With any type of motor work that increases torque/power output, the risk of 3rd gear destruction rises. Unfortunately, this often results in catastrophic motor damage as the small pieces are thrown around the crankcase. There have also been reports of this occurring on stock bikes with heavy track use.
There are 3 options to prevent this issue, which are all cheaper than trying to salvage a motor with shattered transmission.
- replace entire transmission with HRC hardened, micro-finished, close-ratio unit ($$$$)
- replace entire transmission with Nova Racing aftermarket unit ($$$$)
- replace 3rd gear with Nova Racing after market unit ($)
Some issues for HRC replacement are the need to use a speedometer correction device. This is because the gear read by the VS sensor has fewer teeth on the HRC transmission so the speed will always read 31.8% lower than actual. Also, the HRC transmission limits the top speed of the bike to 170mph with stock gearing, but a built motor will pull to 200+ mph. Unfortunately, this requires 17/38 gearing with a 114 link chain to maintain stock wheel-base. 38 sprocket rear is not readily available except for a 520 aluminum PBR.
An alternative is to replace 3rd gear with 33 tooth Nova Racing equivalent but replace 1st with a 31 tooth (vs OEM 32 tooth) gear. The taller 1st gear reduces the large gap from 1st to 2nd so it's the same as 2nd to 3rd.
-1 tooth 1st gear (output shaft)
Here are the parts I ordered overseas. Also, I recommend replacing the output shaft seal. Shipping took 2-3 days via DHL.
Disassemble the motor to the point where the crankcase halves are split, and remove the larger set of transmission gears that the front sprocket bolts to. Both gears to be replaced are on the output shaft, so we do not need to tear down the smaller set of gears next to the shift drum. Service manual does not go into detail on how to tear down transmission, so I have outlined the procedure.
The 1st time I did this, I tried several different sets of snap-ring pliers and had to put up with the constant frustration of snap-rings slipping off our pliers or flexing sideways on the shaft. Issue is snap-rings are right up against a flat surface and have to be lifted with the snap-ring pliers to come off. I recommend the Channel-Lock 927 that has removable tips. Take the copper colored ~1mm tips and grind down the tip so it's tapered as shown. This allows the pliers to engage snap-rings that are flush against the gears AND
apply lifting force without slipping off.
Place the shaft in a vise (aluminum soft jaws) so it's straight up/down. I have labeled the gears for reference.
The ball bearing, washer, 1st gear + needle bearing, washer, and 5th gear slide off without any tools. Note the orientation of all the washers/snap-rings.
Next, we encounter a snap-ring. These snap-rings do not have holes and are flush against a flat surface underneath, which is why it's so important to use pliers that have a taper at the tip. Gently expand the ring while keeping the ring level with your other hand and lift the ring out of its groove. The goal is to expand the ring as little as needed. Once the ring is out of the groove and level, lightly expand the ring and slide it off the shaft.
This is as far as we need to disassemble the transmission. All the snap-rings in the rest of the transmission can be removed with the same process. I cannot emphasize the importance of tapering the tips
of snap-ring pliers enough. If you try to use straight tipped snap ring pliers, it will constantly slip off and drive you insane.
Transmission can be assembled using aftermarket 3rd gear and 1st gear from Nova Racing. Compared to the OEM 3rd gear, the NR gear is thicker in several critical areas.
|OEM 3rd Gear||Nova Racing 3rd Gear|
|Inner Diameter (*)||64.1mm||61.7mm|