The very first thing to do is get a service manual for the engine that you will be working on. Consult the manual for specific recommenations and engine tuning. Then follow the steps below for a guideline on how to rebuild the engine in your tractor or lawn mower.
Once the engine has been drained of oil and removed from the equipment, it should be cleaned. Dirt is your greatest enemy. Once the outside of the engine is cleaned, move it to clean work space and use your service manual to disassemble the engine. A good tip is use your video camera to tape yourself taking the engine apart. It's great to be able to look at the tape to see what this thing is and where it came from.
As you are taking the engine apart, inspect each part for signs of wear. Make notes and/or talk to your camera about the the parts and their condition. Important things to look for are signs of discoloration (caused by heat baking the oil into the part), aluminum that has melted and stuck to other components, scrapes and scaring. You want to know what has been happening inside the motor to be able to avoid another breakdown. If you do not fix the cause of the failure it will probably happen again. If at any point you realize the cost of the rebuild will be more than 50% of the cost of a New Engine, it's a rule of thumb to replace the engine.
Now that the engine is apart its time to take some measurements. Consult your owner's manual for specific measurements. Taking accurate measurements of the cylinder bore is critical to ordering the correct part. While the engine was operating the piston rocks from side to side as it travels up and down the bore. This causes the bore to become egg shaped. Also the bottom of the bore wears more than the top. New Rings will not seal in a out-of-round cylinder. Cylinders can be renewed by boring to Oversize. Oversize is usually .010", .020" or .030" larger than the standard bore which will be listed in your manual. You will want to use the smallest OS that will repair the cylinder so that the engine could be rebuilt more than once. The crankshaft may also need to be reconditioned. If you do not have the tools to take these measurements, just take the parts to the local machine shop and have them reconditioned. The machine shop will then tell you what "oversize" piston and ring set you will need. They will also tell you what Undersized connecting rod will be required if the crank was turned down to correct wear.
Its time to order the parts that you need. Some engines have rebuild kits available. With others you will need to order individual parts. The parts you will certainly need are gaskets. While your at it, rebuild the carb with a Kit. And by all means replace the Oil Filters, Air Filters and Spark Plugs.
Don't forget the proper oil. Run the engine for about 5 minutes at almost full throttle and then allow it to cool some. Change the oil to remove the first break-in cycles metal deposits. Some engines require that the head bolts be re-torqued after the engine cools down completely. Again check your manual for the manufacturers recommendations. After the first 10 hours or less change the oil again.
Check the oil frequently in a new engine. New engines and rebuilt engines can and will use more oil during their break-in period. Good Luck!
Jack's Safety Tips: Before servicing or repairing any power equipment, disconnect the spark plug and battery cables. Remember to wear appropriate safety glasses and gloves to protect against harmful chemicals and debris. View our Disclaimer.