine out of 304 stainless as seen below.

       I cut a new gasket and test fit the flange. I will work on this more later...

       During my mockup of the engine mounting location I dropped the grille housing. Ouch!!! It cracked on the side :o(

       I broke out the TIG welder and blobbed on some aluminum to make a repair. Boy, I’m not graceful with welding aluminum :0/

       A little grinding and it looks good as new !!!

       The engines motor mount will sit farther forward than the Tecumseh so I had to modify the grille casting with new mounting hole locations.

       I welded on two tabs to support the grill farther forward to make room for the engine (the grille itself will stay in the same place as stock).

       You can see the original mounting slots below and where the new hardware is located...

       And now for the forward support bars and hood for a test fit.

       The engine fits under the hood like a charm !!!

       The next step was to build the pulley system. I need two pulleys; one to drive the transmission and one to run the starter generator, For this I used a 7” mower deck pulley (in black - $10) and a 3.75” cast iron electric motor pulley (in gray - $54)...

       The gray motor pulley/sheave has a wedge style hub that securely mounts it to the shaft. They are kind of expensive but work really well for this application...

       I needed to join the two pulleys together so I started machining a large hole in the hub of the mower deck pulley.

       To secure the 7” pulley to the cast one I needed to cut a shoulder to center the hub on as seen below...

       I also cut the back side flat to accept the hardware for the 7” pulley (the second pulley groove will not be used for a belt).

       The next step was to drill the mounting holes into both pulleys. For this I used my digital readout (DRO) on my mill to locate and drill the holes...

       After finding the center of the bore I plugged in the measurements for six holes 60° apart.

       I used a spot drill to “start” all six holes (not shown) and then chased them with a 1/4” bit.

       I haven’t had the chance to use my DRO that much but in this case it was really useful.

       The new holes are perfect and can be indexed in any position with precise accuracy !!!

       I use some 1/4 - 20 stainless hardware to finish the job.

       All I have to do now is mount it for a test spin !!

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