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Haynes final-drive ratios, and the one alternative front sprocket a seller actually lists.
Before you start
Your own risk. Wrong jetting or timing can seize the engine. This is information, not advice.
How many steps: 2
Step 1 of 2
Count the teeth. The two engines are geared differently.
Haynes 4298: Type 122 final drive 2.353:1, 40 rear and 17 front. Type 123 final drive 2.785:1, 39 rear and 14 front. Chain size 520. Slack 25–35 mm. The slack figure does not change because you fancy a different sprocket. Fitting a 123 pair to a 122, or the other way round, is a different gearing even before you call it an upgrade. Count what is on the bike.
Step 2 of 2
A 13-tooth front sprocket is sold. Speeds are not copied.
Radical Racing lists a JT 13-tooth sprocket, pitch 520, for Rotax 122 and 123 engines, and describes it as fitting the RS 125 from 1997. The listing gives an inner diameter of 24/22 mm and 22 teeth on the inside. Those are the seller’s dimensions for that part. The same page prints speed tables and marks a standard pair in red. The red cell does not survive as text, so this site does not copy a tooth pair or a speed from it. Fewer teeth on the front sprocket shortens the gearing. Whether that is what you want, and which rear sprocket you already have, is To verify by counting.
This guide is done. Your place stays on this device.
Haynes 4298: Type 122 final drive 2.353:1, 40 rear and 17 front. Type 123 final drive 2.785:1, 39 rear and 14 front. Chain size 520. Slack 25–35 mm. The slack figure does not change because you fancy a different sprocket.
Fitting a 123 pair to a 122, or the other way round, is a different gearing even before you call it an upgrade. Count what is on the bike.
Source: Haynes manual 4298
Radical Racing lists a JT 13-tooth sprocket, pitch 520, for Rotax 122 and 123 engines, and describes it as fitting the RS 125 from 1997. The listing gives an inner diameter of 24/22 mm and 22 teeth on the inside. Those are the seller’s dimensions for that part.
The same page prints speed tables and marks a standard pair in red. The red cell does not survive as text, so this site does not copy a tooth pair or a speed from it. Fewer teeth on the front sprocket shortens the gearing. Whether that is what you want, and which rear sprocket you already have, is To verify by counting.
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