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Piston/Piston Ring wear

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I have a 2014 kx250f that my dad bought used from a guy who only road trails, and never changed the oil or cleaned air filter. He had the bike serviced at a shop recently and they said something about the bike needing a new piston head and rings and they could hear the sound. My dad didn't ask if it was ok to ride though. Is the bike ok to be raced this weekend?  

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You can easily do a compression test to see the condition of the piston rings sealing. If the shop is hearing piston slap, or piston knock, due to a worn piston, or cylinder, then YES it is time to replace those parts, and NO, you should not race it this weekend!

If the piston is too loose in the cylinder you can easily break the skirt off and then you will be in real deep doo-doo. Do not risk it. If you don't know how many hours are on the bike, pull the top end off the bike, measure the piston, measure the cylinder. If the clearance is still in spec, put new rings on & ride it. If the cylinder, or piston, are worn, replace all out of spec parts with new.

Could you imagine hitting a double, or a triple, and the motor seizes up right as you are on the face of the jump? Is your life worth that kind of risk? Don't do it. Check everything and replace all worn parts. This is something you and your dad can do together. I would not leave it to a shop. First off, they are not going to get it done as quickly as you can. Second, they are going to charge you a lot more than what the parts will cost you if you do it yourself. Lastly, learning how to work on your bike yourself is PRICELESS.

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24 minutes ago, Kawabuggy said:

You can easily do a compression test to see the condition of the piston rings sealing. If the shop is hearing piston slap, or piston knock, due to a worn piston, or cylinder, then YES it is time to replace those parts, and NO, you should not race it this weekend!

If the piston is too loose in the cylinder you can easily break the skirt off and then you will be in real deep doo-doo. Do not risk it. If you don't know how many hours are on the bike, pull the top end off the bike, measure the piston, measure the cylinder. If the clearance is still in spec, put new rings on & ride it. If the cylinder, or piston, are worn, replace all out of spec parts with new.

Could you imagine hitting a double, or a triple, and the motor seizes up right as you are on the face of the jump? Is your life worth that kind of risk? Don't do it. Check everything and replace all worn parts. This is something you and your dad can do together. I would not leave it to a shop. First off, they are not going to get it done as quickly as you can. Second, they are going to charge you a lot more than what the parts will cost you if you do it yourself. Lastly, learning how to work on your bike yourself is PRICELESS.

Alrighty thanks for the insight. Looks like I'll probably be on the Honda again. Will I need any special tools to get into it myself? I am a beginner at engine work and the most I've managed to do is check valve clearances and put in a new clutch. My dad knows less than I do at this type of stuff. I have a feeler gauge and torque wrenches but other than that just a normal toolset. How exactly do I do a compression test? I can't find any good videos about it.

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38 minutes ago, Kawabuggy said:

You can easily do a compression test to see the condition of the piston rings sealing. If the shop is hearing piston slap, or piston knock, due to a worn piston, or cylinder, then YES it is time to replace those parts, and NO, you should not race it this weekend!

If the piston is too loose in the cylinder you can easily break the skirt off and then you will be in real deep doo-doo. Do not risk it. If you don't know how many hours are on the bike, pull the top end off the bike, measure the piston, measure the cylinder. If the clearance is still in spec, put new rings on & ride it. If the cylinder, or piston, are worn, replace all out of spec parts with new.

Could you imagine hitting a double, or a triple, and the motor seizes up right as you are on the face of the jump? Is your life worth that kind of risk? Don't do it. Check everything and replace all worn parts. This is something you and your dad can do together. I would not leave it to a shop. First off, they are not going to get it done as quickly as you can. Second, they are going to charge you a lot more than what the parts will cost you if you do it yourself. Lastly, learning how to work on your bike yourself is PRICELESS.

 

Also what exactly does the piston knocking sound like compared to a normal bike

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You normally will notice piston knock, or slap, on cold start up. When cold, the piston is just a tiny bit smaller than it is when it is at full operating temp. So, start the bike when it is dead cold, and put your hand on the cylinder and feel for a little knocking, and listen for a sound that sounds almost like a rod about to let go in the bottom of a car engine. As the bike warms up you may notice this noise subside a little bit. On really bad examples you may still hear the piston rattling around even after the bike is warmed up. I've seen 4 strokes where the piston got so worn that it rolled over in the bore! Destroyed the cylinder, the head, the rod, etc;

Your local parts store will have a compression gauge. You remove the spark plug, screw the end of the flexible hose from the gauge into the spark plug hole, disconnect the coil wire, hold the throttle wide open and kick the ever lovin' crap out of it until the gauge stops climbing. Some bikes have a decompression release that must be disabled in order to get an accurate reading. Read your manual and it will discuss how to do this. What'd you say? You don't have a manual? Better start there.. Here's a video

 

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