Air-cooled engines are easier to find for a few dollars (so to speak) than those specially prepared for R/C boats. The pics bellows show how to make a water-cooled engine from a stock air-cooled Stihl 62cc saw. We do not recommend such engines because they run to the wrong side: clockwise.

Starting disassembling the stock engine. Look the huge quantity of parts that will not be necessary at the final assembly.



Same engine, already disassembled.



The flywheel, besides working with the coil to make a spark is also responsible for air cooling the engine. So, it has fins to move air. Water-cooled, those fins only make drag.



Taking the fins out on a lathe. In other engines the task is quickly: we use a metal saw and pliers and sand the flywheel. In this engine, considering the fins thickness we choose the lathe, to prevent any damage to the engine.



The flywheel, with the fins after the lathe.



The cylinder, after part of the fins is sew up, also goes to the lathe.



The best and easier way of fixing the cylinder to the lathe is trough this part: made from wood, it does not scratch the cylinder and is tough enough for the job.

The cylinder, turning in the lathe.



The cylinder, ready for the water jacket.



A brass tube at the lathe: it makes a fine water jacket.



The jacket, almost ready. This engine has a slanted plug - another complication. Engines with the plug at the top are far easier to work on.



The jacket already closed - preparation for the plug.



Opening the plug hole. Its a critical work - the hole must be perfectly in line.



The water jacket ready, assembled on the cylinder and the carb manifold. On the original engine, the carb is fixed to a discarded part and connected to the intake trought a rubber joint, so we had do make the manifold. Another dificult on working with a chain saw engine.



The carb pump was also feed trought a pulse opening in another discarded part. Tapping the carb was the solution.



A complete engine view. Still (no pump intended with the engine brand) too bulky, by owner's decision, who intends make use a lot of the non discarded parts in his project, including the oil pump that was use to oil the saw.



Carburetor and manifold detail



Looking for this side, it comes clear the reason for the manifold lenght - otherwise the carb could touch the pull starter.



At least, it will be easy to access the carb, when necessary



Job done, the engine was sent to his owner. He sent us some pics, showing how is going the works. The drive train must be able to run the boat backwards, when necessary.



The complete drive train disassembled



All the parts together. Looks like he will be able to get what he wants to.