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1978 Sears GT/18 Hydrodrive - Converted to Yanmar 3cyl Diesel

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#1 Hugh OFFLINE  



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Posted April 25, 2017 - 10:06 PM

By now, I am sure some of you have seen this thing, but since I haven't posted anything here yet, I figured I would put up a thread about it. The full build thread can be found on another site, one which I am slightly more active on, but nowhere near what I once was - due to school, work, etc etc. Videos can also be found on my YouTube channel, http://www.youtube.c...r/pacificdrumma


The base for the project is a 1978 GT/18 Hydrodrive. This machine (same as a Roper 18T) originally was powered by a B48M Onan, and had a Sunstrand hydrostatic transmission mated to a Dana GT120 axle. I had bought, sold, and bought back the tractor over a 3 year period, and it was a good worker. About a year before the swap started, the throttle cable had snapped, and out of laziness I neglected to fix it. So I ran the engine at a constant speed, 2/3 throttle or so, which was fine to operate the FEL I had on it at the time. Over time, running at a constant speed caused junk to build up inside the carb. So I installed the loader on another machine, and took the carb off of the GT to clean, rebuild, etc. 


And then the diesel stuff started. A local small engine guy I was friends with was preparing to move down south, so he invited me to come over to his place and see if there was anything that I was interested in. I wound up leaving with a 14hp Kubota D600, the engine found in Cub Cadet 782D tractors. For the price, I could not pass it up. I did not have any idea what to do with it, but I knew it would go in something. The D600 is a small engine, and it got me thinking I could perhaps shoe horn it into the GT. So I removed the Onan, and began to cut a rectangular hole in the frame to accommodate the automotive-style oil pan. I was able to get the engine to "kind of" fit, to the point where I deemed it a feasible swap. So I enlisted the help of my good friend Kyle (likely the best friend I will ever have) and we set about getting the Kubota engine running. Luckily, Kyle is a OEM-trained Cummins mechanic, so this little 3-cylinder was a walk in the park for him. We were able to get the engine running, but only on 2 cylinders. We determined there was a stuck injector, and that the injection pump needed work as well. A local fuel injection shop quoted me $80 to do the injector, and $450 to do the pump. Considering that my initial investment in the engine was $250, I did not want to nearly triple my investment that early on. 

Frame before cutting: 




First cut, I thought this would be it (lol): 




Kubota D600 sitting in frame:




So I returned to my small engine friend, and explained the situation. He told me he had a John Deere he would let go of for the right price, this time with a Yanmar. The story goes that he had acquired a 330 with intentions of taking the power steering, two spool valve, and split brakes from a 318 and making a 332 copycat from the two machines. Other projects prevented him from completing it, and I had actually sold him a nice one-owner 330 in the interim, so the 332 project fell by the wayside. We struck a deal that netted me the 16hp engine and radiator, power steering box, steering cylinder, front axle, and 2-spool hydraulic valve. I was instructed to sell the Kubota engine and use the proceeds to pay for the Deere bundle. Armed with a larger, more powerful engine that already ran, as well as a slew of power steering parts, it was full steam ahead. 


Donor "331" tractor:




The 16hp Yanmar is still a small engine, but is slightly larger than the Kubota was/is. We knew that in order to make everything fit, the frame would need to be stretched. We determined that 4" was an appropriate amount - it would give us the room we needed without totally losing proportional length and making it look absurd. So the frame was cut and stretched 4" on either side. Next, we had to make room for the larger oil pan, which meant cutting out yet more frame. Those of you familiar with what a Suburban frame looks like, the mule drive and steering cross members were cut out completely. We boxed in the front half of the frame wherever possible to reduce flexing. The engine is somewhat structural, but we definitely weakened the frame by cutting out as much as we did. 


Frame cut:




4" extensions added: 




Yanmar sitting in frame, no mounts: 




With the hole made in the frame to fit the engine, we next had to worry about mounting the engine itself. One thing we struggled with throughout the course of the build was that the Deere frame is about 16" wide and the Sears frame ranges from 10.5" to 11". This meant that we were truly cramming 10lbs of stuff into a 5lb sack. To mount the the engine, we harvested the factory Deere motor mounts. The front mounts were a "cut" and "paste" type operation, and we made mounts for the rears out of 1/4" plate. I joke, but the rear mounts would likely support a SBC if needed. 


More cuts to the frame in preparation of dropping the engine in: 




Engine in:




After mounting the engine, we mounted the front axle and setup the power steering cylinder. The Deere axle actually bolted into the Sears frame with little modification. The bore of the pivot on the Deere axle is slightly larger, so we did have to ream out the Sears frame, maybe about 1/8" or so. The axle has maybe 1/16" slop in it, but it is not terrible and will likely get a machine bearing in the future to tighten it up. Mounting the cylinder was very easy. Again, 1/4" plate. The early fabrication work had me extremely thankful for the quality welder, plasma cutter, and grinders available for use. 


Deere PS axle in:




Steering cylinder bracket:




Steering cylinder mounted:




One hurdle we had to overcome was the length of the steering shaft. On the Deere, the PS box sits much farther down in the frame, and consequently has a longer shaft than we needed for the Sears application. So using a cutoff wheel and the TIG, we took about 8" out of the steering shaft. The box mounts to the back side of the dash, and the steering wheel is comfortably placed in the operator's lap. Due to the caution we took when completing this part of the build, the shaft is not crooked, and operates fine. 


Steering shaft cut:




Shaft beveled:




Shaft held in place to weld:




Welded (fused technically): 




Box mounted to dash:




We had to cut more of the frame out to accommodate the plumbing for the steering box. As you can see, the frame is a skeleton of its former self at this stage.




Next came the task of figuring out the drivetrain. The reason why I initially decided to use the GT was because of its Sunstrand hydrostatic transmission. I was under the impression that I could remove the pump from the transmission and convert it to shaft drive, and run longer hoses/lines to the transmission. Like an older zero turn mower with separate pumps and wheel motors. We would run into issues down the road causing us to use an entirely different transmission. Nevertheless, we had to learn somehow. So out came the engine, and we flipped the tractor on its side - the "factory service position" as I refer to it - and removed the transmission.


The service position:




Once we had the transmission out, we determined that the engine and transmission spun opposite of each other, so I found a gear box that had counter rotating shafts and was power-rated. The gearbox created an issue of excessive length. With the lovejoy couplers and gearbox, there was not enough room to fit in the frame. We were committed to the frame being the length it was that point, I did not want the tractor to be any longer. So the lovejoy/gearbox method was out. Another issue we determined was that the factory transmission did not have power ports to run things such as power steering or hydraulics. More on this later. 


The pump: 




Pump mounted to gear box:




After scrapping the idea of the gearbox and lovejoys, we decided to pursue a mule drive of sorts to power the transmission. This meant putting the transmission back to its stock location, and preparing to run a belt to it. So we started with a 4" drive pulley from TSC, and built the mule drive. 


Pulley tacked onto a 4" metal disc we cut out:




Pulley installed: 




Around the same time, we fabbed up the battery tray, and installed the air filter. I kept the filter off of the Kubota. It is a heavy duty Donaldson unit complete with the little mushroom-shaped intake. By shoe horning it under the battery and in front of the engine, it prevented us from running it externally or having to fit it elsewhere.


Air cleaner installed:




Battery tray installed:




I also spent some time cleaning up the frame in anticipation of being boxed in:




I was able to get a GM power steering pump off a buddy of mine to use for mockup, so we started the installation of it. It would later turn out that it would not work due to limited belt contact area, and the belt slipping on it. 




The tractor was starting to take shape:




Fuel return added to the tank:




Electric lift pump added:




Fuel return complete:




Everything starting to come together:




Starting work on the exhaust:




Steering box support added:




Exhaust complete:




That's all for tonight. I will pick it up tomorrow and probably finish the story. Thanks for taking the time to read.







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#2 Cvans ONLINE  

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Posted April 25, 2017 - 10:50 PM

Boy you spit a mouth full there. Thanks and I enjoyed reading it. Looking forward to the next installment.  :thumbs:

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Posted April 26, 2017 - 07:03 AM

I followed you on youtube during the build, nice tractor.

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#4 ericjr16 OFFLINE  

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Posted April 26, 2017 - 07:26 AM

Hey Hugh! Glad to see the beast did well! :) You will figure out the cooling.

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Posted April 26, 2017 - 10:19 AM

Very nice custom piece.
Glad you got it roadworthy for PA Plow Day.
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