Small Engine Repair

A lawn mower engine engine is relatively easy to repair. These small engines have small engine parts and are easy to move around after being uninstalled. Common Briggs and Stratton, Kohler and Tecumseh style engines come in just about every type of lawn vehicles or yard tools made. Small horsepower models all the way to appx. 20 hp v twins, are all of similar style. There are two common types of 4 stroke lawnmower engines.The L-head, which were the older style engines and overhead valve engines. L-head engines did well for their purpose but can’t keep up with tightening emissions regulations.

 

L-head engines have the camshaft located in the crankcase along side the crankshaft. The lifters go along side the bore, as do the valves. The valves open along side the cylinder bore and create hot spots especially between the exhaust valve and the bore. This is why the bore clearance is looser due to the bore distortion. The head chamber is also not shaped very well, limiting compression to mild. The emissions suffer from the poor chamber shape. This is due to the combustion not reaching the outer parts of the long and narrow combustion chamber. The chamber also gets cold on the outer reaches of the chamber. OHV (over head valve) engines are the same design as a car engine. The chamber shape can be ideal (round) with the valves placed above the piston and the spark plug centered in the chamber. The compression can be much higher, which is means more efficiency. The chamber is even and there are no cold spots where fuel can condense and collect. Repair of either style of small lawn mower engine is not very difficult. To repair a bore that is really worn out, first use a ridge reamer to remove the excess material at the top of the bore. A hone can be purchased cheaply, which can be attached to a drill, rotated slowly up and down to create a cross hatch pattern in the bore. This will make the bore round again and help to seal the new rings to the bore. The bore will also be honed to accommodate the new clearance due to the new oversized pistons. Even if the bore doesn’t look that bad, it will still need to be ball honed to eliminate the glaze on the bore. If the pistons are o.k. the rings will still need to be replaced. If the bore has been slightly oversized,slightly larger rings can be purchased to fit on a standard piston. To work on the valvetrain first remove the cylinder head. The valve stem retainers are slotted and will have to be removed. First the spring will have to be compressed by hand or with a spring compression tool. This is best to do when the spring is not compressed by the cam. Rotate the engine until the cam is not compressing it. The retainer can then be pushed down and slid up the slot until the retainer will come off the valve stem. After the retainer and spring come off, the valve will lift up through the block on an l-head engine. The valves will come out through the head chamber on an overhead valve engine. The valves can be ground on a valve grinder to round the valves and match the valve angle. A cheap tool can be purchased to grind the valve seat to the correct angle. This tool indicates off of the valve stem and is rotated by hand. The engine end case should be removed which will allow the crankshaft to be removed. The rod bearings and main crankshaft bearings will need to be replaced. The i.d. of the rod and the o.d. of the crankshaft both need to be measured so that the bearing clearance can be determined. After the crankshaft rods bolts have been removed they will need to be replaced due to the fact that they stretch. If the rod bolts are not replaced they will break over time. The head gasket should be replaced dry. All the other gaskets should be replaced using a small amount of silicone lightly smeared on both sides. The spark plug gap should be set and the coil gap to the flywheel should be checked. This should be all it takes to rebuild the average small lawn mower engine.