How to Read a Dirt Bike Sprocket Part Number & Pick Gearing

Short answer: a sprocket listing tells you four things, and you need all four to match. They are position (front or rear), material (steel or alloy), the bike family and year range it fits, and the tooth count, usually written like "-14T-" or "-52T-". Buy the stock tooth counts if you just want the bike back to normal. If you want to change how it pulls, a smaller front or larger rear sprocket gives more acceleration and less top speed, and a larger front or smaller rear does the opposite.

Reading the listing and the part number

Many of our dirt bike sprockets are from ProX, and their listings follow a consistent pattern. Take two examples from our catalog:

• ProX Frontsprocket Kx450F '06-12 -15T- 07.FS44006-15
• ProX Steel Rsprocket Yz125/250+Wr250 -52T- 07.RS22080-52

Read them left to right:

1. Position and material. "Frontsprocket" is the small countershaft sprocket on the engine. "Rsprocket" is the rear, on the wheel. Rears are marked "Steel" or "Alloy".
2. Fitment. The model names and years (Kx450F '06-12) are the bikes the mounting pattern was made for. Sprockets are often shared across several models, so the name lists the main ones. The product page is the final word on fitment.
3. Tooth count. "-15T-" means 15 teeth.
4. Part number. In the ProX numbers we list, the letters after "07." follow the type: FS on front sprockets, RS on steel rears, RA on alloy rears. The digits in the middle identify the mounting pattern, and the number after the dash repeats the tooth count. So 07.RS22080-52 and a 07.RS22080-50 are the same sprocket in two sizes.

That last point is the useful one. Once you have found the part family that fits your bike, you can change the number after the dash to shop other tooth counts for the same hub or shaft.

Other brands use their own numbering, but nearly all of them end with the tooth count. If a listing does not show the tooth count clearly, do not guess.

Check the chain size first

A sprocket only works with one chain pitch. Dirt bikes commonly run 420 or 428 chain on small bikes and 520 on full-size bikes. The chain size is stamped on the chain's side plates, and your owner's manual lists it too.

Do not assume the size from engine displacement alone. Confirm it on the bike, then make sure the sprocket you order is for that chain size. If you are replacing the chain at the same time, which is the right time to do sprockets, order all three parts for the same pitch. A new chain on worn, hooked sprocket teeth wears out early.

How gearing works

Your final drive ratio is the rear tooth count divided by the front tooth count. A higher number means more acceleration and less top speed at the same rpm. A lower number means the reverse.

The 13/50 row is only an example. Look up your own bike's stock counts in the manual, or count the teeth on what is fitted now.

Two rules of thumb help:

• One tooth on the front is a big change. It is roughly equal to three or four teeth on the rear, depending on the sizes involved. Use the rear for fine tuning.
• Divide to compare. Any two combinations with the same ratio will pull about the same, so work out the ratio before you buy.

Which way to go

Tight woods, technical trail, hills. Go shorter, meaning one or two more teeth on the rear, or one fewer on the front. The bike is easier to ride slowly without slipping the clutch.

Motocross. Gear so you are not shifting in the middle of key corners or jumps. This usually means one or two teeth either way on the rear from stock, depending on the track.

Open desert, fire roads, dual-sport miles. Go taller, with one more tooth on the front or a few fewer on the rear, for lower rpm at cruising speed.

Not sure. Stay stock. The factory gearing is a sensible middle.

Three practical cautions:

• A big change in tooth count can need a longer or shorter chain. Check that your axle adjusters have enough range before you ride.
• A very small front sprocket bends the chain more tightly and can bring it closer to the swingarm slider, which speeds up wear. If you need a big change, do more of it at the rear.
• On a street-legal bike with a wheel-driven speedometer, the reading is not affected. On bikes that read speed from the gearbox, changing gearing will change the reading.

Steel or alloy rear

Steel rear sprockets are heavier and last longer. They suit trail riding, dual-sport use and anyone who would rather not change sprockets often. Alloy rears are lighter, which is why racers like them, and they wear faster. Front sprockets are steel either way.

What we carry

• Sprockets: the full range, front and rear.
• Front Sprockets, for example the ProX Front Sprocket Kx450F '06-12 15T, 07.FS44006-15 and the ProX Front Sprocket Rm250 + Dr-Z400 14T, 07.FS33082-14.
• Rear Sprockets, in steel such as the ProX Steel Rear Yz125/250 + Wr250 52T, 07.RS22080-52.
• Alloy rears such as the ProX Alloy Rear Kx80/85/100 51T, 07.RA41086-51 and the ProX Alloy Rear Yz80/85 93-12 47T, 07.RA21093-47.
• Chains, including KMC chain such as the KMC 420-110, and Connecting Links.
• Sprocket Guards, for example the Psychic Front Sprocket Cover MX-03443BK.

Check the fitment on each product page before ordering. Snowmobile track drivers are a different part and live in Drive Sprockets.

FAQ

Q: What does the number at the end of a sprocket part number mean? A: On most brands it is the tooth count. For example, a part number ending in -52 is a 52-tooth sprocket, and the same number ending in -50 is the 50-tooth version for the same bike.

Q: Should I change the front or the rear sprocket to adjust gearing? A: Use the rear for small adjustments and the front for large ones. One tooth on the front changes the ratio about as much as three or four teeth on the rear.

Q: Do I need a new chain when I replace sprockets? A: It is the best practice. Chains and sprockets wear together, so a worn chain will quickly wear new sprockets, and worn sprockets will do the same to a new chain.

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