Rebuilding the top end of a dirt bike may seem like a complicated and daunting task that scares most dirt bike owners. However, how to do top end rebuilding on a dirt bike can be done with a little patience and effort.
What does top end mean on a dirt bike? Top end dirt bike rebuild means redoing the heads on the head houses on your dirt bike leaving the pistons, cylinders, valves and ball bearings are left intact.
Failing to perform top end rebuilding on your dirt bike, you risk catastrophic engine failures.
Top end dirt bike rebuild should be part of regular maintenance such as changing gaskets and filters hence it should be done according to the manufacturer’s recommendation.
With some guidance, top end dirt bike rebuilding should be a relatively straightforward endeavor.
It includes two major steps which are the engine disassembly and the reassembly after fixing the heads.
Related: How to Break in a 2 Stroke Dirt Bike
You will need several tools to perform both functions including service manual, compression tester, sockets, wrenches & hand tools, drain pan, exhaust spring tool, gasket scraper/razor blade and micrometer (calipers), t-bore gauge & flat feeler gauge.
How much does it cost for a top end rebuild? An engine rebuild will cost between $2,500 and $4,000 depending on the cost of parts and labor.
Dirt Bike Top End Disassembly
Dirt bike engines are different and not every step in the list below will apply specifically to your dirt bike.
The following steps are a generic guide to top end dirt bike disassembly:
Clean the Bike
You should thoroughly clean your bike before starting the engine’s disassembly.
If any debris falls into the wrong engine component, it could severely damage the engine and render the rebuild meaningless.
Washing your bike is especially crucial for dirt bikes as they are ridden in harsh environments.
You should also ensure that you plug the dirt bike’s exhaust and air cleaner.
Drain the Coolant
You need to ensure the bike is cool before you start draining the coolant.
You should then put a drain pan under the dirt bike’s water pump and slowly remove the drain bolt.
If you want to drain the coolant faster, slowly open the dirt bike’s radiator cap but you might need to adjust the drain pan’s position to capture all the coolant.
Reinsert the drain plug once you have drained all the coolant.
Depending on Your Dirt Bike Model, You Should Remove the Gas Tank, Seat, Radiator Shrouds and Seat Frame.
To remove some dirt bikes’ carburetor, you need to disconnect it from the engine while the fuel line is still connected to the engine.
You can then move the carburetor away from the bike.
For other dirt bikes you will have to disconnect the carburetor completely.
Regardless of whether it is partially or completely disconnected, you need to ensure that the carburetor is totally out of your way when doing top end dirt bike rebuild.
Remove CDI Box and Radiator
Removing the CDI box and radiator may not be a must but not damaging them will be beneficial for the future.
Moreover, with them out of the way, you have way more operating room to perform your top end dirt bike rebuild.
Using a wrench and exhaust spring tool, remove the exhaust from your dirt bike.
Remove Top Motor Mount
You should also remove your dirt bike’s top motor mount using the same tools as in step 7.
Disconnect Radiator Hose and Spark Plug
While disconnecting the radiator hose from its head, you should have a dirty cloth nearby to capture any remaining coolant handy.
After cleaning the radiator head, disconnect the spark plug cap and remove the plug wire.
Remove the spark plug and then put it far from the bike.
Remove Cylinder Head
You should loosen the cylinder’s head bolts in a crisscross or stepped manner.
The reason is so that the cylinder and the head do not become distorted.
Once you remove the cylinder head, you can subsequently remove the head gasket or O-rings depending on what your engine includes.
Try to remove as much of the gasket as possible if you have one.
Remove Power Valve and Disconnect Linkage
Before you can remove the cylinder jug, you need to severe its connection to the power valve.
Most dirt bikes will have the option to access the power valve externally.
First, remove the power valve cover then remove the linkage bolt.
Some dirt bikes will contain a groove under the bolt that can act as a holding tool.
It will offer protection from damage for the linkage as you remove the bolt. After you remove the linkage, you should also remove the spacer.
Remove Cylinder Jug, Nuts and Base Gasket
With the power valve disconnected, you can now remove the cylinder jug.
You will need a wrench or socket to loosen and remove the cylinder nuts.
Once you remove the cylinder nuts, strongly pull the cylinder jug from its studs and locating bowels.
If tugging at the cylinder does not work, use a dead blow mallet to tap it mildly.
After removing the cylinder jug, support the connecting rod and piston.
It tends to come free which can cause your engine significant damage.
Also, place a rag or plastic bag over the empty case so debris does not stumble into it.
Finally, remove the base gasket trying to use your hand as much as possible.
Remove Circlips and Wristpins
At this stage, you should remove the circlips that hold the wrist pin in the piston using a small pick or pliers.
Subsequently use a socket of equal size to push the wrist pin through the piston.
If the socket does not work, you may have to use a piston pin puller which is a tool designed specifically for this job.
Remove Wrist Pin Needle Bearing & Piston
Once you have removed the wristpin, the piston you should easily come off the connecting rod.
After removing the piston, the needle bearing should just as easily come off the connecting rod.
Clean Gasket Surfaces
At this stage, the top of end of your dirt bike engine is completely disassembled.
Thoroughly clean all gasket surfaces and remove all of the dirt off the old gaskets to create space for a new seal.
You can use a razor or a gasket scraper to clean the gaskets.
However, you need to be careful not to damage the surface of the gaskets while scraping them.
Inspect your engine and ensure all parts are in working order.
If not fix them or buy new ones and then reassemble your top end.
Dirt Bike Top End Reassembly
You should very diligent during top end reassembly so as not to forget any components.
The top end reassembly process is as follows:
Install Piston Rings
Before you install piston rings, check the ring end gap.
If the gap is acceptable for the piston rings, you can install the piston rings and if not, you will have to file the rings to fit the gap.
The side of the rings with letters or markings should be up.
Place one end of the ring at the bottom of the grove near the locating pin and press down with your thumb repeating the process with the top piston ring.
Install Needle Bearing, Piston, Wrist Pin & Circlips
It is easier to install the pistons if you first install one of the circlips on it.
You should then insert the needle bearing into the tiny bore of the connecting rod.
Install the piston in its rightful place ensuring that it is facing the correct direction.
You should check the instructions alongside the piston kit to find out which is the correct way to install it for your bike.
Install the wristpin on the side of the piston that does not have the circlip and once in place, install the last circlip.
Keep the piston’s case covered while doing this so nothing falls into it.
Install Base Gasket and Cylinder
At this stage install the base gasket on the top end case’s cleaned surface then place the cylinder over the piston to reinstall it into the engine.
The engine parts need to be slightly lubricated for smooth movement and correct placement.
As you slide the cylinder over the pistons, press the piston rings with your hands until they are lined up with the locating pins.
Slide the cylinder down until it reaches the case.
You will also need to torque the cylinder nuts on to the studs as specified by the service manual.
Install Head Gasket/O-rings & Head
Place the head gasket or O-rings (according to your dirt bike) on the clean cylinder deck.
Ensure the cylinder head is on correctly and the torque the head bolts as per the service manual.
Install Power Valve Linkage
Connect the power valve linkage and install the spacer if there was one. Bolt the case cover back on.
Install Remaining Parts
Install all the parts you removed in the beginning of the disassembly including spark plug, carb, radiators, hoses, CDI, exhaust, top motor mount, plastics, subframe & seat.
Fill with Coolant
Fill your dirt bike with engine with coolant for proper functioning.
Breaking in the engine after top end rebuilding can be done in a variety of ways.
However, most experts recommend starting the bike and then letting it run for a few minutes several times before riding it.