The simple design of 2 stroke dirt bike engines are a blessing and a curse. They are relatively easy to work on, but they do need more attention than their 4 stroke counterparts. Routine maintenance is very important with a 2 stroke dirt bike, but once you've gone through the motions a few times, it becomes second nature. Part of this process is rebuilding the top end of the engine.
Without doing top end rebuilds, you run the risk of doing major damage to the engine. One such problem is a piston seizure, which can be catastrophic to many other engine parts as well. In turn, you will be forced to do a complete rebuild or just scrapping the whole engine if it is bad enough.
Top end rebuilds can seem like a daunting task for someone who has not done it before. The thought of tearing into the internal workings of an engine may seem frightening, but it is actually not that bad. Rebuilding a 2 stroke is fairly simple, especially with a little guidance along the way. Follow the steps down below as a general guide to complete the rebuild and your dirt bike will be performing at its best longer.
When to Rebuild Your Dirt Bike Top End
There are no real hard and fast rules on when to rebuild the top end of your dirt bike engine. It is largely dependent on how the bike is ridden. If the bike is raced hard, it might need to be rebuilt after each racing session, whereas a lightly ridden bike can go a year or more without the need for one.
The amount of hours on the engine's top end are the best indicator of when to do a rebuild. If you keep up on the maintenance, a bike can safely be ridden from 40 to 60 hours before a rebuild. Some bikes can be ridden much longer, while less than optimally maintained bikes might not make it that far.
A compression test can be a very helpful indicator if you are unsure of the amount of hours on the current top end. Use a compression tester to see if the compression is close to the specifications that are listed in the service manual. If they are too low, you will need to do a rebuild, or at the very least re-ring the piston.
One tell-tale sign that you have low compression is if the bike kicks over very easily compared to what it used to. Another sign is if the bike has a lot less power than it had before. Constantly fouling spark plugs is another warning sign. Also look for any leaking coolant, as that can lead to major problems if it is allowed to leak internally. If you experience these issues, it's time to, at the very least, do a compression test.
If while maintaining the air filter you notice debris in the intake, it might be time to tear down the top end. This debris can be sucked into the engine and can do significant damage, which is why it is so important to take good care of your dirt bike's air filter.
Top End Disassembly
Follow the steps below as a general guideline to disassemble the top end of your dirt bike engine. Some engines may be slightly different and some steps may not pertain to your specific bike model.
Sockets, Wrenches & Hand Tools
Exhaust Spring Tool
Gasket Scraper/Razor Blade
Micrometer (Calipers), T-Bore Gauge & Flat Feeler Gauge
Wash Thoroughly- Before you start the tearing down process of your dirt bike engine, you should take the time to wash the bike completely. An unnoticed chunk of crusty mud could fall down into the cylinder or into the case, further damaging the engine that you are trying to fix. Any abrasive debris inside your engine can eventually cause major damage. When you are washing your dirt bike, make sure to plug the exhaust and cover the air cleaner along with any overflow hoses.
Drain Coolant- Before draining the coolant, make sure that the engine is cool. To start, place a drain pan underneath the water pump and remove the drain bolt. Slowly open the radiator cap to speed up the draining process. You should be prepared to move the pan in order to catch all of the coolant. Remember to reinstall the drain plug once all of the coolant is out.
Remove Gas Tank, Radiator Shrouds, Seat & Subframe- In order to access your dirt bike engine, it will help a great deal to remove your gas tank, radiator shrouds, seat and possibly subframe, depending on your bike model.
Move or Remove Carburetor- The carburetor will at the very least need to be moved out of the way for all dirt bikes. To do this, you will have to disconnect it from the engine with the fuel line still attached. It can then be moved out of the way. For some bikes, it will have to be completely removed. You may also want to remove the carburetor in either case so that it is completely out of your way. If you do remove the carb, be sure to cover the fuel line and fuel inlet on the carb to prevent any debris from entering.
Remove CDI box & Radiators if necessary- Removing these items may not be absolutely necessary, but it can really pay off in the long run. With the CDI and radiators out of the way, you will have much more operating space while trying to remove and install the cylinder.
Remove Exhaust- With an exhaust spring tool and a wrench/socket, you can remove the exhaust from the engine.
Remove Top Motor Mount- Next, you will need to remove the top motor mount on the engine with a socket or wrench.
Disconnect Radiator Hoses & Spark Plug Cap/Remove Spark Plug- As you disconnect the radiator hose from the head, it is a good idea to have a shop rag handy to catch any coolant that may be left in the hose. After you remove the hose, you can disconnect the spark plug cap and move the plug wire out of the way. Next, remove the spark plug and set it aside.
Unbolt & Remove Cylinder Head- While loosening the head bolts it's important to do so in a crisscross and stepped pattern so that the head and cylinder do not become warped or distorted in any way.
Remove Head Gasket/O-rings- With the cylinder head removed, you can now remove the head gasket or O-rings depending on what your engine has. If it is a gasket, try to remove as much of it as possible.
Remove Power Valve Cover & Disconnect Linkage- Many engines allow you to access the power valve externally. For these engines, the linkage must be disconnected before the cylinder jug can be removed. To access the linkage, you will need to remove the power valve cover first. To disconnect the linkage, you will need to remove the linkage bolt. Some models have a groove for a holding tool underneath the bolt. This will allow the linkage to stay in place while you remove the bolt, protecting it from being damaged. With the linkage removed, make sure you remove the spacer as well.
Remove Cylinder Nuts, Cylinder Jug & Base Gasket- With the power valve linkage removed, the cylinder jug can now be removed. To start, loosen the cylinder nuts with a box end wrench or a socket and remove them. With the nuts removed, you can now pull the cylinder off of the studs and locating dowels.
You may have to break the cylinder loose by tapping it with a dead blow mallet. If this is necessary, be careful not to hit the cylinder too hard and damaging it. While pulling the cylinder off, it is important to support the connecting rod and piston as it comes free. With the cylinder off, you can now place a clean microfiber rag or plastic bag over the case opening to prevent debris from entering. Next, remove the base gasket, trying to peel as much as can by hand.
Remove Circlips and Wristpin- You can now remove the circlips that hold the wrist pin in the piston. To do so, use a small pick or a needle nose pliers. You can then use a similar sized socket to push the wrist pin through the piston. If the wristpin does not move freely, you may need to use a piston pin puller, which is designed specifically for this task.
Remove Wrist Pin Needle Bearing & Piston- With the wristpin removed, the piston should now come free from the connecting rod. The needle bearing should slide out from the connecting rod once you have the piston removed.
Clean Gasket Surfaces- Now that you have the top end of your dirt bike engine disassembled, you can now completely clean all of the gasket surfaces. It's very important that you remove all of the remnants of the old gaskets so that a tight seal can be created with the new ones. Use a gasket scraper to remove the stuck gasket parts. A razor blade can also be used in a pinch. Be very careful to not gouge or damage these surfaces while scraping the old gaskets off.
It's important that you inspect all of the engine parts that you can now see to check for excessive wear and tear. If you do see some issues inside the case, you might have to do a complete engine tear down to fix the problem. If you let the issues go unfixed, you run the risk of ruining the engine completely, requiring you to do a complete rebuild anyways. Below are the things that you can check while the top end is removed.
Inspect Needle Bearing- A new needle bearing may come included with the top end rebuild kit, but it is still a good idea to check out the old one before you discard it. If the bearing is missing needles or overly damaged, you can expect that there might be damage to the cylinder and head. If the kit doesn't include a new needle bearing, be sure that the old one is completely intact.
Inspect Connecting Rod & Rod Bearings- Take a look at the bore of the connecting rod on the small end and the actual rod itself to check for any pitting or damage. If any of this exists, the connecting rod should be replaced.
Next, check the rod bearings. To do so, you will need the engine to be at Top Dead Center. Now grab the connecting rod and try to move it up and down without turning the crankshaft. If there is any play, the bearings will need to be replaced. You can also check the side play of the rod by inserting a flat feeler gauge between the rod and thrust washer. You can then cross reference that measurement to the specifications given in your service manual.
Inspect Cylinder- You will need to inspect the cylinder for uneven wear, scratches and gouges. First, make sure that the cylinder is absolutely clean. If you have a brush hone, you can use that to remove all of the debris and gunk that might be filling in the cross-hatching or scratches. If the cylinder is plated, look for any nicks. If there are, you will need to get the cylinder re-plated. If it is a sleeved cylinder and there are scratches or gouges, the cylinder can be honed or bored out larger.
If the cylinder checks out good for nicks, scratches and gouges, you can now measure the bore. You will need a t-bore gauge and a micrometer to take the measurements. Calipers can be used if they are all that you have, but they will not give you the most accurate measurement. First measure the top of the cylinder in all directions, then the middle and then the bottom of the cylinder. Make sure that the cylinder is not out-of-round or is wider at the bottom than the top or vice versa. If it is, the cylinder will need to bored out to be fixed. If the bore is uniform for the whole cylinder you can now check your measurements to see if they match up with the allowed tolerances in your dirt bike's service manual. If the bore is too large, you may have to use an oversized piston.
Inspect Cylinder Head- Use a parts washer or a wire brush to clean the carbon and other gunk from the cylinder head. Once it's clean, make sure that there are no scratches, gouges or indents in the head.
Inspect Cylinder Studs & Locating Dowels- Make sure that the cylinder studs and locating dowels are not rusty or damaged in any way. Replace any that are compromised to ensure that your cylinder will stay flush and secure once reinstalled.
Top End Reassembly
Now that you have all of your top end components cleaned, ready and up to spec, you can begin the reassembly process. Pay careful attention so that you do not forget any components while putting the top end back together.
To ensure that all of the parts go back together smoothly and run with minimal friction when the engine is first started up, you should apply a thin layer of 2 stroke oil to all of the moving parts and the inside of the cylinder.
Install Piston Rings on Piston- Before you install the rings onto the piston, you must first check the ring end gap. To do this, place the piston ring into the cylinder by itself, making sure that it is squared. Use a feeler gauge to measure the ring gap. Check to see if the gap is acceptable for that ring. If it is too small, you will need to file the ends.
With the correct ring gap, you can now install the rings on to the piston. Begin by making sure that the side of the ring with any lettering or markings is facing up. Next, place one end of the ring in the bottom groove near the locating pin. Use your thumb to press down and work your way around until all of the ring is in the groove. Do the same with the top piston ring.
Install Needle Bearing, Piston, Wrist Pin & Circlips- To make the process easier, it helps to install one of the circlips on the piston before installing it. You can use your thumb to install the first one. Be careful not to bend the clip and that it is perfectly seated with the ends either facing up or down. Next, slide the needle bearing into the small bore of the connecting rod.
Then slide the piston in place, making sure that it is facing in the correct direction. There is often an arrow on the top of the piston, which should be pointed to the exhaust side of the engine. This is not always the case, though, so check the instructions that come with the piston kit.
You can now slide the wristpin through the side of the piston without the circlip. Once the wristpin is in place you can install the last circlip. It's a good idea to keep the case covered while doing this so none of the parts can fall down into it.
Install Base Gasket & Cylinder- You can now install the base gasket to the cleaned surface of the case. With the gasket in place, you can begin to slide the cylinder over the piston to reinstall it to the engine. Make sure that you have lubed the inside of the cylinder, the piston and the rings with 2 stroke oil before you do this. As you begin to drop the cylinder over the piston, you will need to compress the piston rings with your free hand, making sure that the rings are lined up with the locating pins. Continue slowly by slipping the cylinder straight down until it reaches the case. Carefully torque the cylinder nuts on to the studs according to the service manual specifications.
Install Head Gasket/O-rings & Head- Place the head gasket or O-rings into place on the clean cylinder deck. Next, place the cylinder head on correctly and torque the head bolts to the specifications of the service manual.
Install Power Valve Linkage- Bolt the power valve linkage back into place. Be sure that you do not forget about the spacer if there was one. Then, bolt the cover back on.
Install Spark Plug, Carb, Radiators, Hoses, CDI, Exhaust, Top Motor Mount, Plastics, Subframe & Seat- Continue reassembling your bike with the remaining parts that you needed to remove in the beginning. This step should be fairly straightforward as it is just putting the remaining components back into place.
Fill With Coolant- Don't forget to fill your dirt bike back up with coolant.
Breaking In The Engine
It's very important that you take the time to break in the newly rebuilt dirt bike engine. This process will help seal the piston rings to the cylinder walls and also ensure that all of the parts are properly lubricated before it is exposed to hard work.
There are a variety of accepted ways to break in a dirt bike engine. One of the most popular is the heat cycle. Here is how to do a basic version of this.
Start engine and let idle for ten minutes. Shut off and let the engine cool.
Start engine and run the engine slightly above idle for ten minutes. Shut off and let the engine cool again.
Start engine and ride slowly for ten minutes.
The bike should now be ready to ride as normal.
At this point, it's not a bad idea to perform a compression test to get a new baseline that you can refer to the next time you check for compression.
Your dirt bike should now be ready to rip once again with the new top end installed. Make this a regular process and your bike will last much longer.