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Lawnmower Engine Repair -- Engine Re-Assembly From "Weekend Mechanic" episode WKM-104

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Figure A Figure B Figure C Figure D Figure E Figure F Figure G Figure H Figure I Figure J Figure K Figure L Figure M Figure N

With the engine disassembled, and the cylinder checked for wear, it was determined that the engine block needs to be re-bored. The rod journal -- where the piston rod attaches to the crankshaft -- was checked and determined to be in good shape.

The engine block has now been sent to the machine shop and had the cylinder bored, and the valve-seats have been re-ground. An oversized piston will be used to replace the old one, and the valves will be replaced with new ones. The engine-rod will be replaced as well.

With the engine block back from the machine shop, the block inspected to see the work that was done. In this case, the block was bored to 10/1000's oversized -- well within the tolerance for installing a new piston. In addition to re-boring, the machine shop cut new valve seats and cleaned the entire block (figure A).

Engine Re-Assembly

• The first step in the re-assembly process is to install the rings on the new piston. Refer to your mower's service manual, since there can be variations to this procedure. In our case, there are three separate rings and three corresponding grooves on the piston: oil, wiper and compression. The rings are fragile, so the best way to install them is with a piston-ring expanding tool. Start by installing the bottom (oil) ring first. The expanding tool makes it much easier to stretch and align the ring and place it in the proper groove (figure B). Repeat the process for the middle and upper rings.

• The next step is to install the rod onto the piston. Again, refer to your service manual for this process. Proper orientation of the rod can vary depending on the mower you have.

• It is essential to lubricate the components first, using a light coat of oil. Once the parts are lubricated, install the rod into the piston by inserting the piston pin through the piston and rod (figure C).

• Install the piston-pin retainer rings to hold the piston pin in place (figure D).

• With the piston-rod assembly complete, engine re-assembly can get underway. During re-assembly, remember to oil all friction surfaces as you go.

• Re-install the crankshaft into the engine (figure E).

• With the crankshaft back in place, the new piston can be inserted. Rotate the piston rings so that the gaps in the rings are staggered (figure F). This will help extend the life of the engine.

• With the piston rings are rotated properly, a piston-ring compressor is used to size the rings and pistons, enabling them to be inserted into the cylinder more easily.

  Important: Oil the rings liberally before placing the piston in the compressor. Installing a piston "dry" will practically guarantee failure at startup.

• With oil applied, insert the piston in the compressor and tighten it down with the tool provided with the compressor (figure G).

• With the rings compressed onto the piston, make sure that the piston and rod are oriented the right way prior to installation in the block. Check your manual for the proper orientation. Once you're certain that the components are oriented properly, carefully insert the piston into the bore (figure H).

• As you insert the piston and rod, guide the rod from the other side of the block to prevent the rod from contacting the crankshaft (figure I).

• With the piston in place, install the lower piston-rod cap. Check your manual for the proper specs, and torque-tighten the bolts in place (figure J).

• Lubricate and install the tappets (figure K) and camshaft.

• Make sure that the camshaft and crankshaft are properly aligned (figure L).

• Install the governor (figure M).

• Finally, re-install the oil pan (figure N) and secure it with the retaining bolts.

• As always, replace all gaskets and seals when replacing components that call for them.

In the segment that follows, the work on the engine is completed, and the engine is re-installed in the mower.

RESOURCES :

The Haynes Small Engine Repair Manual
ISBN: 1850106665
Author: Curt Choate, John H. Haynes
Haynes Publishing Group

Chilton's Small Engine Repair Up to 20 Hp
Model: 0801983258
Author: Kerry A. Freeman
Nichols Publishing

Small Gas Engine Repair
Model: 0830641424
Author: Paul "How to Troubleshoot" Dempsey