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how to Know when to rebuild top end based of tests

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So I have a 2005 YZ 250  I've been riding it for about a year now and Thinking about doing the top end.  When I first got the bike my compression was 185 with two different gauges,  8 months later with the same gauge I'm getting 245, 250.  seems odd to me. The guy I bought the bike from said it had a new cylinder head and motor was gone through, maybe the cylinder head was not honed before installed causing the low numbers,  I'm not really sure.  Those to me seem like really good numbers,  The bike runs awesome now, easy to start and I take care of it pretty well, I just hate replacing parts with out knowing that they are bad.  I've heard compression isn't always the best way to check a Top end.  Can anyone tell me what are good compression numbers and how to do a Cylinder leak down test on this bike. I've heard if you have anymore than 3 percent replace the top end but I don't know how to measure percentage with what I have seen online.  Thanks for the help

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could someone post a link on how to measure percentage of leak down on a two stroke.  I have a 05 yz250 with 250 compression but wanted to do a cylinder leak test to determine if rings are leaking or what not.  I here compression not always a true test to determine ring condition.  I have heard you don't want more than 3 percent leak down but just don't know how people figure out that number.  looks like people just pump up cylinders and spray and look for leaks doesn't really tell you conditions of rings.  I don't really know, just trying to get some tips and advices thanks.

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Comp testing give you a pretty good idea if the engine is bad or not but doesn't guarantee the engine is good . 250 is a bit on the high side of things but not dangerously high just make sure to keep a good tune and use good high octane fuel and it should be fine . Best way to check is to tear down and measure everything but otherwise looking into he exhaust port or taking the head off to inspect the bore and piston side is the best you can do to look for wear . If it looks like the cross hatching is gone or their are any gouches of any type then it's time to tear down .

However it's effectively impossible to measure bottom end health with the motor assembled as if any symptoms develop its already too late.

A properly built two stroke will show lower numbers to start then improve as time goes on as the rings and bore seat to each other .

Best thing to do is figure out your hours on the bike and follow rules of thumb on rebuild intervals . A 250 can be from 10-20 for a raced motor to double that and a little more if its ridden softly.

Remember that not only do the piston and rings wear but the crank bearings and rod bearings too ....

Measuring the health of these bearings is next to impossible as ball bearings like most bearings will wear minimally then once any sort of wear starts they're good as dead

Edited by Swes

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In the first place, leak down testers are not standardized, and the differences in how the tool is configured (test orifice size) can change the numbers it yields compared to other testers.  That makes the numbers a little bit "fuzzy".  Still much better than cranking compression tests.

 

97% sealing is a target for a race motor build, and it's hard to achieve in the first place, never mind to be used as indicator of being worn out.  It's also noteworthy that with normal, gapped rings and no dead run time, the engine won't show that kind of sealing immediately, either.  Like the compression test numbers, it takes an hour or so of solid run time to get to that point. 

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If your compression is testing at or above spec your rings are fine.  Leakdown testing will confirm that.  If you had low comp a leakdown test is a good next step but its just going to tell you what you already know in this case.

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You can see that in that picture?  I doubt it.

 

Do you see any damage? 

 

His compression test confirms that his rings are sealing.  What would make you think theres anything wrong with the bore?

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Do you see any damage? 

 

His compression test confirms that his rings are sealing.  What would make you think theres anything wrong with the bore?

 

I don't see much of anything, no.  Not a good enough picture to judge by, IMO.

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Which is why I said "from what I can see".  Not much, but certainly nothing to justify a teardown if we're going to go by that pic.

 

Plating shows normal wear but some crosshatch is still visible up to the TDC for the top compression ring.  Thats what I'm looking at.

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I think I'm going to wait a little longer before I change it. I'll just keep checking conpression until it drops to what number? What top end kit do you guys recommended? I saw the dark line in the pic is why I thought it might be going bad

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If you're referring to the vertical marking on the cylinder wall its just scuffing.  Not unusual.  I would bet its just a blemish and cannot be felt with your fingernail.

 

Plating will usually show wear around ports or at the top of the bore first and progress from there.  In terms of when to service it, since you don't know how many hours it has on it you could do it whenever you feel like it really.  Rings aren't the only reason to do the top end.  With a two ring piston it'll usually take a very long time before it wears to the point that the compression goes away.  The piston itself is susceptible to degradation to to heat and fatigue.  If you run it too long it becomes increasingly likely the skirt will crack off and trash the bottom end and cylinder.

 

I am servicing mine on about an 18 month interval using stock cast pistons.  I ride on average once a week with at least a good hour of hard motoing each time out.  Thats an absolute minimum of 72 hours between top ends and its probably actually well over 100.  They come out looking great.  You could do them every 50-75 hours on a 250 two stroke and never have a problem.

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No matter how many hours you get between rebuilds, could vary from less than 20 to 100+,

a good rule of thumb is to refresh your top end when a few mid season compression tests show

you have dropped a maximum of 15% of the 'newly rebuilt' compression value.

 

Say a 125 gets about 170-180psi on a new a top end, so by 150psi consider it's time to refresh it.

Likewise say a 250 gets about 200-220psi on a new a top end, by 180psi consider it's due to refresh.

Edited by mlatour

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245-250psi seems really high, I'm thinking you compression tester is perhaps out of calibration.

 

Try doing another test with a different gauge, as mentioned 200-220psi is about what a healthy YZ250 should get.

Edited by mlatour

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