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2 stroke ring replacement

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hey i was thinking of replacing my rings in my 1997 kx 125. last time that i hecked the compression it was at 150psi (around middle of summer) so i figure now that its winter i should do that. My question is how long will they last if i just replace the rings and not piston also? i never really hear of people just doing rings, always doing complete top end. Thanks for anyones input.

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i do full top end every 25hrs on my yz125 but i change rings at 10 and 20 hrs, its probably overkill but i like knowing everythings as tight as it can be in there when im at the track pinning it!

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Pistons can wear out before they break. The piston skirts wear and can cause piston 'slap', like a knocking noise. This would lead to an oval cylinder and/or broken skirt. Also, the ring landings (the grooves where the rings fit) wear out.

If it was an easily ridden 250 I'd say you could throw in a set of rings, as long as you already had a forged piston installed, and run it for a few more hours. However, those 125's need to be rung out like a wet sock, so the fresher you can keep that cylinder the better.

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ok -- bored at work, so I will let a little of the nerdy engineer out.

The piston rings, we are primarily concerned about wear. The wear on the piston rings can be measured by measuring ring end gap. On my 03 KX250, the normal expected ring end gap is 0.25-0.45 mm. The service limit is 0.75mm.

Like Jeekins said for the pistons, we are concerned about wear, but also metal fatigue.

http://en.wikipedia.org/wiki/Fatigue_%28material%29

Basically all metal parts slowly weaken over time with repeated stress loads. Its dependent on the stress during each cycle, and how many cycles it goes through. Pistons go through a lot of cycles. If you assume an average of 5000 RPM and model it with each RPM being 4 load cycles for a piston, then there are 1.2 million load cycles in 1 hour. The chart below has on the Y-axis the breaking strength of a piece of brittle aluminum. The X-axis is the number of cycles it goes through.

BrittleAluminium320MPA_S-N_Curve.jpg

As you can see, by the time we get 25 hours on a piston, with something like 30 million cycles, the piston can sustain signifigantly less stress than it could when it was new. It might not fail for 50M, 100M or even many more cycles depending on its maximum stress you are putting it at. At some point, the piston will fail.

Now -- snap out of nerdy engineer and back to reality --

If you fail a piston while riding, the very least that is happening is that you are going to ruin a ride, loose riding time, push back to truck, etc. What if that failure occurs on the take off ramp for a double? What is the cost if the piston grenades ? cylinder? head? crank? cases?

For me on my fairly easily ridden 250, I do piston, rings, pin, clips, gaskets, etc all at the same time to prevent possible catastrophe. It might still happen, but at least I am minimizing the chance. Its like insurance.

On a 125, the odds are much higher in my opinion.

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Not wasting time and money - replacing the rings is good, but the point is, while you are there, you might as well do the piston, pin, clips, rings, bearing as well.

If you assume that your time and the cost of gaskets and the rings is already included, how much more can the other parts be?

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i do em every year if they dont get done before... but if i get to 25 hrs i always change everything! so am i putting too many hours on it at 25?

ive never had any problems! alot of ppl tell me that what i do is overkill

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On a 125, yes. If I'm opening it up, I'm going to put a new piston in. On a 250, I will do a couple sets of rings before replacing the piston, so long as the piston still measures out well and has no visible wear. On my 200, I will probably never replace the piston again, assuming no catastrophic failure.

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I replace the top end in my KX250 and son's KX65 every winter (60-80 hours of XC/trail riding) which may be overkill but I have never had either bike quit and make us walk. The only time I have replaced rings only (and wrist pin bearing) was when I bought a used YZ125 that had a new top end in it that the PO had "broke it in" and I wanted to make sure the the rings were sealing right. He couldn't explain to me what he did to break it in. Like they say - one failure that causes a cylinder replate will pay for 3-4 new top ends. $125 a year is cheap insurance.

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i do em every year if they dont get done before... but if i get to 25 hrs i always change everything! so am i putting too many hours on it at 25?

ive never had any problems! alot of ppl tell me that what i do is overkill

No, 25 hours is not too much...I'm no expert, maybe you shouldnt listen to me. :moon:

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hey i was thinking of replacing my rings in my 1997 kx 125. last time that i hecked the compression it was at 150psi (around middle of summer) so i figure now that its winter i should do that. My question is how long will they last if i just replace the rings and not piston also? i never really hear of people just doing rings, always doing complete top end. Thanks for anyones input.

What kind of riding are your doing?

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No, 25 hours is not too much...I'm no expert, maybe you shouldnt listen to me. :moon:

lol i was just curious... i dont ride my 125 very often unless i just wanna have a good time at the track and rip around! so 25 hrs is appx what i get out of a piston per year! i also do both 250s every year along with the ring changes but those end up with about 50-60 hard riding hrs a peice on the pistons!:cheers:

fyi i let my buddies borrow my bikes too which would explain the massive amount of hrs! i only wish i had 200 hrs to ride every year not including winter!

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I like to trail ride but alot of the time im opening it up there not really tight trails. What is a good compression reading that is safe to say that i dont need a new top end and just do the rings, for my bike that is? and is there an easier way to measure the size of my piston instead of buying a micrometer?

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Nope, no easy way without a mic.

http://cgi.ebay.com/ebaymotors/KAWASAKI-KX125-KX-125-VERTEX-PISTON-KIT-STD-BORE-95-97_W0QQcmdZViewItemQQhashZitem439c1d4236QQitemZ290381972022QQptZMotorcyclesQ5fPartsQ5fAccessories

Good price on a vertex piston for your bike. As cheap as you can find 125 piston/ring kits, might as well drop in a new piston while you have it opened up.

J>

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Well, there's not much reason to "think" about what the bore size is. Pop the jug off and check it out. If you've NEVER had the top end open yourself then it is decidedly past time to do so. It's just part of owning a 2 stroke :moon:. There's nothing to it really, just do it. The piston size will most likely be stamped in the top of the piston. Clean it off and find out. You could also just pull the head if you just wanted to see the size of the piston.

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You could always just pull it off and take it to a small local machine shop, I'm sure they would measure it for you, it would take someone 30 seconds.

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yeah ill try taking off the head to see if its stamped on there. if im going to put a new piston in ill just wait until sometime in the summer cause i know it still has good compression.

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compression doesnt mean much... if you dont know how many hours are on that piston its def worth replacing it before it grenades on you! these 125s have to be ridden so hard that the pistons take ALOT of abuse! they need changed often! i never put more than 25 hrs on a 125 piston. your asking for trouble.

piston kit= $150

top and bottom end along with anything else that the pieces from the piston mess up= well just use your imagination!

thats the reason these guys are telling you to replace your piston now! obviously you dont know too much about the top end on that bike... thats the first thing i do anytime i buy a used bike is replace the top end! itll save you alot of time and $$$ down the road and no one will be able to say "i told you so". it happens all too often when people get a new bike, they think they can just dump gas/premix in it and go! but it doesnt work like that, they need to be maintained! its all part of the game!

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yea i can see where your coming from. Ill be sure to change it alot sooner than i intended. i deffinately dont have the money to repair a blown engine.

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