Tuning and Modifications: Installing new rear gearing


1.  Let me first start with the fact that I am NOT a mechanic.  I have been working on motorcycles and scooters for a long time and know a little about what I am doing.  I simply take pictures for other people to use as a guide line to help them modify their own motors.  Any modifications you perform are done completely at your own risk and liability.

2.  Tampering with your motor in any way may void your warranty.  Make sure this is a sacrifice you are willing to make before you begin working on your own vehicle.

3.  Before you begin, make sure you have a nice clean area to work in.  A small amount of dirt can be a big problem when working on the innards of your motor.  Keep your hands, motor, and any other parts you are handling very clean.

4.  This example was performed on a 2002 Aprilia SR50 DiTech.  If you own a scooter with similar rear transmission gearing, the same steps will apply.

5.  ALL bolts and socket head cap screws are in metric! Don't try to work on your vehicle if you don't have metric tools, you will just end up messing up the heads.

The purpose of new gearing is to give you more top end while at the same time not changing your acceleration. The peak horsepower of you engine is generated at about 8000-8500 RPM. Stock, you are doing about 8500 RPM at 40-45MPH. From then on, you are increasing your RPM's, and decreasing your horsepower. The gearing just uses the full potential of your variator and makes it so when you are at 8500 RPM your peak horsepower, you will be going 50-55 MPH instead of 40-45 MPH.
First start by removing the cover off the left side of you scooter.
This is what you will see behind the cover:
Remove the bolt holding the clutch assembly on:
Remove the Clutch bell housing and the clutch assembly with the pulley:
Remove the drain plug, and drain the oil from your tranny. There isn't much oil in there, so it doesn't take very long.
Next you have to remove your exhaust to remove your rear tire.
Remove the two bolts holding on the brake calipers. I found it extremely difficult to actually remove the calipers from the disk. So I just left the calipers on the disk. There is enough hose that you can remove the tire without having to take the calipers off the disk.
With the exhaust removed, remove the cover on the back wheel so you can get to the bolt holding it on.
Remove the bolt holding on the rear wheel.
Remove the entire rear wheel from axle. Then remove the bolts holding the tranny cover on.
Once the covers bolts have been removed, gently pry the cover off. There is no gasket around the edges, there is a large o-ring instead. Be sure not to damage this when removing the cover.

When you pull the cover off, be careful not to drop any gears or washer shims. YOU WILL NEED TO RE-ASSEMBLE THE GEAR BOX EXACTLY LIKE YOU FOUND IT. There may be several washers/spacers that are on different gear shafts. Don't loose them or mix them up! Be sure to note where each one was located.
Once the tranny is off, you will notice a large gear, with smaller gears located behind it. Just pull these gears out gently. They will simply slide right out.
These are the gears you should have pulled out.
Now for the most difficult part. You have to press the gear off its shaft. You can use a gear puller (Not an easy task), or if you have a heavy drill press, put a large socket behind the gear, and use the bottom of your drill chuck to push the shaft out of the gear.
Once you have pressed the gear off the shaft, you must now press the new gear onto the shaft. Be sure to line the shaft up perpendicular on all axis when you do this. You DO NOT want the shaft to be cock-eyed in the least when you start to press the shaft through the new gear.

Press the shaft through until the gear comes in contact with the smaller gear on the shaft. After the new gear is on, re-assemble the two gears into the gear box.
Now you need to remove the clutch shaft and gear from the swing arm. Simply use a rubber mallet and pound the shaft through the back.
Here is a comparison of the old gear and the new gear.
Now you have to put the new clutch shaft / gear in. Simply push it in from the back side of the tranny. A few whacks with the rubber mallet to ensure proper seating was needed.
That is it for the gear replacement. Now you need to re-assemble the tranny case (be sure to put all spacers and washers back where you found them), put the rear wheel and exhaust on, and re-attach the clutch assembly. Put your drain plug back in, and fill your tranny case up with new gear oil. I found that tilting the scooter on it's side to fill was easiest. Once I filled it up, I up righted the scooter and let the excess oil drain until there was no more oil coming from the fill hole. Put your cover back on and take it for a spin!

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