2 Stroke Engine Rebuild

Hot Rods’ complete bottom end rebuild kit for 2 stroke bottom end rebuilds.
This is the Hot Rods bottom end kit we used to rebuild our 2 stroke engine. It also came with a Vertex Piston.Adam Booth

Put enough hours on a 2 stroke engine and something will let go. For this Yamaha YZ250 motor, it was the crank after 250 hard hours. It made dreaded noises and came to a skidding stop in the middle of the California desert. We feared catastrophic failure, but in the end, it was simply roasted crank bearings.

We contacted Hot Rods and ordered up a bottom end kit, which comes with the crankshaft, main bearings and seals, and a complete gasket set. It also included a Vertex piston and rings. We sent the cylinder off to Millennium Technologies for a re-plate. Here is a look at our rebuild of the 2 stroke engine. We highly suggest having a service manual by your side when working with any engine and wish you good luck!

Rebuilding A Top And Bottom End On A 2 Stroke Engine

Spark plug showing the sign of severe fouling, with a crust on it, removed during a 2 stroke engine rebuild.
The plug looked a little crusty; it should have been a golden brown. We used a new one when putting the engine back together.Adam Booth
Disconnecting the power valve on our rebuild.
We pulled the head off, loosened the cylinder bolts, and disconnected the power valve before sliding the cylinder off.Adam Booth
Pushing out the wrist pin that holds the connecting rod and piston together.
The old piston wasn’t too bad, which is good because we figured the worst when the engine locked up. Here we are pushing out the wrist pin.Adam Booth
Smashed rod bearings being inspected during a 2 stroke bottom end rebuild.
Holy smokes, the rod bearings were smashed square! There was a ton of play up and down on the rod, but thankfully it didn’t grenade into pieces.Adam Booth
Freezing new bearings before the engine rebuild helps them fit into the case easier.
Freeze all of the new bearings for at least a day before beginning the rebuild. You need them to be cold and a fraction smaller than normal size so they drop into the cases easier.Adam Booth
A tool used to hold the clutch basket and it allows you to grab and hold the flywheel for removal of the center nut.
This tool comes in handy; it doubles as a clutch basket holder and has pins on the opposite side that allow you to hold the flywheel so you can remove the nut.Adam Booth
A flywheel remover tool that takes the hassle out of the job.
This little handy-dandy flywheel remover is a must. On a 2 stroke, the threads are reversed so don’t spend your afternoon trying to thread it in clockwise.Adam Booth
An impact tool helps remove screws that are torqued down tightly, without stripping the head.
If you have problems removing the Phillips head screws, use an impact tool to get them out. If not, you will strip them.Adam Booth
Removing the clutch cover to reveal the clutch pack.
Moving to the clutch side, remove the right crankcase cover. You can handle this; we trust you.Adam Booth
Removing the friction and drive plates from the clutch basket.
Take care when removing the clutch plates.Adam Booth
After removing the clutch plates, inspect the clutch basket for any abnormal wear.
Up close, we are looking for any unusual signs of wear on the clutch basket.Adam Booth
Loosening the clutch basket nut.
Loosening the clutch basket nut.Adam Booth
Holding the clutch basket with a tool to loosen and remove the clutch basket from the engine case.
That cool tool we showed you now comes in handy again to hold the clutch basket so you can remove the clutch basket nut.Adam Booth
The underside of the clutch basket on a 2 stroke engine.
An alternate perspective of the clutch basket.Adam Booth
Removing various washers, gaskets, and nuts during a 2 stroke engine rebuild.
Remove it and make sure you remember where you took it from.Adam Booth
Removing the starter and drive gears from the engine case.
Don’t be afraid to reference your service manual or online manuals when removing the starter gear and drive gears. Out comes the shift shaft!Adam Booth
2 stroke case splitting tool makes the job easier.
Two-stroke cases are much harder to split than 4 stroke cases and that’s why tools like this were invented. A little tip is to tap around the cases with a rubber mallet as you crank on the case splitter to help the cases split apart.Adam Booth
Lifting the left side case after it’s split.
As you lift the left case off, watch for the crank and try not to knock the transmission around too much. It is way easier if you take the transmission out as one cluster and keep it that way.Adam Booth
Signs of the rod bearing having too much play and rubbing the engine cases.
Because the rod bearing had so much play it started rubbing the cases. Luckily it didn’t do much damage because the cost of Yamaha YZ250 cases can run more than $1,000!Adam Booth
Removing the crank bearing seals to have access to the crankshaft bearings.
Pull out the crank bearing seals; heat and fire are on the way.Adam Booth
Use a little torch or a hot plate to heat up the cases around the old bearings for a couple of minutes and when you pound out the old bearing; it’ll be a lot easier.
Use a little torch or a hot plate to heat up the cases around the old bearings for a couple of minutes and when you pound out the old bearing; it’ll be a lot easier.Adam Booth
Pounding out the crankshaft bearings using a bearing driver and hammer.
Using a bearing driver, put the beatdown on the old bearing and knock it out of there. If you have a press, it is easier and less violent than using a hammer. With lots of heat, the bearing won’t be too hard to remove.Adam Booth
Staying organized when you take apart a 2 stroke engine makes putting it back together easier.
Stay organized throughout the build; life will be a lot easier.Adam Booth
Add the new crankshaft bearings to the heated cases.
The new ice-cold and shrunken bearings will drop right into place without force after heating up the cases. We heat the cases for at least five minutes and place the bearings in a freezer overnight. You might have to lightly tap the bearing in, but done right it will drop right into place.Adam Booth
Install the crankshaft bearing seals when the cases have cooled.
Wait until the cases cool to put in the new seals, and use the proper tools if possible to install the seals.Adam Booth
A new crankshaft from Hot Rods for a 2 stroke bottom end rebuild.
Pulling out the new crank.Adam Booth
Lubing up the new crankshaft before it is installed in the rebuilt engine.
Lubing it up before it finds its home.Adam Booth
Installing the new crankshaft in the engine.
Placing the new crank.Adam Booth
When installing a crankshaft properly, use a crank puller to ensure proper fitment.
To get the new Hot Rods crank seated in the case use a crank puller/installer. Don’t hit it with a hammer unless you want a mushroomed-out crank end.Adam Booth
Reinstall the transmission on the Yamaha as a contiguous unit, making sure that everything is there before moving on.
The Yamaha is more tricky than other transmissions so putting it in as one complete unit is the best way. Use assembly lube on all contacting points.Adam Booth
Install a factory gasket or use a gasket maker in between the two sides of the cases.
We use Permatex gasket maker gray between the cases, spreading a smooth thin layer.Adam Booth
When putting the cases back together, make sure to use the crank puller. It will make ensure the crank is fully seated and won’t stress the bolts.
It is important to use the crank puller/installer when putting the cases together. The last thing you want to do is use the case bolts to try and pull the cases together without the cranks fully seated. No bueno.Adam Booth
Check your torque values as you tighten everything in your 2 stroke engine rebuild.
Always refer to the service manual for torque specs on bolts. We use the handy Motion Pro gear locker to make it easy to keep the internals from spinning.Adam Booth
Reinstall the stator in the same position that you took it off.
When installing the stator, line it up in the same position you took it off. If in doubt, refer to that manual we’ve been telling you about.Adam Booth
Before reinstalling the clutch plates, make sure to soak them in oil so that they absorb it and aren’t dry when you start the bike for the first time.
Soak the new clutch plates in oil before installing; you’ll thank us later.Adam Booth
Install the left case cover on your 2 stroke engine, and torque the bolts to spec before installing the top end of your rebuild.
Once everything is back to normal, install the left case cover, torque the bolts, and get ready to install the top end. The end is approaching!Adam Booth
Gather the remaining pieces for your 2 stroke top end rebuild.
Another piece to the puzzle.Adam Booth
The inside of a 2 stroke single-cylinder engine.
A better view of the cylinder.Adam Booth
Installing the circlip inside the piston before putting it on the connecting rod.
It is easier to install one of the circlips with the piston on the workbench so you only have to do one with the piston on the rod.Adam Booth
Find the letter on the piston ring to ensure which side should be facing up.
Look for the letter on the ring; it faces up when installing it on the piston.Adam Booth
Line up the rings on the locator pins on the piston before you install it to the connecting rod.
Line up the rings to the locator pins and double-check the clearance of everything.Adam Booth
Lube the rod bearings with some 2 stroke oil.
Time for 2 stroke oil on the rod bearings.Adam Booth
Lubricate the wrist pin bearing with some 2 stroke oil to aid in the installation to the connecting rod.
Two-stroke oil on the wrist pin bearing isn’t a bad idea either.Adam Booth
Assembling the 2 stroke piston to the connecting rod in a top end rebuild.
Vertex piston and wrist pin finding their home on the rod.Adam Booth
Lubricate the cylinder before installing the piston and the rest of the top end.
Lube up the cylinder before installing the piston.Adam Booth
Make sure the arrow on top of the piston is pointing toward the exhaust port.
There is an arrow on top of the piston; that should point toward the exhaust port, in this case, the front of the engine.Adam Booth
Assembling the cylinder in the 2 stroke top end rebuild.
With the rings compressed into the piston, located in the located pins, the cylinder will slide on effortlessly.Adam Booth
Make sure to torque the cylinder head nuts in the proper sequence as noted in your service manual.
Torque the cylinder head nuts to proper torque and sequence.Adam Booth
Install your head gaskets before sealing the top of your cylinder.
Time for new head gaskets, which come in the Hot Rods kit.Adam Booth
Install the top of your cylinder and torque the head nuts to proper spec.
Torque the head nuts to proper torque per the manual. You’re so close to finishing that you can taste it!Adam Booth
A top and bottom end rebuild on a 2 stroke engine.
With the carburetor back on and oil in the transmission, this engine is ready to go; just like it did when it was new with the help from Hot Rods, Millennium Technologies, and Vertex.Adam Booth
With the 2 stroke engine rebuilt, your bike is back out and racing in competitions.
Racing the rebuilt YZ250 at National Hare & Hound races, racking up the hours on the fresh engine.Mark Kariya