Story Scot Gustafson and Adam Booth Photos by 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 (www.hotrodsproducts.com) 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 (www.mt-llc.com). 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!
Here is the Hot Rod bottom end kit we used to rebuild our YZ250 engine. It also came with a Vertex Piston. www.hotrodsproducts.com
The plug looked a little crusty, it should have been a golden brown. We used a new one when putting the engine back together.
We pulled the head off, loosened the cylinder bolts and disconnected the power valve before sliding the cylinder off.
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.
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.
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.
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.
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.
If you have problems removing the Phillips head screws use an impact tool to get them out. If not you will strip them.
Moving to the clutch side, remove the right crank case cover. You can handle this, we trust you.
That cool tool we showed you now comes in handy again to hold the cutch basket so you can remove the clutch basket nut.
Careful when removing the basket and keep inventory on where everything sits and watch for falling washers or bearings.
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!
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.
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.
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!
Pull out the crank bearing seals, heat and fire is on the way.
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.
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.
Stay organized throughout the build, life will be a lot easier.
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.
Wait until the cases cool to put in the new seals and use the proper tools if possible to install the seals.
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.
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.
We use Permatex gasket maker gray between the cases, spreading a smooth thin layer.
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.
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.
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.
Soak the new clutch plates in oil before installing, you’ll thank us later.
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!
We sent the cylinder off to Millennium Technologies and for $199.45 they stripped it, re-plated it and used a diamond hone to bring it up to perfect spec. We sent the Vertex piston along with the cylinder so the two would be matched. If welding or repairs had been needed it would have totaled $262.45. Millennium Technologies; www.mt-llc.com or call (920) 893-5595.
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.
Look for the letter on the ring, that faces up when installing on the piston.
Line up the rings to the locator pins, double check the clearance of everything.
Time for two-stroke oil on the rod bearings.
Two-stroke oil on the wrist pin bearing isn’t a bad idea either.
Vertex piston and wrist pin finding its home on the rod.
Lube up the cylinder before installing the 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.
With the rings compressed into the piston, located in the located pins, the cylinder will side on effortlessly.
Torque the cylinder head nuts to proper torque and sequence.
Time for new head gaskets, which come in the Hot Rods kit.
Torque the head nuts to proper torque per the manual. So close you can taste it!
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.
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.