Two-Stroke Rebuild

Hot Rod bottom end kit
Here is the Hot Rod bottom end kit we used to rebuild our YZ250 engine. It also came with a Vertex Piston.Adam Booth

Put enough hours on an engine and something will let go. For this YZ250 two-stroke motor it was the crank after 250 hard hours. It made dreaded noises and came to 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 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 engine. We highly suggest having a service manual by your side when working with any engine and good luck!

crusty plug
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
sliding cylinder off
We pulled the head off, loosened the cylinder bolts and disconnected the power valve before sliding the cylinder off.Adam Booth
pushing out wrist pin
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
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
Freeze all the new bearings for at least a day before beginning rebuild, you need them to be cold and a fraction smaller than normal size so they drop into the cases easier.Adam Booth
tool used as clutch basket holder
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
flywheel remover
This little handy dandy flywheel remover is a must. On a two stroke the threads are reversed so don\'t spend your afternoon trying to thread it in clockwise.Adam Booth
removing phillips head screws
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 right crank case cover
Moving to the clutch side, remove the right crank case cover. You can handle this, we trust you.Adam Booth
removing crank case cover
Easy, now.Adam Booth
after removing crank case cover
Up close.Adam Booth
center crank case nut
The clutch basket nut.Adam Booth
removing crank case nut
That cool tool we showed you now comes in handy again to hold the cutch basket so you can remove the clutch basket nut.Adam Booth
perspective of crank case
An alternate perspective.Adam Booth
removing motorcycle parts
Remove it!Adam Booth
removing gears
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
splitting apart cases
Two-stroke cases are much harder to split than four-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 off left case
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
rod bearing rubbed 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 over a $1000 bucks!Adam Booth
pulled out crank bearing seals
Pull out the crank bearing seals, heat and fire is on the way.Adam Booth
torch heating up cases
Fire is dangerous. You are officially warned. Use a little torch or a hot plate to heat up the cases around the old bearings for a couple minutes and pounding out the old bearing will be a lot easier.Adam Booth
beating down old bearing
Using a bearing driver, put the beat down on the old bearing and knock her 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
organized motorcycle parts
Stay organized throughout the build, life will be a lot easier.Adam Booth
add new bearings
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
add new seals
Wait until the cases cool to put in the new seals and use the proper tools if possible to install the seals.Adam Booth
Pulling out the new crank.
Pulling out the new crank.Adam Booth
assembly lube
Lubing it up.Adam Booth
Placing the new crank
Placing the new crankAdam Booth
Hot Rods crank
To get the new Hot Rods crank seated in the case use a crank puller/installer. Don’t hit with a hammer unless you want a mushroomed out crank end.Adam Booth
installing new crank
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
Permatex gasket maker gray
We use Permatex gasket maker gray between the cases, spreading a smooth thin layer.Adam Booth
using a crank puller
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
putting top end back together
Always refer to the service manual for torque specs on bolts and we use the handy Motion Pro gear locker to make it easy keep the internals from spinning. Now get to getting and put all remaining parts on and then work that top end back together. Don’t forget to double and triple check your work and most of all, have fun roosting.Adam Booth
installing stator
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
soaking new clutch plates in oil
Soak the new clutch plates in oil before installing, you’ll thank us later.Adam Booth
install left case cover
Once everything is all 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
motorcycle parts
Another piece to the puzzleAdam Booth
motorcycle part up close
A better view.Adam Booth
It is easier to install one of the circlips with the piston on the work bench so you only have to do one with the piston on the rod.Adam Booth
letter on the ring
Look for the letter on the ring, that faces up when installing on the piston.Adam Booth
ring lined up to locator pins
Line up the rings to the locator pins, double check the clearance of everything.Adam Booth
oil on the rod bearings
Time for two-stroke oil on the rod bearings.Adam Booth
oil on the wrist pin bearing
Two-stroke oil on the wrist pin bearing isn’t a bad idea either.Adam Booth
vertext piston and wrist pin
Vertex piston and wrist pin finding its home on the rod.Adam Booth
lubing up cylinder
Lube up the cylinder before installing the piston.Adam Booth
positioning piston
There is an arrow on top of the piston, that should point towards the exhaust port, in this case, the front of the motor.Adam Booth
sliding on cylinder
With the rings compressed into the piston, located in the located pins, the cylinder will side on effortlessly.Adam Booth
torquing cylinder head nuts
Torque the cylinder head nuts to proper torque and sequence.Adam Booth
new head gaskets
Time for new head gaskets, which come in the Hot Rods kit.Adam Booth
torquing head nuts
Torque the head nuts to proper torque per the manual. So close you can taste it!Adam Booth
finished engine
With the carb back on, 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
rebuilt YZ250
Michael Allen out racing the rebuilt YZ250 at the National Hare and Hound races, racking up the hours on fresh engine. Photo by Mark Kariya.Mark Kariya