Lawn Mower Engine Overhaul


Author and Copyright

Author: Samuel M. Goldwasser

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Copyright © 1994-2008
All Rights Reserved


Improper use, testing, or repair of gasoline powered equipment can result in explosion, fire, injuries including loss of limbs or worse, as well as total destruction of your spouse's prized flower bed.

We will not be responsible for damage to equipment or property, your ego, or personal injury or worse that may result from the use of this material.


When does this information apply?

This chapter deals with the following:

  • Indications for the need for an overhaul.
  • Engine disassembly down to the last nut.
  • Inspection of major parts for wear and damage.
  • Basic replacement or repair of any broken or damaged parts.
  • Engine reassembly.
  • Post overhaul testing.

For detailed instructions on valve grinding, cylinder reboring, or main bearing reaming, for example, you should refer to one or more of the books listed in the section: References. However, this chapter will give you the general feel and basic information needed to perform many common types of simple overhaul operations and to evaluate the need for more drastic action - such as a trip to the new lawn mower store!

Depending on your particular problem(s), only a subset of these sections may apply. For example, inspection and cleaning of the valves and combustion chamber - even valve regrinding (but we said we weren't going to talk about that!) can be done with a minimum of engine disassembly.

Do you need an overhaul?

Many common problems can be remedied without going into the deep dark recesses of the engine. However, some will require either a partial or total overhaul. Eliminate all other possibilities from consideration before considering an overhaul - it will not be a fun afternoon (or weekend, or week, or month,....).

The following are indications that at least a partial overhaul may be needed:

  • Mechanical damage - broken, damaged, or bent parts resulting in inability to start or even turn the crankshaft for starting or excessive vibration while running. In most cases, this will be obvious - the mower died very suddenly - possibly with a loud clunk or p-ting and now the crankshaft hits something really really solid inside when attempting to pull the starter.

  • Low compression - this is due to wear or abuse (lack of oil) of parts like the rings or valves or due to a blown head gasket. Perform the compression test described in the section: Compression testing. Symptoms would be difficulty in starting and unusually little resistance when pulling the starter cord, and perhaps, loss of power once you get it started. If rings are bad, there may be excessive oil consumption and blue exhaust smoke. If only the valves are involved, only the cylinder head may need to be removed.

  • Excessive oil leaks - a failure of the oil seals (the lower one on mowers at the PTO/blade end, most likely) will result in oil dripping or pouring from under the mower deck. The blade will be coated with oil and there will be a puddle where the mower is stored. Of course, if this is severe enough or neglected, you may end up with much more serious problems when the internal parts fail due to lack of lubrication. Replacing an oil seal is not difficult. The old seal is removed by piercing its thin metal shell with an awl or ice pick and carefully prying it out. Take extreme care not to scratch or dent the mounting surface or crankshaft. This may be possible without extensive disassembly. The new one is then pressed on. In fact, installing the new seal is best done with the crankshaft in place as there will be less likelihood of damage to the new seal and it can then be driven in straight. There is a special tool for this but a piece of pipe that just fits over the crankshaft cut off square will work just as well. Remove any burrs on the crankshaft to prevent damage to the new seal and take care that any rubber lip on the seal does not get folded over.

  • Excessive noise - knocking, banging - while an engine powered piece of machinery is not exactly quiet, there should not be unusual or excessive mechanical noises. Such noise can be an indication of an excessively worn engine or of some part that is about to fail. Should you strip the engine based on this? I cannot say - it is a judgement call. It something about the sound suddenly changed, then investigating the cause is certainly warranted.

In some cases, multiple problems may be present and/or there may just be excessive wear of parts like the cylinder, rings, and piston. Under these circumstances, the cylinder may need to be rebored to accept a replacement oversize piston and ring set. The cost of the parts and labor (you really don't want to rebore a cylinder) will likely be more than you want to spend. This is when a new engine or mower is the best option.

Comments on engine rebuilding

While the specific question dealt with a medium size snowblower engine, the comments should apply to other yard equipment as well.

"Is it economical or feasible to properly rebuild a 7 HP Tecumseh engine on a snowblower? Compression seems fine. Has been burning oil to some degree for the last 3yrs, but this year its' burning a lot - maybe 1/2 pint oil for each gal of gas. Until last year, was using 5W30. This year, switched to straight SAE30. I could get a new Tecumseh SnoKing engine for about $350 including shipping."

(From: Mother ([email protected]).)

As a finalist in the All-American Engine Repair Championships formerly held at the Outdoor Power Equipment EXPO (an industry trade show) in the Tecumseh division, and as a Briggs and Stratton Master Service Technician, it has been my experience that:

  1. Yes it is POSSIBLE to rebuild one successfully, although if it is not an HH model with cast iron bore, it probably will not hold up (single H models are aluminum bore).

  2. It is not cost effective to do so.

  3. Short blocking this engine requires special tools if it is more than ten years old, as the ignition timing is not fixed, as it is on newer, solid state models.

  4. Engines from companies like Northern Hydraulics may be adaptable to your unit, but will likely not just bolt on.

If this engine is on a top of the line product, such as Ariens, Snapper, or BearCat, it is probably worth repairing, as a new comparable product is big bucks (and overpriced).

If this engine is on a mid-range product, such as Toro, Simplicity, John Deere, Husqvarna (European product, not USA built), etc., then repair is probably still a good option, due to the overpricing of similar replacement products.

If it is on a Murray, Noma, AMF, Dynamark, Ultra, Sears, MTD, YardMan, White, Husqvarna (USA built by Murray/Noma), or other discount store brand, go buy a new machine. The cost of a new unit is not much more than the cost of the engine repairs, and then you won't have a worn out piece of discount store equipment to break down again in three weeks when something else goes bad...