NotTaR of small Gasoline Engines and Rotary Lawn Mowers : Rod disasters - or why the oil and gover..  
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Rod disasters - or why the oil and governor are kind of important

A combination of low oil (well, actually, almost no oil) and probably too high RPMs resulted catastrophic failure of the connecting rod and cap on my garage sale Eager 1 Craftsman mower. I had just completed cleaning the carburetor and was testing it when p-ting!! and it stopped dead - the rod had broken and it was dead-dead. Extremely embarrassing since there was no excuse for such a disaster.

The primary cause was likely a lack of oil - I should have checked it before attempting to run the engine for more than a few seconds. I have no idea whether someone had actually drained the oil for who knows what reason or it was just very low. In addition, I may have accidentally put the governor link back in the wrong hole permitting the engine to run at an abnormally high (and dangerous) speed.

There was no warning. The rod cap just exploded into e pieces (and this was at normal speed) and took a nice chunk out of the interior of the crankcase. Based on a post mortem of the rod, it appears as though one of the cap screws just loosened and backed its way out totally - there was no evidence of thread damage that would be expected if it were ripped out - and fell into the sump. With only one screw holding the rod and cap together, eventual failure was inevitable. Due to the offset design of the cap, this probably worked for a while since most of the force is on the rod.

Discoloration indicated excessive heating but no obvious bearing damage was evident that could be attributed to the lack-of-oil condition. The bearing was not in pristine condition but the type of scoring seemed to be more due to just poor general maintenance - lack of regular oil changes - than to this incident in particular.

Lessons: Check the oil level no matter what if there is any question or you are working on an engine of unknown history. Double check the governor linkages - take notes during disassembly - and be aware of what a normal speed sounds like for your type of engine (2 stroke or 4 stroke). If in doubt, install the link in the hole that would result in lower RPMs - closer to the carburetor. You can always move it later.

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