Gasket Kit (Overhaul) | Honda CT90 / Trail 90

Honda CT90 / Trail 90 Gasket Kit (Overhaul)

Whether it started with a bang or an oil leak, you're looking at a disassembled CT90 engine and wondering what to put back in. This is the top tier of three CT90 gasket kits — every paper gasket, O-ring, and crush washer you'll touch during a full top-end and bottom-end rebuild, including BOTH styles of head gasket and BOTH cam chain chamber rubbers so you can match whichever year-era your CT90 actually has.

Why You Need This

An Overhaul kit is what you order when the cylinder head comes off — for a head gasket leak, a valve job, a piston replacement, a stuck cam chain, or a full bottom-end rebuild. Every paper gasket compresses when it's first torqued, and every O-ring takes a permanent set against the surface it sealed against. Putting an engine back together with the old gaskets is a guaranteed oil weep within the first few heat cycles.

The CT90 OHC single-cylinder is built to last, but Honda's service data calls for top-end teardown well under 8,000 miles — and most of the bikes coming through our shop are well past that. When you're already this deep into the engine, the cost of the gasket kit is small money compared to having to crack the case again for a leak you could have prevented.

Specifications

Note — Letter X includes BOTH Early and Late style head gaskets (qty 2). Honda updated the head gasket design in the K3/K4 era (around 1971–1972) when the cam chain tensioner architecture changed. Rather than ask you to guess which one your bike uses, we ship both. Install the one that matches your bike's casting; keep the other as a spare.

Note — Letter AD is a single exhaust valve stem seal. The CT90 only seals the exhaust valve stem — the intake valve runs dry from the factory. If your kit looks like it's "missing a seal," it isn't — that's how Honda built it.

Note — Letter V (Oil Pump To Crankcase) and Letter W (Oil Pump Cover) are two different gaskets. V seals the oil pump body to the crankcase; W seals the oil pump cover to the pump body. Both are oil-pump-related but they live on opposite faces of the pump. Use both during reassembly.

Note — this kit covers gaskets, O-rings, and sealing washers only. It does not include pistons, piston rings, valves, valve guides, valve springs, the cam chain, or the cam chain tensioner. We sell those separately: Pulsar piston kit (standard + overbore), CT90 piston rings, "Airlock" CT90 valves, CT90 cam chain, CT90 cam chain tensioner.

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 gasket footprint may differ — 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.
• K0–K3 (1966–1971) use a hydraulic-only cam chain tensioner. K4 and later (1972–1979) use a mechanical tensioner with an external adjuster. The kit ships both styles of cam chain chamber rubber (Letters Y and Z) and the mechanical tensioner's adjuster cover seals (Letters AB and AC) — install only the parts your bike uses; the others are spares for the next rebuild.
• Does NOT fit ST90. The ST90 uses a manual clutch and different transmission internals from the CT90's centrifugal auto-clutch and dual-range sub-transmission — its gaskets are not the same.
• Does NOT fit CT110, C90, S90, or any non-CT90 Honda 90. The CT-series engine layout is unique to the Trail 90 line.

What's in the Kit

This Overhaul kit includes everything from the Oil Change kit (Letters A–F) and the Service kit (Letters A–T, no M), plus 14 additional gaskets and O-rings (M and U through AG) for the cylinder head, cylinder base, oil pump, cam chain chamber, crankcase split, and valve guide work — the gaskets you only touch when the head and cases come apart.

Installation

Difficulty: Advanced

Tools needed: Standard hand tools, torque wrench (kg-cm or in-lb + ft-lb scales), valve spring compressor (Honda valve spring tool or equivalent), 9mm wrench for the valve adjuster jam nut, feeler gauge set, single-edge razor blade, acetone, Honda Bond 4 gasket sealer, anti-seize lubricant.

Don't use RTV silicone anywhere in this engine — pieces of RTV migrate and plug oil galleries. Honda Bond 4 (or 3M ThreeBond equivalent) only.

1. Drain the oil and remove the engine from the frame for full top-end + bottom-end work. The cylinder head can come off in-frame, but a crankcase split needs the engine on a bench.
2. Remove the exhaust system. Take off the muffler bracket bolts at the frame first, then unbolt the two flange nuts at the cylinder head. The Letter M Exhaust Crush Gasket will be at the head port — clean the port surface, install the new crush gasket, and on reassembly tighten the muffler bracket bolts first, then the flange nuts last so the pipe doesn't bind.
3. Pull the carburetor and intake manifold. Letters H + I + Q (paper gaskets + O-ring) install at the carb-to-manifold and manifold-to-engine interfaces. Light grease on the new paper gaskets keeps them in place during reassembly.
4. Remove the right crankcase cover (the CT90 has no separate filter access panel — the full cover comes off). This is the same cover the Oil Change kit's Letters A–F service. The clutch and centrifugal auto-clutch live behind it.
5. Disassemble the cylinder head. Remove the four 8mm head nuts (factory torque on reassembly: 2.0–2.5 kg-m / 14.5–18.1 ft-lbs in a cross pattern), lift off the head. Letter X gives you both Early and Late style head gaskets — the Late style was introduced around the K3/K4 cam chain redesign. Install the one matching your bike's casting and keep the other as a spare.
6. Inspect head flatness with a straightedge and feeler gauge — spec is 0.05 mm (0.002 in) maximum warpage at the mating surface. If your head is over the limit, send it to us for a deck job via our Cylinder Head Machining Service (CT90 supported) before reinstall.
7. If pulling valves: Compress the valve springs with the spring compressor tool, remove the cotters, retainers, and old exhaust valve stem seal (Letter AD — only one seal, on the exhaust valve; the intake runs dry by design). If you're replacing valve guides, Letter AG gives you the two valve guide base O-rings that seat where the new guide presses into the head. Walk-through video for valve spring work: Remove and Install Valve Springs on Your Vintage Honda.
8. Remove the cylinder. The Letter U Cylinder Base Gasket and Letter AE Oil Passage Rubber both live here. On reassembly, apply a small bead of Honda Bond 4 around the oil passages on the upper crankcase and the bottom of the cylinder before laying the new Letter U gasket. Do not apply sealer to the head gasket itself (Letter X) — head gaskets install dry. Walk-through video: Cylinder Jug to Piston Installation.
9. If splitting the crankcase (bottom-end rebuild): remove the cylinder, head, clutch, gear shift spindle, and split the right crankcase from the left. Letter AA (Crankcase Split Gasket) seals between the halves — clean both mating faces with acetone, apply a thin coat of Honda Bond 4, and seat the new gasket with the two factory dowel pins fitted. Mating surface flatness spec is 0.05 mm (0.002 in) on both halves.
10. Oil pump service (Letters V + W). The oil pump unbolts with three 6mm screws plus a special hex bolt. Letter V seals the pump body to the crankcase; Letter W seals the cover to the pump body — install both during reassembly.
11. Cam chain chamber (Letters Y + Z) and tensioner (Letters AB + AC). Both styles of cam chain chamber rubber are in the kit; install whichever matches your engine's chain tunnel. For K4 and later (mechanical tensioner), use Letter AB sealing washer under the tensioner adjuster cover bolt and Letter AC O-ring at the adjuster body. For K0–K3 (hydraulic-only tensioner), AB and AC are spares for a future rebuild — not installed.
12. Reassemble in reverse order. Set valve timing: rotate the crankshaft so the rotor's T mark aligns with the index mark before installing the cam chain on the cam sprocket. Tighten cylinder head nuts in a cross pattern to the spec above. After the engine is back together, set valve clearance to 0.05 mm intake and exhaust (cold), set point gap to 0.012–0.016 in, and set ignition timing to the F mark.
13. Refill with oil. 0.9 L / 0.95 US qt SAE 10W-40 100% mineral plus 2 oz of zinc additive per quart. Avoid the semi-synthetic diesel oils (Shell Rotella T4, Chevron Delo 400, Mobil Delvac 1300 — all reformulated to semi-synthetic in late 2023): friction modifiers cause the wet centrifugal clutch to slip.

Installation Tip: Clean all surfaces with a razor blade and acetone before any gasket goes near them. Lightly grease new paper gaskets — it keeps them in place during reassembly and lets the joint come apart later without tearing the gasket. Use anti-seize on the cover bolt threads.

The cylinder base gasket procedure CMC uses on the bench: razor and acetone to clean the mating surfaces, a small bead of Honda Bond 4 only at the oil passages (never RTV), and the cylinder lowered onto the pistons with the rings already compressed. Multi-model demo on a CB350 — technique applies to every vintage Honda single and twin.

Frequently Asked Questions

Why are there two head gaskets in the kit?

Honda changed the head gasket design partway through CT90 production — around the K3/K4 era (1971–1972) when the cam chain tensioner architecture was revised. Rather than make you tell us which year your bike is to ship the right one, we put both styles in every Overhaul kit. Install the one that matches your bike's cylinder/head casting and keep the other one as a spare for next time.

Why is there only one valve stem seal? Did I get half a kit?

No — that's how Honda built the CT90. Only the exhaust valve gets a stem seal from the factory; the intake valve runs dry. Letter AD covers the exhaust seal; the intake doesn't need one. The same is true on most small single-cylinder Hondas of this era.

Letter V and Letter W are both oil pump gaskets — what's the difference?

Two different gaskets on two different faces of the oil pump. Letter V seals between the oil pump body and the crankcase (where the pump bolts to the engine case). Letter W seals between the oil pump cover and the pump body (the cover you remove to access the pump rotors). Install both during reassembly — one without the other will leak.

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 for a full tune-up: points cover and base, carburetor intake, left crankcase cover, 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 14 more (M + U through AG): exhaust crush gasket, cylinder base, head gasket (both Early and Late styles), oil pump pair, cam chain chamber rubbers, crankcase split, tensioner adjuster seals, exhaust valve stem seal, oil passage rubber, oil pipe crush washer, and valve guide base O-rings. The full top-end-plus-bottom-end rebuild kit.

Does this kit include pistons, rings, valves, valve springs, the cam chain, or the tensioner?

No. This is a gaskets-and-O-rings-and-crush-washers kit only. We sell the wear parts separately so you can mix and match what your specific engine actually needs: Pulsar piston kit (standard, 0.5mm, and 1.0mm overbore), CT90 piston rings (matched to the piston size), "Airlock" CT90 intake and exhaust valves, CT90 cam chain, cam chain tensioner, and tensioner roller.

I have a 1966–1968 CT90 K0. Will this kit fit?

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, send us the frame serial number before ordering and we'll confirm.

Does this kit fit my Honda ST90, CT110, S90, or C90?

No. The ST90 uses a manual clutch and different transmission internals. The CT110 is a later, bigger engine. The S90 and C90 are entirely different model families. The CT-series engine layout in this kit is unique to the Trail 90.

When do I need this kit instead of the Service kit?

When the cylinder head is coming off, or the cases are getting split. Specifically: head gasket leak, valve job, piston or ring replacement, cam chain replacement (the engine has to come apart far enough to swap the chain), oil pump service, crankcase oil leak from the split line, or a top-end-down rebuild after a piston seizure or extended sitting period. If you're just doing a tune-up — valves, points, intake reseal — the Service kit (#4319) covers it.

You Might Also Need

• #4318 Gasket Kit (Oil Change) Honda CT90 — Letters A–F, routine oil change tier
• #4319 Gasket Kit (Service) Honda CT90 — Letters A–T (no M), tune-up tier
• Pulsar Piston Kit Honda CT90 — Standard, 0.5mm, and 1.0mm overbore
• CT90 Piston Rings — matched to piston size
• "Airlock" Intake & Exhaust Valves CT90 — if valve work is part of the rebuild
• CT90 Cam Chain — replace while the top end is open
• CT90 Cam Chain Tensioner — K4 and later mechanical tensioner replacement
• CT90 Cam Chain Roller — companion to the tensioner
• Cylinder Boring & Honing Service — CT90 supported for standard, 0.5mm, and 1.0mm overbore
• Cylinder Head Machining Service — deck the head, valve job, valve guide replacement (CT90 supported)
• Honda Bond 4 Gasket Sealer — cylinder base oil passages and crankcase split mating surfaces (NOT for the head gasket)
• CT90 Exhaust Gasket — the standalone Letter M part if you need spares
• Valve Spring Compressor Tool — required for valve stem seal install (Letter AD) and valve guide work (Letter AG)
• Valve Adjustment Wrench (CT90 / CB175 / CB200) — 9mm jam-nut + square-head adjuster tool
• CT90 Oil Seal Kit — the engine oil seals under the side covers
• CT90 Allen Bolt Kit — JIS-to-Allen upgrade for the right side, left side, and points plate bolts
• CT90 Ignition Contact Points
• CT90 Ignition Condenser
• CT90 Carburetor Gasket Kit — the carb's internal gaskets, separate from this kit's Letters H + I + Q which are external interfaces
• CT90 Carburetor Insulator Spacer Kit (K0–K6 + 1976–1977) — standalone version of Letters H + Q
• CT90 Carburetor Rebuild Kit (K1–K8)
• CT90 Spark Plug (NGK D8HA) — fresh plug at first start-up
• CT90 Spark Plug Boot
• CT90 Oil Filter Screen (OEM)
• CT90 Oil Drain Plug
• CT90 Drain Plug Crush Washer (5-pack)
• Zinc ZDDP Oil Additive — 2 oz per quart at first fill
• Anti-Seize Lubricant — cover bolt threads
• Chain Lube — finish the service

Last updated: May 2026