Gasket Kit (Service) | Honda CT90 / Trail 90

Description

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

Spec	Detail
Fits Models	CT90 / Trail 90 K0 (1966–1968), K1 (1969), K2 (1970), K3 (1971), K4 (1972–1973), K5 (1974), K6 (1975), K7 (1976), K8 (1977), K9 (1978–1979)
Replaces OEM Parts	91301-216-000, 91307-KF0-003, 91307-035-000, 94109-12000, 90407-231-000, 90407-259-000, 11691-028-306, 11691-028-000, 11393-028-306, 11393-028-020, 22112-121-632, 22112-028-000, 22112-028-306, 22112-121-630, 30391-459-952, 30391-028-000, 30391-459-950, 16201-077-306, 16201-077-010, 17119-046-306, 17119-046-000, 11395-053-306, 11395-053-010, 11693-459-962, 11693-053-030, 11693-053-306, 11693-459-960, 30392-028-306, 30392-028-020, 18291-028-306, 18291-028-000, 12394-121-630, 12394-028-010, 12394-028-020, 12391-121-630, 12391-028-000, 12391-028-306, 90441-028-010, 16010-102-305, 16010-102-700, 91304-HB3-003, 91302-200-000, 91302-943-003, 91306-HB3-003, 91303-001-000, 91301-053-000, 91302-HB3-004
Sold As	Kit (19 gasket types, 24 individual pieces — Letters A through T, no M)

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.

Honda CT90 / Trail 90 Gasket Kit Comparison

Kit	Letter	Gasket	Qty
4318 Oil Change Gasket Kit (Includes Gaskets A–F)	A	Stator Rotor Cover Bolt O-Rings	3
	B	Oil Dipstick O-Ring	1
	C	Oil Drain Plug Crush Washer	1
	D	Stator Rotor Cover Gasket	1
	E	Right Crankcase Cover Gasket	1
	F	Oil Filter / Clutch Basket Cover Gasket	1
4319 Service Gasket Kit (Includes Gaskets A–T, no M)	G	Points Cover Gasket	1
	H	Carburetor To Intake Manifold Gasket	1
	I	Intake Manifold To Engine Gasket	1
	J	Left Crankcase Cover Gasket	1
	K	Subtransmission Cover Gasket	1
	L	Points Base Gasket	1
	N	Rocker Arm Cover Gasket	1
	O	Cylinder Head Cover Gasket	1
	P	Point Base Sealing Washers	3
	Q	Carburetor To Intake Manifold O-Ring	1
	R	Valve Cover O-Rings	2
	S	Clutch Lifter Adjuster O-Ring	1
	T	Subtransmission Selector O-Ring	1
4320 Overhaul Gasket Kit (Includes Gaskets A–AG)	M	Exhaust Crush Gasket	1
	U	Cylinder Base Gasket	1
	V	Oil Pump To Crankcase Gasket	1
	W	Oil Pump Cover Gasket	1
	X	Head Gasket (Early and Late Style)	2
	Y	Cam Chain Chamber Base Rubber Gasket (A)	1
	Z	Cam Chain Chamber Base Rubber Gasket (B)	1
	AA	Crankcase Split Gasket	1
	AB	Cam Chain Tensioner Adjuster Cover Sealing Washer	1
	AC	Cam Chain Tensioner O-Ring	1
	AD	Exhaust Valve Stem Seal	1
	AE	Cylinder Header Dowel Oil Passage Rubber Gasket	1
	AF	Oil Pipe Joint Bolt Crush Washer	1
	AG	Valve Guide Base O-Rings	2

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.