When to rebuild 2 stroke?


44 replies to this topic
  • Yokomo

Posted December 20, 2008 - 03:31 PM

#1

I've owned a few 2 strokes over the years but never felt like one didn't have enough power to need a rebuild. So other than loss of power, how do you know if one needs a new top end?

What about hours? How many hours do you guys get out of a 125/250?
I picked up a 125 and he guessed he put 8-10 hours on it last year.
Just curious more than anything.

Thanks!

  • jbusa8

Posted December 20, 2008 - 03:33 PM

#2

piston slap at low RPMs is a big indicator

  • KJ790

Posted December 20, 2008 - 03:42 PM

#3

I always rebuilt mine often, about every 25 hours. Some people will run them for a long time, but sometimes the piston will decide to shatter. I'd rather be safe than sorry.

  • rpt50

Posted December 20, 2008 - 03:46 PM

#4

It depends on what is in it. I don't leave the stock pistons in long. On a 250 with a forged piston, I do the top end about once a year if the bike is used very frequently (like every weekend). If the bike is not used very much, it can probably go a lot longer.

On my 125 (which is trail ridden often and occasionally raced), I swap out the ring every 4-5 months ($13 plus a base gasket for 5$), and put a new piston in every 9 months or so, if the bike is used very frequently. Sometimes I get lazy and stretch these intervals, however.

Also, run your 125 at 32:1, and RE-JET rather than change ratios if you foul plugs! Usually a clip postion change and/or a pilot jet is all it takes.

Since this bike is new to you, and top-ends are so cheap for smokers, I would just go ahead and put in a top-end now so you KNOW everything is tight. Pull it apart after 5-6 months and see how she looks with your riding style, and determine your own schedule.

  • Yokomo

Posted December 20, 2008 - 03:50 PM

#5

I won't be riding it much at all, more of a bike to have for a bud to ride here and there.

  • idacurt

Posted December 20, 2008 - 04:00 PM

#6

What do you guys consider a rebuild?
New rings,piston and cylinder?
Kj790,after 25 hrs are you just doing rings?

  • Chevy74

Posted December 20, 2008 - 04:03 PM

#7

In a 125 I would recommend rebuilding every 40-50 hours.

  • CharleyDarwin

Posted December 20, 2008 - 04:03 PM

#8

wow. I'm way behind then, I probably put 20 hours a week on my KDX 400 over the summer and never rebuilt it. Never had a loss of power and it still sounded good, so why bother?

  • 72gag

Posted December 20, 2008 - 05:01 PM

#9

countless riders have subscribed to the why worry philosophy, most of them eventualy break the piston, my last 2 stroke was a 98 ktm 300 I would tear it down and measure it once a year and every time it was worn beyond the recomended tolerances, my 74 cr 125 needed a top end once a month

  • Josh_dr-z125l

Posted December 20, 2008 - 05:09 PM

#10

wow. I'm way behind then, I probably put 20 hours a week on my KDX 400 over the summer and never rebuilt it. Never had a loss of power and it still sounded good, so why bother?


20 hours? dayumm:bonk:

i've done more tho

  • Ride-2-Live

Posted December 20, 2008 - 05:32 PM

#11

I realized it was time when I started loosing power. It's my first 2-stroke, next time I'll do it sooner.

  • Chokey

Posted December 20, 2008 - 05:49 PM

#12

There is no "magic interval" for how often you should rebuild your engine, be it two-stroke or four. There are way too many variables. It depends on what the displacement is, how hard you run it, how much time it spends screaming at the top of the rev range, how well it's maintained, what oil you use and how much, fuel quality, etc etc etc...

A KDX200 that is only putt-putted around campsites might never need a rebuild. On the other extreme, an 85 that is ridden for everything it has all the time by a blazing-fast up and coming young pro may need a rebuild every 5-6 hours.


The only way to determine how often you need to rebuild your engine is to take it apart at regular intervals and measure all the wear items. Once you've reached the limits of the acceptable tolerances and rebuilt the engine, you will have an idea of the pace of wear in your engine under your usage and conditions.

I find that, with my current useage habits, my riding conditions, and my maintenance, I am typically at the wear limits on my pistons at around the 40-45 hour mark. When I was younger, a lot faster, and a lot harder on my engines, I needed rebuilds more frequently.

  • KJ790

Posted December 20, 2008 - 06:43 PM

#13

What do you guys consider a rebuild?
New rings,piston and cylinder?
Kj790,after 25 hrs are you just doing rings?


I consider a rebuild as piston, rings, wristpin, wristpin bearing, c-clips, and all the necessary gaskets. I find changing just the rings to be pointless, if you are going through the work of tearing it all down, why not pay the extra $50 and put a piston in it?

  • Chokey

Posted December 20, 2008 - 07:19 PM

#14

I consider a rebuild as piston, rings, wristpin, wristpin bearing, c-clips, and all the necessary gaskets. I find changing just the rings to be pointless, if you are going through the work of tearing it all down, why not pay the extra $50 and put a piston in it?

I completely agree. I never re-ring a piston. If I'm tearing the engine down, it's getting a new piston.

  • rpt50

Posted December 20, 2008 - 07:42 PM

#15

I have to repectfully disagree. A ring can freshen things up and result in a noticeable boost on a smoker, and like I said above, for about $20 total (ring and base gasket). "Tearing down" a 2 stroke is nothing! You remove the tank, slide the rads our of the way, and slide the barrel off. Easily done in an evening after dinner. Why spend another $55 to replace a piston when it is within spec?

  • factorymx

Posted December 20, 2008 - 07:44 PM

#16

60 hrs is a good time to rebuild

  • Chokey

Posted December 20, 2008 - 07:47 PM

#17

I have to repectfully disagree. A ring can freshen things up and result in a noticeable boost on a smoker, and like I said above, for about $20 total (ring and base gasket). "Tearing down" a 2 stroke is nothing! You remove the tank, slide the rads our of the way, and slide the barrel off. Easily done in an evening after dinner. Why spend another $55 to replace a piston when it is within spec?

Because my time is worth more than that. If I'm opening the engine, It's getting a new piston, so I don't have to open it again in another twenty hours.

  • KJ790

Posted December 20, 2008 - 08:17 PM

#18

I have to repectfully disagree. A ring can freshen things up and result in a noticeable boost on a smoker, and like I said above, for about $20 total (ring and base gasket). "Tearing down" a 2 stroke is nothing! You remove the tank, slide the rads our of the way, and slide the barrel off. Easily done in an evening after dinner. Why spend another $55 to replace a piston when it is within spec?


My feeling is that if your rings are worn so much that you are experiencing performance loss, then the piston has gone through many cycles of stress. While it may not be likely, I'm not going to risk a piston failure to save $50, especially when it doesn't even save me any time.

  • CharleyDarwin

Posted December 20, 2008 - 08:19 PM

#19

With nothing else to do during the summer in a rural area, we spend every waking moment on our bikes. Except if it's raining. =p and from the time we get home from school until it gets dark. If we've got the gas, we're going riding. So 20 hours a week is really not a big number for me, or any of my friends. When I was riding my XT550, I got great mileage and I was on it from the time i woke up until just after dark. With a small bore bike like the KX 125 I have now though, (once summer rolls around, it's parked till it gets warm) I'll probably rebuild as often as I can afford it. I'm used to having lots of power with the 550, so I'm sure I'll be riding the hell out of the little bike.

  • Chokey

Posted December 21, 2008 - 03:05 AM

#20

The problem with using "power loss" as an indicator of re-build time on a two-stroke is that the power loss is so gradual, by the time the power loss is big enough to be noticeable by the average Joe, some parts are already worn beyond spec. And running worn rings on a piston that may still be in spec is a pretty good way to cause piston failure. The blow-by that gets past the worn rings burns lubricating oil from the bore surface and the piston skirts, and overheats the skirts. Broken pistons or seizures often follow.

And a four-stroke is a completely different animal. They typically run seemingly perfect right up to the time that they just blow up, with little or no noticeable power loss.

I'll say it again. The only way to know for sure when it's time for a rebuild is to inspect and measure everything. It will only cost you a gasket set. Abd after the first rebuild, you will have an idea of the wear rate you are going to experience in your engine to use for future reference.





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