Front Chain Sprocket (15T) | Honda CT90 / Trail 90
The CT90's automatic clutch means the engine is always braking the drivetrain when you slow down, and the front sprocket takes that load every ride. It also sits buried behind the left engine cover, so it is one of the most commonly neglected parts on these bikes — and by the time most owners get to it, it is badly worn. This is a 15-tooth replacement front chain sprocket in top quality steel, made like the original Honda factory sprocket for the 428-pitch drive chain.
Why You Need This
Worn sprocket teeth go thin and sharp and wear unevenly — compare yours against the new-versus-worn photo above and the difference is obvious. Because the CT90's front sprocket is harder to inspect than on most bikes, it tends to run long past worn, and a worn sprocket eats a fresh chain early.
The front sprocket works in conjunction with the CT90's dual-range gearbox — a setup unique to this bike — and the sprocket is the smaller, faster-turning half of the final drive, which is why it wears before the rear does.
Specifications
| Replaces OEM Part Numbers | 23800-053-000 |
| Fits Models | CT90 K0–K8 (1966–1977, see K0 note), CT90 (1978–1979) |
| Chain Pitch | 428 |
| Tooth Count | 15T |
| Sold As | Sold Individually |
Note: This front sprocket fits CT90 K0 models with the dual-range gearbox, from Engine SN 122551 and up. Earlier K0 engines used a different setup.
Note: When you replace the front sprocket, check the rear sprocket and drive chain — worn drivetrain parts wear out their new neighbors, so replace all three together when in doubt.
Installation
Difficulty: Intermediate — front sprocket access on the CT90 is more involved than on most bikes. Plan on at least a Service Gasket Kit to do the job correctly, and an impact driver to crack the factory JIS cover screws loose.
- Remove the entire left side engine cover, then the sub-transmission cover and gears to reach the front sprocket.
- The CT90 runs two oil seals, one on either side of the front sprocket — replace them while you are in there with the CT90 oil seal kit.
- Before installing the new sprocket, spread engine oil generously on the inner brass bushing, and lightly coat the inner lips of the oil seals so the sprocket pushes through without tearing them.
- Going back together, consider swapping the factory screws for the Allen-head bolt kit — future services get much easier.
Since the left side comes apart for this job, it is a natural one to do at oil-change time.
Our Sprockets 101 video covers how sprockets wear, why the front goes first, and why front, rear, and chain get replaced as a set.
Frequently Asked Questions
Why is the front sprocket harder to replace on the CT90?
It sits behind the entire left side engine cover plus the sub-transmission cover and gears, so getting to it is most of the job. That is also why it gets neglected — out of sight, out of mind — and why ours tend to arrive at the new-versus-worn photo stage before anyone looks.
What else should I replace while I am in there?
The two oil seals on either side of the sprocket, and at least a service gasket kit for the covers you opened. Check the rear sprocket and chain at the same time — if either shows wear, replace all three together.
Will this fit my early CT90 K0?
Only from Engine SN 122551 and up — those are the K0 models with the dual-range gearbox. Earlier K0 engines used a different arrangement, and this sprocket does not fit them.
Do I need a new chain and rear sprocket too?
If the chain or rear sprocket shows wear, yes — a new part running against worn ones wears out early. The matching 45-tooth rear sprocket and 428 drive chain cover the same models.
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Last updated: June 2026
