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Yes, no, or maybe. A lot depends upon what brand and what model engine you are attempting to convert. If the engine has a pressure-type lubrication system, go find something else to do! I used to teach some 9th grade Industrial Arts power mechanics and metalworking classes. It wasn't uncommon for a student to be enrolled in both classes in the same semester, and to decide to do some extensive modifications on a small engine. It's relatively easy to convert the little 3-4 hp Briggs & Stratton engines made in the 1960's through the 1980's from vertical to horizontal. These engines had a splash lubrication system, and sometimes the only lubrication modification needed was to add the little "oil slinger" for a horizontal engine to the end of the connecting rod. Leave the little rotary "oil slinger" from the vertical shaft engine in place. It won't hurt a thing. Sometimes it is necessary to file or grind a small notch on the inside of the main bearing boss to allow oil to enter from the top. You may decide to do this to the cam bearings also, but I've seen students NOT do it, and end up with an engine that lived anyway! Some engines have a little drain hole drilled in the bearing area to drain oil away from the back of the seal which prevents leakage into the case holding the breaker points. If you choose to not put a new hole in there on what is to become the bottom surface of that area, you'd better hope the crankshaft seal is a good one! The fuel system is a different matter. The tank and carburetor for a 3.5 Briggs can often be turned 90 degrees and re-mounted on the intake tube. It will be necessary to bash the lower part of the gas tank with a hammer a couple of times to let it settle in against the flywheel shroud enough to line up properly. Apply some perforated plumber's strap or a small piece of sheet metal bent at the proper angle and riveted or welded to the shroud for a tank support. Hooking up a governor is not easy. It can be done, but it will be an engineering exercise you likely won't enjoy. It is also possible to invert the intake tube and elbow on the block and re-attach it with the tube pointing out the other side of the engine. If this is done, the carb can be mounted on the same side as the exhaust pipe. If you're serious about some decent performance, turn the intake tube in this manner and scrap the original top-of-the-tank carburetor and tank asembly. Braze an adaptor plate onto a shortened intake tube, and mount a carburetor from a Tecumseh (or other brand) engine. Run the fuel line to a tank mounted on the other side of the engine. Vertical engines typically have lighter flyweels than horizontal units. You won't have a lawnmower blade to help keep the engine turning at low speed, so you'd better plan on swapping flyweels. You MAY be able to start and run it on the original flyweel, but conditions have to be pretty nearly perfect, and it WON'T run smoothly at low speed. If it "kicks back" when you try to start it, or when shutting off, you may shear the flywheel key, resulting in no ignition until it's replaced. Make sure the taper on the crankshaft matches the taper of the flywheel, and that the magneto is compatible with the flywheel. Mounting the engine is another engineering problem. The "quick and dirty" method is to use some 1.5" angle iron bolted to the original base and then welded to two pieces going back under the engine toward the flywheel (parallel to the crankshaft) for mounting to whatever you'll use it on. This isn't pretty, and it sometimes is difficult to square up if you don't have everything perfectly in place when you weld it! Alternatives that will work include welding heavy aluminum angle directly to the block, or making a special base which is bolted to the bottom of the block with LARGE FENDER washers inside and lots of gasket sealer to keep them from leaking. If the washers aren't large enough, or the mounting surface is uneven, the bottom of the block WILL break out due to vibration. If you bolt a base into the bottom of the block, do the bolting through the flat surface or it WILL leak. Make sure the bolt heads aren't in the way of the oil slinger as it comes by! In addition to all this, remember that the vertical shaft engine likely has a 7/8" or even a 1" diameter shaft. This is typicaly larger than found on a horizontal engine, and will likely not fit whatever pulleys, gears, or clutches have been used in that application. The shaft can be turned down in a lathe to the right diameter, but then you will lose the keyway. Sometimes it's adviseable to swap crankshafts, but that isn't always possible due to the length of the bearing surface. Of course you may be able to swap the vertical engine sump plate for the side cover or bearing plate of a horizontal engine, but that gets to be a lot of mixing and matching of parts. If you're looking at a Tecumseh engine, or one of the many variations of Tecumseh engines made for Sears, the same sort of techniques may be used. The fuel system will require some extensive parts searching, and possibly you'll have to MAKE your own intake manifold. The flywheels on those engines are definitely too light without the help of a blade, and swapping flywheels is a lot more complicated on these than on a Briggs engine because there are more different series of crankshaft tapers and taper lengths involved. The vertical Tecumseh engines used a combination pressure (sort of) and splash system for oiling the critical parts. You can leave the pressure system in place, but it won't help anymore, so concentrate on finding ways to throw LOTS of oil into the air on the inside of that thing. This sort of activity can be a LOT of fun, and provide some really neat learning and engineering challenges. I'd recommend you not try it unless you have access to some machining equipment, such as a drill press, lathe, small milling machine, and a welder. Some parts manuals can be a big help also!
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