Wednesday, December 17, 2014

Workshop Wednesday-Lore of the Rings

 Piston rings. Boring stuff if you work behind a desk but for a small engine mechanic they are very important. They seal the piston to the cylinder wall allowing compression to build in the combustion chamber.  The rings separate the combustion chamber from the crankcase, preventing blowby and keeping oil consumption down. They are also partially responsible for transferring heat from the piston to the cylinder where cooling fins or a water jacket carry the heat away. Thats a lot to ask of a couple of thin strips of metal.
 In a two stroke engine the ring grooves in the piston have pins to prevent the rings from spinning. This is because a two stroke cylinder has ports and a ring end would catch in a port if lined up with it.

 In this pic you can see the lower ring ends line up on each side of the pin in the groove. This is the intake side of the piston. Those ring ends never uncover the intake port, the piston skirt does. Look at the upper ring. Somehow the ring is able to pass under the pin, allowing it to spin in the groove. Spin in the Groove. Wasn't that a hit from the 70's?

 This is the exhaust side. That upper ring spun around until the end got caught in the exhaust port. It scratched away at the port opening. Bits of piston and cylinder started grinding away at each other and at some point the ring just hooked so hard it broke off that chunk of piston.

 Here you see the damage around the exhaust port.
 So what? Well the owner is gonna want to know what happened to his thousand dollar concrete saw. This was Als project so he got to make the phone call. What would you tell the owner the cause of the failure was?
 I never asked Al what he told the guy. All I know is the guy told him to scrap it.
 It would be easy to just claim lack of oil but I don't think we have an oil issue here. First, I know this contractor has a lot of equipment and this is the only two stroke with a problem. Second, that scuffing on the intake side of the piston skirt is most likely from dirt, not lack of oil. This is a concrete saw and they work in extremely dusty environments. I don't think the dirt was the cause because the lower ring is closer to the dirt and that ring is fine.
 There is no evidence of blow-by. 
 The locating pin in the top groove is loose. How did the hole the pin is pressed into get worn? A couple things I think. An aluminum piston and a steel pin. I'm guessing these expand and contract at different rates. A lean condition or dirty cooling fins could raise the temp in the cylinder. As the ring passes the exhaust port it bulges out just a hair. As it moves back out of the port into the cylinder it gets compressed again. This tiny movement is realized at the ring ends and works on the pin. This happens thousands of times a minute and just might be cause.
 It's rare that we see something like this so I doubt it's a manufacturers defect. If this would have happened early in the life of the saw then maybe I would suspect a defect but this saw has been in service for many years. I think this saw just wore out. I mean shit don't last forever. It had a hard life and died. (a country music hit from the 90's)
 There. Now you know what bounces around in my head during the day when I don't get any riding time in.

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