Wednesday, February 18, 2015

Piston Lock

 People bring broken equipment into our shop everyday. Sometimes the equipment just fails. Sometimes it wears out. Other times the owner tries to fix it on his own and creates more problems. We are going to talk about one of those times. Not to belittle the guy that made the mistake. No, he feels bad enough. I just want to put this out there so anyone reading this doesn't make the same mistake.
 Before we get into it I need to post this pic of Al.

 I couldn't believe he let me take this pic so I of course had to post it. Things are slower in the winter and sometimes we have time to be goofy. I want to be clear, Al is NOT the person that made the mistake with the chainsaw.

 Back to our broken saw. The customer wanted to remove the clutch and do some repairs on the oil pump. To remove the clutch you need to prevent the engine from turning over so you can unscrew the clutch from the crankshaft. A popular way of locking the engine is with a piston stop. This could be a special tool, a stopper that replaces the spark plug or even a piece of rope pushed into the cylinder through the spark plug hole.
 Our guy tried something else. There was a note on the repair card about the customer using a skrench (that tool you get with some equipment that is a screwdriver and a spark plug socket in one) to hold the piston. At first I didn't realize what was being said but then it was pointed out that he admitted to putting the screwdriver into the spark plug hole, across the top of the piston and into the exhaust port. I'll try to explain better with this pic.

 Now certainly this will lock the engine. I'm sure he felt pretty good about his repair until he tried to start the engine. What happened?

  This is a look from the exhaust port where the screwdriver was sticking out. You can barely make out the dent on top of the piston.

 Here you can see where the screwdriver crossed the top of the piston. The burr created from the screwdriver jammed in the port and the pressure applied when removing the clutch created a raised edge and then this scoring. Now the piston ring is seized in the groove and can't seal very well to the cylinder. The engine only develops 90 psi compression and that's not enough for the engine to run.
 This saw will get a new piston and cylinder. A costly mistake that you will never make.


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