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So, how precuationary is a top end rebuild on a second hand bike?

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do a compression test. That will give a good indication of the rings & cylinder. That's the easiest test. Otherwise if you, or someone you know, has the ability to do a leakdown test, that will be more accurate of the condition of the motor. Other than that, you can choose to either run it till it blows or just rebuild it for a peace of mind.

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+1 compression test

You have more time over the winter to rebuild rather than having the bike sit in the summer.

And you really can't see much unless it is already broke.

Edited by gsa102
more info

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I wish I would have done a rebuild when I bought mine. Was told it had about 40 hrs on the top end, I put about 15 more on it when the rings let go. Oh well at least it's not a 4t. Sleeved and top end kit for about $350 and I'm back in the saddle again.

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I wish I would have done a rebuild when I bought mine. Was told it had about 40 hrs on the top end, I put about 15 more on it when the rings let go. Oh well at least it's not a 4t. Sleeved and top end kit for about $350 and I'm back in the saddle again.

So it had 55 hours on the top end and your surprised it blew up?

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Not suprised, just wished i would have rebuilt it when i bought it. I believe it had alot more hrs on it. I now rering it at 40 hrs and piston is in spec and looks new (old piston looked bad ). Might just rering it again at 80hrs, depends on its condition. I have a buddy who rebuilt his 98 yz250 for the first time this year, probably has 400hrs on it,Didnt even replace the piston, so each situation is different.

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to be honest i had an '88 cr125 at 13 and i knew that needed a desperate top end rebuild, i know its easy to turn over a 125 by hand but you could feel the gasses passing the rings, and taht bike ran for ages. i think these 2 stroke motors take more than we give them credit for.

its a nice feeling knowing theres nothing wrong with your top end though

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start it up and you can hear a rattling sound it means the piston has been scuffed and is rattling around in there.

Example:

Don't ask me why that guy is riding an 85 but yeah that clicking sound is what you don't want to hear.

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thanks. does anyone have the figures i need for a compression test on the 98 kx250? still waiting for my clymer to get here

Above 150 is still pretty good. A good one is above 190. Below 125 is a serious power loss and on it's last leg.

But in all honesty, a worn top end, i.e., rings, isn't the culprit of a catastrophic failure. It's only the first sign that it's time for some parts. The parts can only take so much abuse before stress and fatigue starts to take part in in leading to the failure. So, if you're that unsure, do a full rebuild so you start fresh with new crank, mains & topend.

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Well I will add another sad story to this, when I bought my 99K250 the guy told me it had a new cylinder and a new piston in it so I pulled the head and yes it was all brand new so I put the head back on and rode it. Well a half hour in to my first ride on my new to me bike it quit, no compression. Took it home pulled it apart and a wrist pin keeper had came out and destroied the new cylinder........ so I say take a GOOD look at what you just bought.

Ed

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Not new to me, Ed, I had the same thing happen to my daughters 65 2 years ago. But hey, Stuff happens.

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Picked mine up a few months back and was told "it had a top end a few months ago." I wanted to be confident in the condition of the motor and was looking for an excuse to get more familiar with the bike. Pulled down the top end and the cylinder was scored and the rod had excessive side to side play. After re-plating, a top end ,and bottom end I now have a fresh motor and have a bunch more confidence in what I'm riding.

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There's no amount of testing that's gonna tell you if the piston skirts are fatigued. My friends YZ had plenty of compression when the piston skirts let go and destroyed it.

At a minimum, pull it down and inspect!

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There's no amount of testing that's gonna tell you if the piston skirts are fatigued. My friends YZ had plenty of compression when the piston skirts let go and destroyed it.

At a minimum, pull it down and inspect!

Exactly what I was thinking.

Same thing happened with a friends YZ. Bike ran perfect, started first kick, no excessive smoking, KAPLOOOIE! Stock/cast piston let go. He got lucky and didn't ruin the cases, but cost him a crank and cylinder....and a long ass push back to his truck.

The idea is to keep the piston fresh/tight. When a worn piston flops around in a cylinder it will wear out the cylinder. Keep a fresh piston in there to get more life from your cylinder. It's not all about whether it starts or not.

I do a top end on my 250's annually. A 125 I'd do twice a year, maybe more depending how you ride it.

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I try to tell a friend at work that pistons have a limited life span, as in 40 hours. MX bikes are performance machines, they are not scooters! He says "i havent had a problem with it yet". He also runs crappy ass 2 stroke mix, the cheapest stuff at canadian tire!

When you replace a piston you then have a reference point/time for life expectancy for your piston. It always ends up less expensive then a re-re top and bottom end.

my 2 cents canadian

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How often do you find someone selling a used bike with a brand new rebuild? Not many people put a couple hundred into it just to sell it for a grand or two. My point is... if it runs fine, ride it a little then take it apart and freshen it up, then you'll know for sure what your riding.

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A bit off topic but... IMO common sense is your best guide to used bikes. Is the bike clean, like new, minimal scuff’s, etc. (requires knowledge of what the bike looked like when new. Anything less than that is going to require a bit more in-depth investigation.). Look at the seller. Does he have the means to maintain the bike ( liven in a rat shack or the up-scale side of town)? Can he ride or is he a Spode?. Test ride the bike. Is it tight, straight and feel right (of course this requires that you know what “right” is)? My point is just be aware and don’t be afraid to walk away. Understand this and a lot of these problems won’t happen to you.

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I bought a 2001 CR250 over the summer. I rode it once to make sure I like the bike (I do). After the one ride, I took apart the top end to replace the piston, rings, etc. I found a hairline crack on the intake side of the piston (a common problem, I believe). I'm very glad I spent the money/effort to replace the top end. It was the original cast piston (not that many hours on the bike) and the cylinder hadn't been off the bike since it left the factory. Taking it apart and checking is the only way to tell. It's a two stroke; it's really not that difficult. Even if the piston is okay, I'd bet the power valve parts still need to be cleaned. You should also verify the power valve parts are assembled correctly. The sub/side valves in my KDX were installed incorrectly when I bought it (they must have went by what the factory manual says :thumbsup:).

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