D-bore sprockets with a single flat will not fit the double D-bore shafts.
We recently added a 9 tooth sprocket with 10mm dual D-bore that fits the Razor MX500, MX650, and SX500 dirt bike motors.
It is our item # SPR-2509C and it is available on our #25 Motor Sprockets Page.
A 9 tooth sprocket for #25 chain has a maximum recommended RPM of 2860 though and this is approaching the speed of the MX650 motor so this 9 tooth sprocket should be used experimentally to see if it works well with the bike or not.
Update Sept. 2020: Several of our customers have now used this 9 tooth sprocket to modify their MX500 MX650 and SX500 dirt bikes and the results have been hit or miss. Some people have reported that the 9 tooth sprocket causes the chain to miss or jump a tooth at high motor speeds and a couple of people have had the 9 tooth sprocket break in half after using them for a little while. Due to this we are not recommending that the SPR-2509 sprocket be used on Razor MX500 MX650 and SX500 dirt bikes and if it is then there is no guarantee of its suitability or reliability for this application. If it is experimentally installed then a new chain should be used with it to give it the highest chance as possible of successfully working on the bike.
Instead of installing a smaller motor sprocket a larger wheel sprocket can be installed to lower the gear ratio. We now make custom wheel sprockets and can make wheel sprockets for the MX500 MX650 and SX500 dirt bikes for #25 chain up to 185 teeth and for 8mm (T8F) chain up to 146 teeth, however, we do not know what the largest rear wheel sprocket size is that will work with the bike's frame and swingarm, so that will need to be determined before ordering. The rear wheel sprocket bore and bolt hole pattern for the MX500 MX650 and SX500 dirt bikes is 4-Hole Freewheel (F4). Here is a link to our custom sprocket price quote form: https://electricscooterparts.com/custom-made-sprockets.html
A smaller motor sprocket will not cause motor overheating. The smaller the motor sprocket is the lower the gear ratio will be and the cooler the motor will run.
The risk of overheating the motor is when larger motor sprockets are installed and the gear ratio is raised higher because that puts more load on the motor.
I ended up overvolting my mx650 with 48 volts. I love it but it still struggles with some hills. I don’t need a high top speed so I installed a 89T rear sprocket. I’d like to get even a little more low end by going with an 8T or 9T front sprocket but I can’t find one that will fit with the dual flat hole. Will the single flat fit? Do you have something close that I can modify?