How To Build A Stock Moto Engine
Part III: Teardown

Ok, you've found yourself a suitable CR125 for the Stock Moto conversion. This write-up is based on the 1999 CR125, but the steps involved are applicable to all years. If you use a 1998 engine, you'll need to sell the ignition (CDI and stator) and buy 1999 components. If you use a 2000-2002 CR125, you'll need to swap the ignition and the head/cylinder.

Here's my '99 CR125 engine. I paid $465 shipped for this engine, but it did not include a carburetor. That's ok because the stock carburetor isn't legal anyway. At this point, I've remove the spark plug and power valve (Honda Power Port aka HPP) covers.
Start by disconnecting the HPP actuator arm. This can be done by removing the clip on the far right. Once the circled items are removed, you can pull the HPP assembly out of the cylinder.
Here is what the cylinder will look like after the HPP assembly has been completely removed. The hollow "slot" on the left side will be filled by a power valve plug.
With the HPP assembly out, we turn our focus to removing the ignition. For this, I recommend the flywheel holder. It makes this job a total breeze.

Universal flywheel/sprocket holder
Honda P/N: 07725-0030000
Sources: Honda of Troy, eBay
Cost: $20-$40

To remove the flywheel, you will need the Honda (or universal) flywheel puller. This is one tool that you absolutely must have.

Flywheel Removal Tool
Honda P/N: 07733-0010000
Source: Honda of Troy, eBay
Cost: $15-$25

The flywheel holder has small studs on one side for holding the flywheel. If you turn it over, it has large studs for holding a sprocket. I removed the factory sprocket.
With the sprocket removed, pull of the collar that is on the end of the countershaft. If you do not remove this collar, the cases will not properly separate.
With the ignition out, I turned my attention to removing the head and cylinder. I removed the head so that I could inspect it and was pleased with the results. My seller had sent me several pictures of the head, but it's always nice to see it in person.
After removing the 4 12mm nuts that secure the cylinder to the cases (do not pry the cylinder off with a screwdriver! Tapping with a plastic or "dead blow" hammer will usually do the trick), I removed and had a look at the bore. I was again pleased with what I saw. If your engine is a 1990-1997 or a 2000-2002, it won't really matter what you see since you'll be selling the head and cylinder to buy a '98/'99 head/cylinder combination. The cylinders are on national backorder, but John @ Fastech Racing has them in stock.
I flipped the cylinder over to have a look at the ports. Everything looked great! No grinding has been done to any part of the cylinder.
Perhaps more important than a stock cylinder are stock cases. Replacing a set of cases will run you around $400. Cases cannot be modified in Stock Moto. These cases are bone stock.
Before I put the cylinder away for awhile, I cleaned it up. This is what 5 mins of scrubbing in hot water and Castrol Super Clean will do.
Here is a quick peek at the SwedeTech power valve plugs that we'll be installing later. SwedeTech's plugs use an o-ring to seal. This makes them one of the best choices on the market.

SwedeTech power valve plugs
Source: Fastech Racing, SwedeTech Racing Engines
Cost: $65

Here is the reed cage assembly removed. Honda used carbon fiber reeds in the CR125. They perform well and last a long time. Also notice that the reed stoppers are missing. This seems to be a common mod in the MX scene. They can be replaced for $15 or better yet, ask your seller to include them (assuming they were removed).
It's not necessary to remove the outer clutch cover to get access to the clutch, but I removed it along with the water pump cover so that I could keep everything bagged. The long shaft that goes through the cases is used to engage/disengage the clutch. The cap on the end has a little steel ball inside of it. Be careful not to lose it!
Here is the clutch pack. 7 disks and 6 floaters. This thing was designed to handle some serious abuse. Most times, the clutch is in good shape and all it really needs is a set of springs.
Now it's time to remove the clutch basket bolt and the drive gear nut on the end of the crankshaft. I try to avoid using an impact wrench if possible since it can damage the threads of the previous owner went crazy with thread sealant. Sit the engine in your motor mount and put the motor mount bolts in (no need to tighten them). Clamp the engine onto your chassis.
This is my trick for removing the clutch basket nut. Put the transmission in 5th gear to minimize the torque multiplication through it. The gear holder will rest against the axle as you remove the nut from the other side.
After the clutch basket nut's retaining washer has been flattened, use 27mm socket to remove the nut. Sockets this large are common in impact socket sets.
A nickel wedged between the clutch basket gear and drive gear will allow you to loosen the 12mm drive gear nut.
With the clutch basket nut removed, the clutch basket slides right off. Make sure you grab the needle bearing and collar, too. Remove the drive gear nut and the collar behind it. With those things out, the kickstartar gears are ready to come out. We will not be re-using these. Although they are of little value to a karter, you can expect to sell this stuff for about $25 on eBay.
The last step before splitting the cases is to remove the drum shifter assembly. Be careful when removing it since there are two very small springs packed in. Go nice and slow so that these don't fly off into some corner of the garage where you'll never find them again.
The tool used to split the cases is a luxury, but not mandatory. Once you build your Stock Moto engine once, you will not need this tool any more since the crank will "slip fit" into the bearings. If you do not have this tool, you can usually get the cases apart using 10 M6x100mm bolts. Pull all 10 case bolts out and screw the 10 10mm bolts in as far as they will go (this is important to keep the threads from getting damaged). Tap with a plastic hammer. The cases will usually come apart.

If you would prefer to buy the tool, here is the info:
Universal Case Separator
Honda P/N: 07937-4300001
Sources: Honda of Troy, eBay
Cost: $25-$50

As the big bolt on the tool is slowly turned, I lightly tap the countershaft with a plastic hammer to keep the cases separating evenly. As mentioned before, that collar on the countershaft must be removed or the cases will not separate.
Here we are after the halves have been separated. Pull the shift fork rods out and toss them into a Ziploc bag. Next, remove the forks themselves. Put zip ties or rubberbands on the ends of the gear shafts to hold everything together. Tip the case up on its side and then pull the shafts out.

The last thing we have to do before diving into the gearbox conversion is remove the crank from the right-side case half. This can be done with either a hydraulic press or if you're lucky, you can tap it out with plastic hammer. Soaking the bearing collar with PB Blaster generally seems to help free the crank up. As a last resort, you can simply knock the crank out with a hammer, but you should not do this unless the crank is known to be junk. A good used crank will fetch you at least $50, so it's worth the effort to properly remove it.

Introduction/Search Criteria Rebuild Overview Teardown Gearbox Conversion Assembly Finishing Touches