5 Signs That Your Dirt Bike Needs A New Top-end – 2 Stroke

Does your 2-stroke dirt bike need a new top-end? Is it lower on power, or continually fouling plugs? In this post we’ll look at the top 5 signs and symptoms that your dirt bike may need a top-end rebuild.

If you buy a 2-stroke dirt bike with a lot of hours or you ride a lot every month, chances are it will need the top-end replaced sooner rather than later. A top-end rebuild is fairly easy to do on a two stroke engine if you take your time and follow the manufacture’s service manual, and the only special tool you’ll need is a torque wrench for most dirt bikes. However, before tearing your engine apart, there’s several signs that can tell you it has a worn top-end.

If you have most or all of the symptoms listed below, you can more than likely count on replacing at least the piston and ring, and possibly a cylinder and head if there is excessive wear. Although, if none of the symptoms relate to your bike, there’s a good chance it won’t need a rebuild yet…

It Takes 50 Kicks To Start My Bike…

This is a common symptom for both two and four stroke dirt bikes. Unless you are riding in extreme climate, your bike shouldn’t take more than a few kicks to get started when properly jetted. Certain bikes require more muscle and/or technique to get started, but a solid running engine will only take 1-3 kicks to start, hot or cold. On the flip side, just because your bike takes forever to start doesn’t always indicate a worn top-end. More on that later…

My Bike Is Gutless

Does your dirt bike feel like it lost its power-band (sarcasm)? A worn piston/ring will make the engine feel weak and low on power even when WOT (wide open throttle). This often happens after a top-end has so many hours on it, and will get worse/less powerful over time. A 2-stroke piston can last over a hundred hours if the bike was casually ridden and properly maintained, but an aggressive motocross racer can wear out a top-end in less than 20 hours of ride time.

Shattered CR80 Piston

This Thing Eats Spark Plugs

How often are you replacing the spark plug? If you have to put a new one in every ride, stop and inspect before riding again. If the jetting is right, a fouled spark plug can also be caused by a bad top-end or blow-by from the worn piston/ring/cylinder. This definitely indicates a new top-end is required, as well as a new spark plug. I have personally seen a CR80 that fouled plugs because of a bad top-end. It turned out that the piston was cracked and it eventually just lost compression. The cylinder was used again because it was surprisingly still in good shape!

Look For Scoring

A scored piston or cylinder can be caused by multiple things. A lack of lubrication from not enough 2-stroke oil in the gas or lean jetting can damage the piston and cylinder, as well as the crank assembly and bearings. Scoring is caused by metal to metal contact, which is common on 2-strokes when they aren’t warmed up properly. It makes me cringe when people try to warm up their dirt bike too quickly, and I see it quite often. A lot of top-end damage is done if you start your bike and immediately ride it hard or rev it without giving it enough time to warm up.

Quick Heat Expansion Causes Problems.

A piston has a certain amount of clearance between it and the cylinder wall. This clearance is for “growth”, because it will get bigger as it heats up. The size difference isn’t much, but if the piston heats up too quickly, it will expand faster and get too big to properly fit in the cylinder bore, thus creating scoring, and possibly a “cold seizure”. You should also note that forged pistons, as opposed to stock cast pistons, are slightly smaller in diameter and require slightly more warm-up time to expand; so take extra precaution.

Peaking In Through The Exhaust Port

You may have to take the cylinder head off to get a good look at the cylinder walls to check for scoring, but there’s a quick and dirty way to give you an idea of its condition without touching the engine itself. Just unbolt and remove the pipe/expansion chamber so you can look in the exhaust port of the engine.

Shine a light in there and see if there’s any noticeable scratches. You can look at the piston as well, but any scratches/scoring that looks like something you can feel with your finger nail is damage and will most likely need repair/replacement. If you can’t see the cross-hatching on the cylinder walls then it is worn and probably out of tolerance.

Top end rebuild
Scored 2 stroke piston

My Bike Is Super Easy To Kick Over

If you don’t know already, this isn’t a good thing. While most 2-strokes are easier to kick over than a four-stroke dirt bike, you shouldn’t be able to push the kick-start lever down with one finger. If it kicks over with next to no resistance, the top-end is probably down on compression. Low compression will usually go hand-in-hand with taking a lot of kicks to start, as well as a weak engine.

Low Compression?

To test your engine, you can get a you can get a compression tester (Amazon) to get an actual PSI number that will tell you what condition your top-end is in. For a tutorial on how to check engine compression stay tuned for an upcoming post.

How Many Hours Before Rebuild?

How many hours does a top-end last in a 2-stroke engine? There’s a lot of variables, but once you start losing more than 10% of your compression is the best time to replace the piston and/or rings off you want to keep it at optimum performance. The number of hours it takes to reach that point will differ for each bike and rider.

The engine components only last so many revolutions before they start wearing out. This is why a 125cc 2-stroke that is ridden by an expert level motocross rider at high RPM consistently will wear out in less time than a 250cc bike that is ridden by a novice rider at a lower RPM on average.

This is why it’s difficult to determine the number of hours you should go before rebuilding the top-end. Going by the book, a 125 should be inspected and/or replaced every 5-10 hours of racing.

In the other hand, if you’re just casually trail riding and don’t care if you lose a little bit of power after a while, you could easily go 50-100+ hours before needing to replace engine parts.

What Parts Should Be Rebuilt?

A new piston ring may be the only thing that needs replacing if the piston and cylinder are still in good shape. Rings only last so long and eventually go out of spec (refer to your manual). If you have the cylinder off, check the ring gap with gauges and compare to what the manual says. If the gap is too big, replace the ring with a new one. You can usually re-ring a piston once before needing a new piston as long as everything else checks out.

These tips are only guidelines. Nothing is set in stone, as every bike is different. Just because your bike is hard to start doesn’t mean it will always need a new top-end. It could be something completely different, such as a dirty carburetor because the bike hasn’t been started for months. I put this list together because these are the most common symptoms of a bad top-end when all of the simple problems have already been checked.

If your bike won’t start, there’s a simple protocol to go through to figure out why. Is your bike getting fuel? Did you turn the gas on? Does it need a new spark plug? Read more on diagnosing a no-start.

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6 Responses

  1. Stephon says:

    Hey, I have a 99 kx 125 2 stroke and my up end given up on me, I look on my bike and it’s said up end bearing PK 1881 so I think I need that one

  2. Webb josh says:

    Put a new clutch in my bike took it for a ride and it locked up the back wheel, the bike will start in neutral but when put in gear it won’t go no wear.. what wrong with my Rm85L

    • mxhideout says:

      I suggest you remove any parts that you installed and inspect them. A part may have not been installed properly and got damaged, or there may have already been a damaged/ severely worn part(s).

  3. Taylor says:

    I have a yz125 and freshly rebuilt the top/bottom end i thought i broke it in okay because it ran better than it ever has for 3 hours on the break in, then i go to ride a week later, it starts up i start riding in first gear and it felt like all the power went away, bogged out and stalled. I got it to turn over one more time then it stalled out again and would not start the rest of the day.

  4. Emmanuel says:

    I have a 2007 yz85 and I was running all gears and I down shifted and was about to stop and it stalled on me I tried to kick it it wouldn’t start and I bumped it also wouldn’t start and I got home and checked the spark plug and it was really messes up and wasant tight on there I can take it off with my hand , does this mean my bike needs a rebuild or just a new spark plug ?

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