One thing is for sure, it is better to rebuild your dirt bike engine too soon, rather than too late. You want to avoid a catastrophic failure of the engine. A catastrophic failure will not only cost you a lot of money, but it can also lead to serious injuries if the engine lets go at a time when your vulnerable on the bike, like in the air or full throttle on a straight away.
There are numerous cases of motocross riders having their dirt bike engine fail and lock up in mid-air, ultimately causing them to crash and seriously injure themselves.
Ok, enough doom and gloom. So how do you know when you need to do an engine rebuild?
I wish there was a set number for everyone, but there just isn’t. Manufacturers have maintenance intervals in the owner’s manual that are often geared toward someone who is racing the dirt bike. If you go by the manual, engine checks and rebuilds are usually done according to a set number of hours or races, whichever comes first. You have to figure those numbers are probably overkill for the average rider.
I am an Intermediate Vet Rider and here is what I generally do on modern 4-stroke dirt bike engines:
These are the intervals I follow for a 250cc and 450cc dirt bike. When dealing with smaller CC engines, consider the fact that a smaller motor is working harder to produce optimum power and will most likely wear out quicker. Again, this is not a one size fits all topic, and some people will go far longer between rebuilds then I do.
Remember, as an engine wears and gets hours put on it, internal parts are wearing in conjunction with one another and they're wearing a certain way. When you throw a new part into the engine, you then change the way the components will wear. For example, if you run your bike for 100 hours and then decide to throw a new piston & rings into the motor, the compression changes. Now your putting more force on a 100 hour crankshaft than what it was used to, which could possibly lead to a crank failure.
Now, I'm not saying you have to rebuild your engine all the time. Just keep this concept in mind when you start to rack up the hours on your motor.
The type of riding you do and how hard you ride the dirt bike will have a significant influence on when you rebuild your dirt bike engine. The more load your putting on the motor, the faster the parts are going to wear out. Off road and trail riders can sometimes get away with longer maintenance intervals, because they are not constantly in high RPM’s and putting significant load on the engine. Motocross riders however, have a tendency to be in higher RPM’s more often. When you consider motocross starts, straightaways, and free wheel revs in the air over jumps, a motocross bike’s engine spends more time at higher RPM’s . Of course, this all depends on the rider’s ability.
Is the dirt bike hard to start?
If your dirt bike is getting harder and harder to start, this could be a sign of wear in the motor as well. In 4-stroke dirt bikes, it may just mean that your valves are out of adjustment and they need to be put back into spec. However, consider the fact that when valves get out of adjustment, it’s usually because of wear on the valve or the valve seat in the head of the engine. Valves don’t just move themselves around and get out of adjustment. Either way, if your dirt bike is getting hard to start on a regular basis, and you’ve kept up on maintenance, it may be time to take a look inside the motor.
Is the dirt bike smoking?
Unless you have a two stroke, your dirt bike should not be smoking from the exhaust. If it is, that means either oil or coolant is getting into the combustion chamber. On a four stroke, I’ve seen this happen mostly when valve stem seals are worn out or improperly installed. However, there could be multiple causes for smoke from the exhaust. Either way, it’s time to tear the engine down and take a look.
Here are a few things you can keep an eye on between regular maintenance intervals to help determine whether or not it is time to rebuild your dirt bike motor.
How many hours has it been since the last rebuild?
If you purchased your dirt bike brand new, then hopefully you put an hour meter on it. You can follow the owner’s manual recommendations or you can make the determination based on your riding and knowledge of your care for the dirt bike. Again, keep in mind the smaller CC dirt bike engine has to turn a higher RPM to create optimum power. Generally, you will get more life out of your larger CC dirt bike than you will out of a smaller one.
Examine the oil and filter.
When you change your oil and filter, it's a good idea to look at the drained oil, as well as the used engine oil filter. While it is normal to have some small metal particles in the oil, if you are seeing larger chunks of steel or copper shavings, this is an indication that something inside the motor is wearing out. You can get a magnetic drain plug to help attract any metal particles in the engine as well. If the oil has white streaks in it or has a milky look to it, that is a sign that water is getting into the oil somehow. This usually happens from a leaky head gasket or a blown water pump seal. If you see this, it is time to take a look inside the motor.
Listen to the motor.
Dirt bike engines can naturally be a little noisy. They are high compression, compact motors that produce A LOT of power for their size. However, listening to your dirt bike engine can give you some clues as well. After you warm up your dirt bike, there’s nothing wrong with giving it a couple twists of the throttle and listening for any odd sounds. Things like ticking, knocking, winding or rattling can all be signs of trouble. Listen to the motor at idle, on acceleration, and on deceleration and just make sure you don’t hear anything out of the ordinary. If you do, listen to your gut and don’t run the bike. I’ve had to cut my ride days short for this very reason and after inspecting the dirt bike, I’m glad I did, because the engine may have failed during my motos.
Check out this RMZ 450F video with some BAD motor sounds:
For many people, going out and buying a brand new dirt bike is out of the question. We look on craigslist and grab the best bike we can with the money we’ve got. When you buy a used dirt bike it’s important to look it over thoroughly and do your best to obtain all the info you can about the engine history on the bike. Unfortunately, unless you tear into the motor and have a look, there’s really no way to know what kind of condition the engine is in.
With a used dirt bike, I would recommend at least putting a new top end in the bike. Just a piston and rings and a fresh cylinder hone will be fine. If you’re working on a 4-stroke, a new cam chain wouldn’t hurt either. By putting in a top end on the dirt bike, you’ll get an opportunity to look at the head, cylinder and piston of the engine and inspect the crankshaft. Whether you have to replace those components or not, you will at least have the peace of mind that you know what kind of condition the engine is in and you can plan from there.
Maintenance is key. If you want to make your dirt bike engine last, perform regular maintenance and keep good records. I change the engine oil and filter every three rides, and clean the air filter every one or two rides. I never go beyond two rides on an air filter before I clean it. Still, with perfect maintenance, engine parts are still going to wear out. So be prepared and set aside a little cash when you can to prepare for the rebuild.
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