NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Engine overhaul procedure               
 Copyright © 1994-2007, Samuel M. Goldwasser. All Rights Reserved. Reproduction of this document in whole or in part is permitted if both of the following conditions are satisfied: 1. This notice is included in its entirety at the beginning. 2. There is no charge except to cover the costs of copying. I may be contacted via the Sci.Electronics.Repair FAQ (www.repairfaq.org) Email Links Page.

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Engine overhaul procedure

The following description applies directly to a large number of Craftsman mowers using Tecumseh engines (most do). However, with minor modifications, it is also applicable to most other mowers using 4 stroke engines.

Mechanically, 2 stroke engines are very similar. In many respects, they are simpler having no camshaft operated valves or oil pump. There is no oil to drain or change. However, needle bearings are used in key spots which complicate matters slightly. Refer to one of the books listed in the section: References for detailed 2 stroke overhaul procedures.

Prepare the engine for removal

While for certain repairs it is quite possible to work on the engine while still mounted on the mower or other yard equipment, it will almost always be much more convenient to disconnect and remove the entire engine to the convenience of your workbench. There are generally only a handful of actual connections. A typical small engine is remarkably light and compact once stripped of the mower deck!

Remove the engine

At this point, the engine should be free of all its attachments to the mower except for its mounting. For a typical rotary mower, there will be three large bolts accessible from under the deck. Removing these with the proper socket will allow the engine to be lifted and moved to your workbench. You will probably be surprised at how light it is! I recommend just screwing the bolts back into their threaded holes finger tight. That way they will not get lost and the threads will be protected. Also, Protect the threaded end of the crankshaft with a bit of rag or paper towel fastened with an elastic band.

Engine disassembly

The following sections provide the detailed procedures for disassembly and initial inspection for major damage. As noted, these apply directly to most Tecumseh engines but most other 4 stroke engines are very similar.

Removal of the accessories

Now it is time to get down to business! As noted, depending on your situation, not every step will be needed.


The carburetor can now be set aside or disassembled and cleaned. (See the section: Cleaning Craftsman (Tecumseh) carburetors.

WARNING: there is still likely a significant amount of gas inside the float bowl. If turned on its side or upside-down, this gas will come gushing out. Therefore, it is best to set the carburetor aside in an outdoor area in an upright position. Plug the intake manifold and fuel pipe with wadded up paper towels or rags to prevent the entry of dirt.


Note that in addition to decreasing the noise from your engine, the muffler serves a very important spark/flame arresting function. Therefore, it is important that it be in good condition.

Some mufflers simply screw into the cylinder using pipe threads. Others are mounted with a couple of bolts.



This can be left in place but will be susceptible to damage.

Cylinder head


You should only need to do this if you are replacing or grinding a valve. In most cases, the valves are undamaged but may appear in poor condition due to carbon buildup - which can be removed in-place fairly easily.

  • Remove the valve cover on the side of the engine next to the valves.

  • Use a feeler gauge to check the gap between the valve lifter and valve stem with the valve lifter in the relaxed - lowest - position. A typical value is .010 inches. Excessive clearance will require replacement of the valve or valve lifter. There could also be too little clearance - which is also bad - but the valves stems can be ground down to repair this problem which is either a defect in manufacturing or a result of a cam shaft replacement which is oversize.

  • Use a valve spring compressor to remove pressure on each valve.

  • The retaining clip or split cup should now be free. Remove these.

  • The valve should now slide out.

  • Loosen the valve spring compressor and remove the spring and any other hardware. Don't interchange the intake and exhaust valve springs.

  • Remove the built up carbon deposits from the valves and valve seats using a soft metal scraper. Some WD40 may help to loosen the caked on carbon.

  • Inspect the valves for serious burning or pitting especially on the seating (angled) surfaces. Once free of any carbon, they should be smooth and undamaged.

  • Similarly, inspect the valve seats for serious burning and pitting.

  • Inspect the springs for rust or other damage. Stand them on a flat surface and check for serious droop/tilt. Measure their free length and compare with your engine specifications.

    Oil sump/crankcase cover

    We are now going into the lower section. I can hear you saying "Joy!".


    Connecting rod




    Oil seals

    Detailed inspection

    A complete overhaul can restore a small engine to like-new condition. Any parts that are found to be damaged or out of tolerance are repaired or replaced.

    Determining this requires a visual inspection and taking measurements of all critical dimensions of bearings, cylinder, piston, and rings. Some of the inspection is subjective - how badly scored a bearing surface is before it must be replaced or reground. A few score marks around the circumference of a bearing surface will not adversely affect operation or wear. How few is a few? Perhaps if less than 10% or so of the surface is affected. You are not going to spend as much to repair the mower as it cost in the first place in any case so don't lose sleep over it.

    Measurements may come up marginal as well. For example, if the limit listed in your engine specifications is .0015" and you measure .002" will this be a serious problem requiring the replacement of expensive parts? Probably not. You may get less than optimal life out of the engine but it will probably still work fairly well and for a long time. So many other factors can affect life that this may have no effect at all.

    The following items should be visually inspected. If any significant wear is indicated, precise measurements should be made:

    Engine reassembly

    Once you have performed whatever magic is required to repair or replace broken or damaged parts, here are the steps that will transform your pile of parts into a (hopefully) working engine.

    If any filing, sanding, or grinding was involved, make sure all traces of abrasives have been removed from every part. The best approach is to clean with soap and water or mild detergent and dry thoroughly. Then immediately coat all ferrous parts with engine oil to prevent rust.

    Where the internal moving parts are involved, liberal use of fresh engine oil will also make things to go together smoothly and help protect the surfaces from damage due to initial lack of lubrication.

    For the following, position the crankcase flywheel/magneto side down on some wooden blocks so that when the crankshaft is installed, it's end will be clear of the table-top.

    For the following steps, position the engine on wooden blocks blade/PTO side down.

    Engine installation

    It will be easier to tighten the flywheel nut to the recommended torque once the engine has been reinstalled on the mower. Therefore, now is a good time to install the engine to the mower deck:

    Initial post-overhaul testing

    Assuming you didn't make any mistakes, the engine should start on the first pull. As you start it, look and listen for any abnormalities and immediately stop it if any are detected:

    Assuming nothing appears wrong, run it for a while at slow speed (if you have the option). Continue to be on the lookout for anything unusual. After a few minutes, stop it.

    Let is sit for 10 minutes or so and then check, and if necessary, top off the oil.

    Now, restart and run it at high. Mow a few lawns.

    Congratulations! Hopefully, your engine will now serve you for many more years - or until the blade hits the next curb!

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