Gasket Kit (Service) | Honda CT90 / Trail 90

Honda CT90 / Trail 90 Gasket Kit (Service)

The middle tier of three CT90 gasket kits. The Service kit goes beyond the routine oil-change scope (Letters A–F) and covers everything you'll touch during a full tune-up — valve adjustment, ignition timing, intake manifold reseal, and left-side cover work — with 19 gasket and O-ring types in one box.

Why You Need This

When you sit down for a real Trail 90 tune-up — cam chain, valves, ignition timing, carburetor — you end up opening half the engine's covers. Even gaskets that look fine should be replaced, because once a paper gasket has been compressed and heat-cycled it will not seat the same way twice. This kit bundles every gasket and O-ring you'll cross during that work, including the routine oil-change parts plus the points cover, intake manifold pair, rocker arm and cylinder head cover gaskets, left crankcase cover and subtransmission gaskets, and the small O-rings on the points base, valve covers, clutch lifter adjuster, and subtransmission selector.

Honda's published service procedures for the CT90 specify replacing these gaskets and O-rings with new parts on reassembly. Reusing a 50-plus-year-old paper gasket is the fast path to oil weeping out of a cover that was sealed yesterday.

Specifications

Note — this kit does NOT include the exhaust crush gasket (Letter M). If you're pulling the exhaust to replace it, add the CT90 Exhaust Gasket separately, or step up to the Overhaul Kit (#4320) which includes Letter M.

Note — this kit does NOT include the exhaust valve stem seal (Letter AD). If you're pulling valves for guide work or replacement, you'll need the Overhaul Kit — only the Overhaul kit carries the valve stem seal. The Service kit's Letter R covers the tappet/rocker inspection cover O-rings (the two small caps you remove for valve adjustment), which is a different part.

Note — this kit covers gaskets and O-rings only. It does not include the cam chain, cam chain tensioner, valves, valve springs, piston, rings, or any wear part. Sourcing those separately: CT90 Cam Chain, Cam Chain Tensioner, "Airlock" CT90 Valves.

Model-Specific Notes

• CT90 K0 1966–1968: The vast majority of K0 frames use the dual-range sub-transmission and fit this kit. There is a very early K0 variant without the dual-range transmission whose points cover may use a different gasket footprint — if your K0 has a 3-speed only (no Hi/Lo lever) and you're unsure, contact us with your frame serial number before ordering.
• Does NOT fit ST90. The ST90 looks similar but uses a manual clutch and different transmission internals — its gaskets are not the same.
• Does NOT fit CT110, C90, S90, or any non-CT90 Honda 90. The CT-series sub-transmission is unique to the Trail 90 line.
• K0–K3 (1966–1971) use a hydraulic-only cam chain tensioner that does not require manual adjustment. K4 and later (1972–1979) use a mechanical tensioner with an external adjuster — check it during the same service.

What's in the Kit

This Service kit includes everything from the Oil Change kit (Letters A–F) plus 13 additional gaskets and O-rings (G, H, I, J, K, L, N, O, P, Q, R, S, T) for the carburetor intake, left side cover, sub-transmission, points base, valve covers, and clutch lifter adjuster. Letter M (exhaust crush gasket) is skipped because it lives in the Overhaul kit, which also adds Letters U–AG for a full top-end and bottom-end rebuild.

Installation

Difficulty: Intermediate

Tools needed: Standard hand tools, 9mm wrench for the valve adjuster jam nut, feeler gauge set (0.002–0.016 in range), single-edge razor blade, acetone, anti-seize lubricant. A timing/test light helps for the ignition timing step.

The CT90 tune-up sequence is cam chain → valves → ignition timing — do them in that order, because adjusting one shifts the next.

1. Cam chain tension (K4 and later only — K0–K3 are hydraulic-only): adjust per our CT90 Cam Chain Tension walk-through. No gasket from this kit is used at this step; if you're going deeper and removing the tensioner adjuster cover, the sealing washers for that are in the Overhaul Kit.
2. Valve adjustment (Letters N, O, R). Bring the engine to TDC on compression (finger over the spark plug hole, T mark on the rotor aligned with the index mark at 3 o'clock on the coil). Remove both tappet inspection covers — intake and exhaust — and check each gap. Spec is 0.05 mm (0.002 in) on both valves. Overshoot to 0.003 in (0.08 mm), then loosen back — thermal expansion is more forgiving when slightly loose than slightly tight. Use a 9mm wrench on the adjuster jam nut. On reassembly, fit the new Letter R Valve Cover O-Rings before bolting the inspection covers back on. If the rocker arm cover or cylinder head cover came off during this work, install fresh Letters N and O. Walk-through video: Set the Valve / Tappet Clearance on the Honda CT90.
3. Ignition timing (Letters G, L, P). Remove the points cover, then loosen the three points-plate screws (these are the screws Letter P x3 sealing washers seat under — replace them whenever the plate comes off). Set the point gap to 0.012–0.016 in (0.3–0.4 mm) with a feeler gauge, then set static timing so the points just begin to open as the rotor's F mark (Fire on compression stroke) aligns with the index mark. The T mark is TDC, not timing. The CT90 is a battery-ignition bike — you need a charged battery in the bike to verify spark. Reassemble with new Letter G Points Cover Gasket and Letter L Points Base Gasket. Walk-through video: Set the Ignition Timing on the Honda CT90.
4. Intake manifold and carburetor (Letters H, I, Q). Pull the carburetor and intake manifold for inspection. The Letter I gasket seats between the intake manifold and the engine; the Letter H gasket seats between the carburetor flange and the intake manifold; the Letter Q O-ring seats at the carb-to-manifold interface as well (paper-and-O-ring stacked seal). Clean both flanges with a razor and acetone before fitting the new gaskets. Light film of grease on the new paper gaskets keeps them in place during reassembly.
5. Left crankcase cover (Letter J). If you're servicing the gearshift mechanism, stator coil, or just chasing a left-side oil weep, drop the cover, replace the gasket, and reseal carefully — be careful to take care with the oil seal at the shift shaft when handling this cover.
6. Sub-transmission (Letters K, T). CT90's High/Low range lever sits in the sub-transmission. Replace the K cover gasket on the cover itself and the T O-ring at the selector shaft seal whenever this cover is opened.
7. Clutch lifter adjuster (Letter S). Remove the small cover on the right side, replace the O-ring, and re-adjust the clutch: loosen the lock nut, turn the screw clockwise about one turn, then back off slowly. Engine off.
8. Oil-change letters (A, B, C, D, E, F). Drain, clean the centrifugal oil filter rotor on the right side, replace the right crankcase cover gasket and the oil dipstick O-ring. Drain plug torque is 2.0–3.5 kg-m (14–25 ft-lbs) with a 17mm wrench. Refill with 0.9 L / 0.95 US qt SAE 10W-40 100% mineral plus 2 oz zinc additive per quart.

Installation Tip: Clean all sealing surfaces with a razor blade and acetone. Apply a light coat of grease to the new paper gaskets during installation — it prevents sticking and lets the joint come apart cleanly next time without tearing the gasket. Use anti-seize on the cover bolt threads, especially on the right-side cover that gets removed every 3,000 mi for filter cleaning.

Brenden walks through the CT90 valve adjustment procedure end to end — finding the T mark, the overshoot technique with the 9mm wrench, and the "if the O-ring is worn, change it" inspection cover reseal that the Letter R O-rings cover.

Frequently Asked Questions

Will this kit fit my 1966–1968 CT90 K0?

Almost certainly yes. The kit fits every CT90 K0 with the dual-range (Hi/Lo) sub-transmission — which is virtually all of them. A very small number of very early K0 frames were built without the dual-range transmission, and on those bikes the points cover gasket footprint may not match. If you're not sure whether your K0 has the High/Low lever on the sub-transmission, send us the frame serial number before ordering and we'll confirm.

What's the difference between #4318 Oil Change, #4319 Service, and #4320 Overhaul?

Three tiers, cumulative letter scope:

• #4318 Oil Change (A–F): Six gaskets and O-rings — what you replace at every routine oil and filter change.
• #4319 Service (A–T, no M): A–F plus 13 more (G–T, skipping M) for a full tune-up: points cover and base, carburetor intake, left crankcase, subtransmission, rocker arm and head cover, plus the small O-rings on the valve covers, clutch lifter, and selector.
• #4320 Overhaul (A–AG): Everything in Service plus the exhaust crush gasket (M) and the 14 more (U–AG) you only need when the head and cylinder come off — head gasket, cylinder base, cam chain chamber, crankcase split, valve stem seal, and oil passage rubbers.

Are valve stem seals in this kit?

No. The Service kit includes the valve cover (tappet inspection cover) O-rings — Letter R, qty 2 — which seal the two small caps you remove to adjust valve clearance. The exhaust valve stem seal (Letter AD, sealing between valve guide and valve stem) is in the Overhaul Kit (#4320) only, because you have to remove the valve to install one.

Does this kit fit my Honda ST90?

No. The ST90 looks like a sibling but uses a manual clutch and different transmission internals from the CT90's centrifugal auto-clutch and dual-range sub-transmission. Its gaskets are not interchangeable.

How often should I do a full Service?

Honda's factory recommendation is 3,000 miles between full tune-ups. CMC's experience with these little air-cooled engines is that 1,000–1,200 miles is the right interval — the CT90 works hard for its 89.5cc and the points, valves, and cam chain all drift faster than the factory schedule suggests. Pair every full service with an oil change using SAE 10W-40 100% mineral and 2 oz of zinc additive per quart. Avoid the semi-synthetic diesel oils (Shell Rotella T4, Chevron Delo 400, Mobil Delvac 1300 — reformulated to semi-synthetic in late 2023): the friction modifiers cause the wet centrifugal clutch to slip.

Is Letter Q the same as the O-ring on the fuel-bowl drain on my carburetor?

No. Letter Q is the O-ring at the outside of the carburetor — where the carb flange meets the intake manifold. The fuel-bowl drain O-ring is inside the carburetor and belongs to our CT90 Carburetor Gasket Kit (a different product). If you're rebuilding the carb itself, that's the kit you want.

Do I need to replace the cam chain or tensioner while I'm in here?

The Service kit doesn't include either — they're separate products. The cam chain on the CT90 is a steady wear item: if you have over 10,000 miles on the engine and you've never replaced it, replace the CT90 cam chain while the top end is opened up for a deeper rebuild. For routine tune-up service (just valves and timing), the cam chain stays. On K4 and later models, you can adjust the mechanical tensioner from outside without disassembly — K0–K3 use a hydraulic-only tensioner that needs no adjustment.

You Might Also Need

• #4318 Gasket Kit (Oil Change) Honda CT90 — Letters A–F, routine oil change tier
• #4320 Gasket Kit (Overhaul) Honda CT90 — Letters A–AG, full top-end rebuild tier (adds valve stem seal + head/cylinder gaskets)
• CT90 Oil Seal Kit — the engine oil seals that live under the side covers (kickstart, shift shaft, transmission)
• CT90 Allen Bolt Kit — JIS-to-Allen upgrade for the right side, left side, and points plate bolts
• Valve Adjustment Wrench (CT90 / CB175 / CB200) — the 9mm jam-nut + square adjuster tool the procedure calls for
• CT90 Ignition Contact Points — service the points along with the gaskets
• CT90 Ignition Condenser — companion to the points
• CT90 Points Plate Allen Screw Set — the three points-plate screws (the same three the Letter P sealing washers sit under)
• CT90 Carburetor Gasket Kit — if you're rebuilding the carb internals (separate from Letters H, I, Q which are the external interface)
• CT90 Carburetor Insulator Spacer Kit (K0–K6 + 1976–1977) — standalone version of Letters H + Q if you need spares
• CT90 Carburetor Rebuild Kit (K1–K8) — if the carb is being fully rebuilt
• CT90 Oil Filter Screen (OEM) — while the right crankcase cover is off, inspect and replace if torn
• CT90 Oil Drain Plug — replace if the threads are damaged
• CT90 Drain Plug Crush Washer (5-pack) — spare crush washers, the kit only includes one
• Zinc ZDDP Oil Additive — 2 oz per quart at refill
• Anti-Seize Lubricant — cover bolt threads
• Chain Lube — finish the service

Last updated: May 2026