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2 Stroke Rebuild or Not?

Joined
May 23, 2007
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#1
Is it really necessary to do the frequent top end rebuilds on a 2 stoke? See below for the long winded Version.


We have a KX60 bought new this year as it was left over on the showroom from 2003. It has - I'm guessing 60 + hours on the original top end (likely more). Compression is 110 pounds. It has lost considerable power, put still hauls butt with a 50# rider. I took it apart at ~ 30 hours and it looked perfect lots of lube all over. Piston and skirt looked great, cylinder walls great with al the original cross hatches still visible. I put it back together just like it was with new circlips. BTW -Circlips are a pain - any suggestions?

I'm reading all these very avid professionals that recommend these little motors be rebuilt at intervals as little as 5 hours. From my experience thus far I would think this is overkill. The Clymer service manuel says the service limit for compression is 109 pounds. this is pretty much where the bike is currently so I am going to install a new top end tomorrow.

This is our first 2 stroke, but I would like to get rid of my xr250 ang get a kx125 or kx250 for myself. But are hesitant still wanting to see how the current 2 stroke hold up. My son rides this little bike pretty hard now and revs the pi-- out of it. Started off slow, but the last 30 hours have been fully in the power band. Just to give an idea - he's going thorugh 5 gallons or more of 32:1 mix a week.

So again-
Is it really necessary to do the frequent top end rebuilds?
 

jim f.

Mi. Trail Riders
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Jul 2, 2005
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#2
It depends on how hard it's being run and if you want max performance. I mainly trail ride, some mx and I put on 1200 miles on it last year before rebuilding it. But when trail riding I rarely rev it way out. It is a CR125.
 

darringer

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#3
It will more than likely "look" fine, with very little scuffing on the piston skirt, or cylinder wall wear, right up until it lets go. The only accurate way to tell whether it needs a top end is to tear it down and measure the piston, ring end gap, and cylinder walls. Compression readings alone are not a reliable indicator of wear in a 2 stroke. Even you said it has "lost considerable power." To tell how often to rebuild it needs to be torn down and measured at a set interval of time to see how far you can go between top ends.
 
Joined
Feb 5, 2007
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#4
the fresher you keep the top end ,the longer the cyl will last.as you let the piston keep getting more and more wore down the cyl is getting more and more egg shaped.keep it fresh.as far as the pro's go,they r getting paid,so they dont need any excuses.the cleaner you keep your filter,the more consistent your power will be and the longer you engine will last.
 
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Oct 19, 2006
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#5
Compression tester is a go,no-go tool,if you check it and it is low,it is ready to implode! For optimal performance the factory recommendations are for a top notch racer. Start there and then venture further out to see how the engine is wearing. Waiting too long can aggravate problems,oval cylinder. Check clearances and take measurements! Use the compression tester,but as reference.
 
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Oct 19, 2006
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#6
To add to the above comment about the go/no-go characteristics of a compression test, if the engine fails a compression test that's bad. If it passes, it may or may not be ok still. I had a severely worn top end that made 180 psi. A new top end makes 185 psi.
 

Jasle

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#8
you did good. you checked everything. Thats the first component. when its apart you might as well swap rings. that one is a no brainer if your already doing the labor depending on what piston you use its fairly cheap to do a ring and gasket set. Especially if your in there changing clips. I'd suggest a new base gasket also. odds of a leak are considerable after taking the cylinder off and trying to reuse the old one. For the circlips you can buy a special tool made just for installing them. or I grooved some mini needle nose pliers and I sort of make a twisting motion while putting them in. not too bad after you've done a few.

I do a new topend and the break it in. I then check the compression every other race weekend. Make yourself a log book! Then when 90% of original I change rings and check piston clearance. No one can quote you a certain compression reading of when to change your rings. I've had 2 different testers back to back give 20lb different readings. I can usually get 2 sets of rings to one piston though. I have gotten 20ish hours out of a set of rings before the 90%. We mainly race mx, much of it at national levels. Taken a few top 3's and won some amateur national level moto's and we put several hours a week on the bikes. I noticed an improvment when we switched to Amsoil Dominator. but I still run 40:1 cause I'm wierd like that. We run 50's 60's 65's and 85's. I tend to push the 85 motors longer but as my rider has gotten faster in that class I have paid with failure when I've gone below 90% I've gone to 80% and then hung a ring!!! Much more expensive than just doing the topend.
Not sure off the top of my head but I'd bet piston clearance for your 60 was right about .002" so when its apart check your manual and clearances.