How To Build A Stock Moto Engine
Part IV: Gearbox Swap

The gearbox swap sounds intimidating, but it is actually quite simple. You will need the 6-speed tranmission parts from a '90-'97 CR125. Later is generally desired since the gearbox will have less wear. The parts needed are:

Mainshaft with all gears
Countershaft with all gears
Shift Forks
Shift Fork shafts
Shift drum
Shift drum head

Where do you find one? I've had best luck with eBay. Once the 5 speed transmission comes out, I put it up for sale on eBay with a note that I'm willing to trade it for an earlier transmission. The 5 speed is considered an upgrade for many MX riders, so it's not uncommon to find someone who is willing to trade with you. Another way to get one is to find a seized up '90-'97 CR125. I've seen them sell for $250-$300. All you're really after is the transmission, so if the rest of the engine is blown up, that's not a big deal. Even if you pay $350 for a blown up '90-97 CR125, you'll easily get $125-$150 out of your 5 speed transmission. So, you're looking at $200-$225 for the 6 speed transmission if you go this route. In general,

Best Case Scenario: $0 (trade your 5-speed for a '90-'97 6-speed)
Worst Case Scenario: $250 (buy a blown up '90-97 CR125 + sell your 5-speed)

The first order of business is to remove the shift drum. Because Honda ships the shift drum bolt with sealant already on it, it will typically take a lot more than the specified 16 ft-lbs to break this bolt loose. I've found that removing the bolt is usually pretty easy if done with an impact wrench. You will need to hold the drum with one hand (cover it with a rag).
Here's the complete 5 speed gearbox that will fetch $125-$175 on eBay.
Here's a look at the 5 speed next to the donor 6 speed transmission from a 1997 CR125. Note that several of the 5 speed gears are wider to take up the space of the "missing" 6th gear.
So you've located a good 6 speed transmission. It is good practice to disassemble the shafts and make sure that everything is in working order. As long as fresh oil is kept in these transmissions, they generally will last forever.
The first thing I check is the condition of the shift forks. If there is any abnormal wear in the transmission, a bent shift fork is a likely suspect. Laying them on a flat surface and using a feeler gauge will allow you to determine whether or not they are straight. After checking for straightness, measure the thickness of the tips of the fork. The standard thickness range is .194 to .197 inches. If the fork is any thinner than .190 inches, it will need to be replaced.
For reference, I write the thickness of the fork tip on every fork. Pictured is a good set of forks along with a fork that came out of damaged transmission. The damaged fork is not suitable for use.
The first shaft to be disassembled is the countershaft. It is the more complicated of the two shafts, so we'll knock it out first.
The countershaft 2nd and 6th gears slide off the left side of the shaft. Inspect the thrustwashers, gears, and collars for signs of damage. As long as the proper amount of oil was kept in the gearbox, these parts are generally fine.
From the right side of the shaft, remove the countershaft 1st and 5th gears. Make sure all thrustwashers are present. Inspect the collars, gears, gear "dog ears", and thrustwashers.
3rd and 4th gear are removed after removing this clip. Using a pair of snap ring pliers, spread the clip just enough to pop it out of the groove. Do not over-extend the clip or you will ruin it. Replacement clips are about $5.00 each. It is good practice to replace them, but if you over-extend them, new ones will not do you any good.
Before reassembling, I usually thoroughly clean all of the parts with very hot water and 409 or Castrol Super Clean. For reassembly, a good moly-based assembly lube works well, but you can use engine oil if you don't have any assembly lube. Reassemble the countershaft using the service manual as a guide and pay close attention to the orientation of the clips and washers.
Next we will inspect the mainshaft. Since 1st gear is cast into the mainshaft, everything slides off of the left side of the shaft.
With 2nd gear and 6th gear removed, inspect the 6th gear collar, dog ears, bearing surface.
Here is a look at two mainshaft 6th gears. The gear on the right came from another transmission. It is damaged and would not be suitable for use. The edge of the gear could be sanded down at a 45 degree angle, but it would make the contact area very small.
The mainshaft has two of spring clips like this one. They are even smaller than the one on the countershaft and easier to mess up. Use a pair of good snap ring pliers and spread the clip just enough to pull it out of the groove.
Once the clip is popped out of the groove, use the 3rd/4th combo gear to slide it off of the shaft. The shaft is made of hardened steel. You won't hurt it.
With the 3rd/4th combo gear removed, you can remove the clip that holds the mainshaft 5th gear in place. Thoroughly inspect and clean everything.
I used new clips and thrustwashers in this transmission. You generally do not need to do this, but I had these lying around.
Use the factory diagram to reassemble the mainshaft. Pay attention to the orientation of the clips and thrustwashers.
As a way of double-checking my work, I like to install the gearbox and shift through all of the gears. Testing this way will help you spot any potential issues before you have everything put back together. At that point, it is much more work to disassemble and fix something!

The whole shift drum head assembly needs to be installed. Since the little springs inside the head assembly can launch parts across the room, be careful when installing them! Use assembly lube or oil prior to installation.

Here's the drum head assembly fully installed. Make sure all four 10mm bolts are torqued to spec.
Install the gearbox assembly into the right-side case half. Hold the two shafts together so that all corresponding gears interlock. The shafts will slide right into the bearings. Put assembly lube on the tips of the shift forks and then install them. On the mainshaft, there is only one fork (marked "C"). The countershaft has two forks (marked "L" and "R"). The fork marked "R" goes into the case first. After the forks are installed, lube up the fork shafts and install them. You will feel them bottom out in the case when they are fully inserted.
You can use a "J" arm or the factory shift arm to test the gearbox. The factory arm is easier to use due to the amount of leverage. Keep pressing down on it until the transmission is in first gear. Spin the mainshaft from the clutch side. The countershaft should spin nice and smooth with no binding. Shift through all of the gears and verify that everything is smooth.

If your gearbox is tight or doesn't want to spin, you probably forgot to install a thrustwasher or you're using one of the wrong thickness. If the shafts spin and then stop, you likely have a gear with some damaged teeth.

Put everything away for now. If the transmission will sit for awhile, coat everything in oil.

With the 6 speed transmission inspected and ready to go, it's time to start putting everything back together.

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