Replacing a lawnmower engine is not as difficult as you might think, but a key requirement is that the new engine has the same horsepower, deck bolt pattern, crankshaft length and power accessory drives as the old one. For instance, a self-propelled mower has a metal shaft that drives the front wheels via a belt or gear. If you install a new engine not so equipped, the self-propelling device will not work. A pull-start model will not have the electrical wiring to operate the starter, although upgrade kits are available. Be careful if you change engine brands. A Tecumseh model may have a different bolt pattern or throttle cable arrangement than a Briggs & Stratton or Kohler version.
Locate the model number of your lawnmower engine stamped on the engine case or metal badge -- you can also use the lawnmower deck label's model number information to obtain the proper replacement engine reference from the mower manufacturer.
Unhook the fuel hose from the tank, or turn the mower on its side and allow it to flow into a drain pan. Choose the method that will minimize gas spillage.
Note that oil might seep out when you turn it sideways; sometimes this cannot be avoided. Return the mower to an upright position after the fuel has drained.
Remove the oil dipstick tube, or drain plug on older models. Turn the mower on its side to drain the newer versions – place the pan under the mower and remove the drain plug with a socket wrench for older models without a dipstick tube. Allow the oil to drain for a couple of minutes. Screw the plug back in.
Put on work gloves and grasp the lawnmower blade -- do not grab the sharpened portion. Hold the blade while removing the center screw with the socket wrench. Jam a wood block between the blade and the deck if the bolt is really stuck, and twist the bolt to get it loose. If it still won't budge, apply Liquid Wrench or a similar penetrating oil all around the blade bolt and allow it to soak in for 10 minutes before loosening.
Remove the blade collar, if applicable. Use penetrating oil and a pry bar to loosen it from the crankshaft if necessary. Discard the old lawnmower blade.
Stand the mower upright and disconnect the throttle cable, handle safety interlock cable and any electrical wires.
Remove the self-propel drive cover on models so equipped.
Unbolt the engine. Rotate the unit to remove the self-propel drive belt, if so equipped, and then pull the engine away. Lift the engine off of the deck and set it aside.
Clean away any debris from under the deck. Lubricate all cable linkages and inspect the electrical wiring for any frays. Use electrical tape to repair exposed wires, or solder them into place if necessary.
Place the new engine on the deck. Remember to turn the engine slightly off-center from the bolt holes and attach the self-propel drive shaft to the belt or gear before securing the engine with the deck bolts. Bolt the engine to the deck. Replace the self-propel drive cover.
Turn the mower on its side. Slide the blade collar on the shaft if so equipped, making sure the key-way lines up with the shaft slot. Install a new balanced blade. Hold the blade and tighten the blade bolt snugly. Turn the mower upright.
Reconnect the control cable and any electrical wires. Install a new spark plug if it's not included.
Fill the engine with oil according to manufacturer's instructions and dipstick markings. Fill the fuel tank and start it to test.