2 stroke top bottom end rebuild dirt bike

2-stroke top end and bottom end rebuild tips for beginners

In Beginners, Dirt bikes, maintainance by darrellsmith4 Comments

When it comes time for an engine rebuild, many riders pass their dirt bike over to their local repair shop to have the work done. Unfortunately, this can wind up costing an arm and a leg, and it really isn't necessary.

With a few tools and a little knowledge, a 2-stroke top end rebuild can be done by any rider in a matter of hours. Bottom end rebuilds will take a lot longer, but they can also be done by anybody with a couple of days to spare.

In this post we'll look at what tools you'll need, including any special tools that you'll need to order before starting the job. And we'll run through both top and bottom end rebuilds step by step, from start to finish to give you some valuable tips to help you along the way.

Before you start your rebuild


Before you start work, it's important to thoroughly wash your dirt bike. Removing all dirt and grit lowers the risk of any of it falling into your engine, where it could cause damage when the engine is started.

Use a powerful jet wash to get the bike as clean as possible, and then use a bucket of water and a brush to remove any remaining dirt from the engine.


Top end rebuild tools

  • Screwdrivers
  • Metric allen key set
  • Metric socket set
  • Torque wrench
  • Metric spanner set
  • Long nose pliers
  • Plastic scraper (an old credit card will work)
  • Feeler gauge
  • Flat needle file
  • Bore gauge

Bottom end rebuild tools

If you're rebuilding the bottom end I'll assume that you will also be replacing the piston, so you will require all of the above tools, plus:

All of the special tools are essential, and you will likely cause damage to the engine if you try to do the job without them. I use the Tusk crank puller (pictured below) and a Tusk case splitter, they work great and you can buy them as a package on Amazon as I did here > Tusk Tools.

I use an EBC clutch holding tool, and I made my own flywheel holder by welding two pins onto a length of 1/2" square bar. If you don't fancy making your own tool, you can find a tool that holds the clutch basket and doubles as a flywheel holder here on Amazon.

The Tusk crank puller is essential for installing the new crank. The puller can be seen in action below during a recent full engine rebuild of my Honda CR 250 project.

2 stroke top bottom end rebuild crank puller

Before carrying out any maintenance, it's also worth having a workshop manual just in case you get stuck. A top end rebuild is quite straight forward, but a bottom end rebuild gets a little more complicated.

If you can't find a workshop manual for your dirt bike, take pictures on your phone as you remove each part, and write a list so you can simply reverse the list to reassemble the engine.

You're now ready to remove your dirt bike's engine. A bottom end rebuild will obviously require engine removal, but many people leave the engine in while carrying out a top end rebuild.

I advise against this, simply because anything that's still above the engine, like parts of the frame, the fuel tank, hoses, etc, can still be dirty. As your hands brush against these parts, small bits of grit can fall into the engine, and wreak havoc when the engine is started.

Removing the engine from the dirt bike

Some of the tools listed earlier will be used to strip the dirt bike down to remove the engine. Start by draining the water from the engine and radiators, by removing the bleed screw on the water pump.

If you're doing a bottom end rebuild, you'll also need to drain the gearbox oil by removing the drain plug on the bottom of the engine. The more oil that's removed the better, so leave the engine to fully drain for 30 minutes before proceeding.

For bottom end rebuilds, it's also worth removing your clutch cover and loosening the spring bolts, as doing this with the engine out can be difficult. Simply put the bike in gear to lock the clutch and they'll come out easily.

With the oil and (or) water fully drained, and the cylinder nuts loosened, you can now begin stripping parts from the engine. Remove the exhaust, hoses, carburettor, clutch cable, electrical plugs, etc, and be sure to remove the main engine bolts last.

Before removing the main engine bolts, loosen the cylinder head and base nuts. These will be very tight, and can be a pain to undo when the engine is freely moving around your bench.

Keep loose nuts and bolts in a jar, and screw them back into the frame to keep them safe wherever possible. Screwing them back in will also help you to remember where each bolt goes when you come to put the engine back in.

Rebuilding the top end

The engine is out and you can now remove the top end. You should have already loosened the nuts, so take them off and remove the head, and gently slide the cylinder off to expose the piston as below.

2 stroke top bottom end rebuild piston

You can dispose of the metal head gasket and the base gasket, as these can only be used once. The contact surfaces will likely have old pieces of gasket stuck to them, this will need to be removed. Don't scrape it off with a screwdriver as this will damage the soft aluminium surface, use the old plastic credit card mentioned in the tools section.

Before cleaning the surfaces, place a rag into the crankcase to stop any pieces of debris falling in.

The next step is to remove the gudgeon pin, which is the pin that connects the piston to the connecting rod (con rod). This pin is held in place by two clips, one either side, that can easily be removed with long nose pliers. You can remove just one piston clip, and push the gudgeon pin from the opposite side.

The piston will now lift off, and you will see a needle roller bearing (the small end bearing) that you can remove from the con rod. This bearing is cheap enough to buy, so it's worth changing it whenever you do a top end rebuild.

Checking the crank

With the piston removed, you can now check the crank for up and down play. Simply grab the con rod and push and pull it while feeling for any movement. There will always be a small amount of side to side play, but there should be no up and down play whatsoever.

If there is play, then the crank will need replacing, which means a full rebuild is needed. If you discover play, but decide not to replace the crank, you risk a big end failure, which can totally destroy the engine, so don't risk it.

You should also check the top of the con rod where the small end bearing goes, known as the rod small end. This part can wear out, and over time becomes oval. This will cause the needle roller bearing to rattle around, and eventually destroy itself along with your engine.

Measure the rod small end diameter, and check it against the manufacturers recommended tolerances in the owners manual. Also look out for any surface marks or damage, if you find anything suspicious then the rod will need replacing.

Checking the cylinder

You will also need to check the cylinder for imperfections. Even a very mild engine seizure can leave aluminium deposits on the cylinder walls, so look very carefully, as this will lead to problems if left uncorrected.

Muriatic acid can be used to clean of aluminium deposits from the cylinder plating. It won't affect the plating, but be very careful not to get it on any bare aluminium on the cylinder, as it eats up aluminium very quickly. After you have used Muriatic acid, thoroughly wash the cylinder with soapy water.

If the Nikasil coating is damaged, the cylinder may need replating. If so, search online for a trusted cylinder repair shop to carry out the required work.

Do you need to hone the cylinder?

Many people will tell you that a cylinder must be honed before replacing the piston. This may be true for older dirt bikes that aren't using a Nikasil plated cylinder, but there is nothing to be gained (and much to lose) by honing a Nikasil cylinder, so don't do it.

Replacing the piston

If you're also rebuilding the bottom end, these steps should be left until last.

To find the correct size piston, you will need to measure the cylinder with a bore gauge. You will find a range of oversized options from all piston manufacturers, and your measurement will need to correspond to one of their sizes. If you can't find the size you need, it's best to give them a call and they will tell you which piston the buy.

When it comes to installing the new piston onto the con rod, you basically reverse the process of removing the old one. Be sure to rub plenty of 2-stroke oil into each new part as you install it, this pre-lubricates it ready for the first start.

Gapping the piston ring

Before you can fit the ring onto the piston, you will need to set the end gap. To do this you will need a feeler gauge, and a small needle file.

The reason for setting the gap is to allow room for expansion, so that the two ring ends do not hit one another and break. At the same time, the gap mustn't be too big as this will cause blow-by, which will reduce compression.

Every manufacturer will recommend a different ring gap, so you will need to look up the measurement for your particular engine; or refer to the replacement piston manufacturers recommendation.

When you have the correct measurement, insert the piston ring into the cylinder, and use the feeler gauge to check the gap. If the gap is too small, you can remove the ring from the cylinder and gently file the ends to increase the gap. You can then reinsert the ring and check again. Do this until the gap is correct, and be very careful not to file off too much of the ring.

When you have the gap set, the ring can be fitted to the piston, but bare in mind that it will need to go the correct way up. The side that has any lettering on will face up, so that the tapered angle faces down. If the ring is installed the wrong way up, it will feel tight, and it will stick in the piston's ring groove, so check that it moves freely before proceeding.

Which way around does the piston go?

The piston will need to be installed with the arrow (marked on top) facing towards the exhaust port. This is done so that the rings join at the rear of the piston, to prevent them expanding and snagging on the exhaust port. Look at the ring groove on opposite side to the arrow, and you will see a pin, this is where you'll place the end gap. This will be the same for all 2-stroke engines.

When the piston is installed, check that both piston clips are correctly seated into their grooves, and that the gap isn't lined up with the dimples.

Replacing the cylinder

Before replacing the cylinder it's always worth servicing, or at least cleaning your power valve. These valves are located in the exhaust port, and they can get very dirty, which causes them to stick.

The first thing to fit is the new base gasket, and you can coat the gasket in grease to help it seal, and to help it come off the next time you remove the cylinder. Or you can use Hylomar blue non-setting gasket & jointing compound; this is used and trusted by manufacturers like Honda. Simply place the gasket over the studs.

Rub a good amount of 2-stroke oil around the cylinder bore and piston before proceeding.

Carefully squeeze the piston ring into the ring groove, making sure the end gaps are in place over the pin, and lower the cylinder over the piston. This should feel smooth, and the cylinder should move down over the piston easily. If you feel and resistance, stop and check that the ring is located correctly.

You can now replace the cylinder base nuts, and tighten them to the specified torque. Now place the head gasket onto the cylinder, this gasket doesn't require any grease or sealing compound. Place the head on, and progressively tighten the bolts down to the correct torque in the order shown below.

2 stroke top bottom end rebuild head
Rebuilding the bottom end

All dirt bike bottom ends vary, so this section will just include the most important tips and helpful information.

An hour before you start stripping your engine, take your new main bearings and place them in the freezer. The reason for this will be explained later.

When you remove the casing on the right side of the engine, the first thing to remove is the crank gear. I recommend removing this gear before the clutch, as there is a little trick you can use to loosen the nut, which requires help from the clutch. This trick works well for me, and you can watch the video I found below to find out how it works.

When the crank gear is off, check to see if there are any other gears that can be removed with this method. You can then remove the clutch plates and clutch basket. If you followed the advice earlier, you will be able to remove the clutch spring bolts without tools, as you already loosened them before removing the engine.

Keep all of your clutch plates in order as you remove them from the basket.

To remove the main nut that is holding the clutch basket, you will need to flatten out the locking tab washer. This can be done with a hammer and a flat head screwdriver.

2 stroke top bottom end rebuild clutch

Removing the nut will require your clutch basket holding tool, these tools grip the clutch basket to stop it turning while you loosen the nut.

Flywheel removal

You can now move around to the other side of the engine and remove the flywheel, this will require the flywheel holding tool and a socket wrench. When the main nut is removed you will need to use the flywheel puller to release the flywheel from the tapered shaft. When the flywheel is off, be sure to grab the small key from the shaft, and keep it safe by sticking it to the flywheel magnets.

Split the cases

When it comes time to split the cases, you'll find that they are very tightly fused together, and you will most definitely need a case splitter to avoid damaging the cases. The Tusk case splitter will make the job a breeze, and it comes with full instructions, or you can find plenty of helpful videos online.

After splitting the case, the crank will still be held on one side. You can simply move the case splitter to the other side to push it out.

Removing the oil seals and main bearings

To remove the old crankshaft oil seals, you should avoid the common mistake of levering them out with a screwdriver. If the screwdriver catches the surface it can cause damage to the aluminium, which may stop the new seal working, or even damage it as soon as it's installed.

A better way to remove old seals is to use a very small drill to make a hole in the seal, and then screw a self tapping screw into it. You can now use the claw hammer to pull on the screw, which will easily remove the seal.

To remove the main bearings you will need to heat the case with a heat gun, or a blow torch. The aluminium will expand more than the steel ball bearing, making it easier to remove them.

If you're using a blow torch you will need to be very careful not to heat the cases excessively, or you risk warping them. Heat all around the bearing to 200 degrees Fahrenheit, and then tap the bearing with a large socket and a hammer to remove it.

Replacing the main bearings and oil seals

An hour before you started stripping the engine, I told you to put your new main bearings into the freezer. This is done to shrink the bearings to make them fit into the heated (expanded) cases easier.

If you're fast, the case will still be hot enough to let you drop the bearing straight in, they rarely need tapping. If you find the bearing doesn't quite go all the way in, be fast before heat is transferred to the bearing, and give it a good tap to seat it. When tapping the bearing, be sure to only hit the outer race, not the inner, as this can damage the bearing.

The seals are very easy to install by tapping them into place with a socket and a hammer, but be careful not to tap them in too far.

2 stroke top bottom end rebuild main bearings

Installing the new crank

To install your new crank you will need to use your crank puller to pull it through the bearings. Never try to tap it through with a hammer, as this can knock it out of alignment, causing it to become tight in the bearings.

If you buy the tusk crank puller, you will find some great instructional videos online, and it comes with full instructions to help you out.

Finishing up

When the cases are back together, it's simply a matter of putting everything back where it came from. Don't forget the flywheel key, and remember to bend the tab back up on the clutch tab washer.

Tighten every nut and bolt back up to the specified torque, and remember to clean your contact surfaces so that the casing seals against the gaskets. Lastly, remember to fill the gearbox with oil, and the radiators with coolant before you use the engine, you'd be amazed at how many people forget.

Thanks for checking out these top and bottom end rebuild tips, any more tips can be added below in the comments to help other readers.

Find out all of the differences between 2-stroke and 4-stroke engines HERE.

Check out some awesome accessories for your dirt bike HERE.

Or return to the homepage HERE.


  1. Hi,

    I plan to replace my crank with a complete crank/rod setup from hot rods. In your procedure there is no mention of what happens with the transmission during the case splitting process. I’m wondering what the go is with the transmission components when splitting the cases to replace the crank? Does the transmission all stay in situ while the cases are being split? Are there any things to watch out for with the transmission during this process such as components coming apart that are tricky to put back together?


    1. Author

      Hi Adam, you’ll need to take the transmission out. You can usually take each shaft out without removing the gears, but be careful not to let them slide off the shaft as you’ll never remember how it all goes; unless you have a manual of course.. But even then it can be tricky as you’ll have spacers between each gear that will also need to go back in the right place.

  2. Checking ring end gap involves inserting the piston ring into the bore and using feeler gauges to determine how large of a gap there is. You should compare your measurement to the spec outlined in your owners manual or piston instructions.

  3. i will be soon be starting a rebuild on my 1999 rm125 first time rebuilding a bottom end any tips or tricks i shpuld know before getting started

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