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I thought I would do a quick, but thorough, right up on my recent engine swap on my 520-H. The Onan has served me well for the past three year with the exception of the manifold leaking and having to run it at half choke to smooth out the idle. Currently this machine is used for my snowblower in the winter and loader in the summer. Knowing that the Onan had life left, but not wanting to risk catastrophic failure on a necessary piece of equipment I decided to look for a replacement. With shear luck I found a brand new 2001 Kohler 25hp on Craigslist for $450 and wasted no time getting it. The Onan will have a new home on a second 520 I was given in desperate need of attention, making it a good candidate for a restomod tower/ hood swap donated from a black hood model. The swap isn't to terrible but more time consuming than I had originally thought. The worst of which was a wiring gremlin that I created running a wire to an accessory power source instead of a full time one. The engine is wider than an Onan making placement a challenge. An inch froward of where the engine is currently bolted you can see two holes. The one to the left side of the tractor is a factory whole that I used for the first round placement of the motor. This left the engine to far forward for the clutch engagement to work properly, but was a good medium for the belt alignment. After moving the wholes back a little over an inch everything worked out great, with exception of the belt guard which I have spaced out with washers for the time being. Next came the wiring. With the diagrams I was able to find I discovered that the Kohler came with a similar setup to the Onan. This includes the oil pressure switch, starter solenoid, a place for the tach. wire. The Kohler has a fuel shut off solenoid that needs constant 12v while the engine is running and is tied into the starter solenoid. On the back of the engine plug there is a post with a red and a blue wire, the red one I connected to the yellow ( coil 12v ) wire and the blue one to the light blue ( starter solenoid ) wire. The fuel shut off is currently my only engine kill as the 520 key switch does not have a post that provides grounding in the off position. Here is the Basic layout: Tractor Kohler CH25S Oil Pressure - Dark Blue Oil Pressure - Green Starter Solenoid - Light Blue Starter Solenoid - Blue Tachometer - Gray Rectifier - AC Post Coil Positive - Yellow Carburator Solenoid - Red - formally connectedt to blue Battery + - Red Battery + - Heavy Red Ground - Black Ground - Black Cylinder Temperature - White Temperature Sensor Kill Switch - White The cylinder temperature probe needs a whole with 1/4 - 20 threads but the Kohler uses fine thread 1/4 so I had to tap one of the throttle bracket bolt wholes for it. this was the best location for a good reading while keeping it away from the exhaust port. The throttle cables worked good, just had to shorten the choke a bit. The oil drain from the Onan was the perfect fit for the Kohler also. As for the pto I needed to bend the top bracket to allow the muffler to line up properly. Last to be modded was the hood. Luckily the hood just clears the filter cover, but I had to notch for the muffler. If anyone has any questions, don't hesitate to ask.
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