Special tools

• Micrometer - many of the measurements of wear to engine parts requires determining the diameter of shafts or bearing surfaces. Except for the piston, most of these can be accommodated by a micrometer with a maximum opening of 1 to 1-1/4 inches. However, in many cases, what is important is not actual diameter but clearance - and this can be determined with the inexpensive 'plastigauge' or a substitute.

• Plastigauge - these are disposable pieces of calibrated plastic used to determine the critical clearance between the rod bearing and crank pin journal.

  You place one in between the rod bearing and crank pin journal and tighten to specifications. When removed, simple measurements on the markings on the squashed plastigauge will very precisely determine the clearance, taper, and out-of-round specifications for your bearing. It is also possible to obtain most of this information by using slips of paper or foil of known thickness but this will not be as accurate or convenient.

• Flatness gauge - a good machined straight-edge and a set of feeler gauges will suffice for checking the mating surfaces of the cylinder and head.

• Ridge reamer - the 1/8" or so above where the piston slides in the cylinder will develop a buildup of carbon. In addition, if the engine has seen really heavy use, the metal in this area will be higher (less worn) than the section below. In order to remove the piston, this ridge must be eliminated or else it either won't come out or you risk breaking the rings.

  Fortunately, the metal ridge is rarely a problem on lawn mower engines and the carbon ridge can be removed with a simple homemade tool which is just a soft metal (i.e., aluminum or brass) piece with a straight edge or inside right angle. You probably will not need an expensive commercial ridge reamer tool. You will not need one at all unless you will be removing the piston.

• Piston ring compressor - when reinstalling the piston and rings, it is virtually impossible to squeeze the rings together to fit into the cylinder without some help. Commercial piston ring compressors are available for about $5 or you can make your own from a large hose clamp and strip of sheet steel (say, 1" x 12" x .020"). This tool is not needed unless the piston is being removed from the cylinder.

• Piston ring expander - this allows the easy removal of piston rings from the piston. With care, you can do this by grabbing the two sections of the ring and guiding it off the piston by hand. In any case, unless you will actually be removing the rings from the piston, this tool will not needed.

• Valve spring compressor - in order to remove and reinstall the valves, their rather powerful spring must be squeezed together tightly. This is almost impossible to do without this tool. I have done it with an improvised clamp designed for holding lab equipment but it was barely up to the task and not fun. However, unless you are going to remove the valves, this tool will not be needed.

• Rubber mallet - the engine overhauler's 'persuader'. In particular, to break free the crankcase/oil sump joint and for reinstalling the piston using the piston ring compressor. A small one will be more than enough.

• Scrapers - to remove built up carbon deposits and stuck gasket material - start with a strip of aluminum. For stubborn deposits, a flat edge paint scraper or straight blade screwdriver will come in handy. Take care not to scratch any machined surfaces. Coarse steel wool can then be used to finish up after the major deposits have been removed. For fine work, an X-acto knife also is useful.

• Wood blocks (4" x 4" x 8" typical) for supporting the engine on your workbench.

• Rags, paper towels, and more rags. Just make sure to dispose of oil soaked material safely. Plenty of old newspaper to protect the ground or table top.