Yamaha Rear Exhaust Kit — Install Guide

GT40 Marine | SKU GT40-YAM-EXH-1819L | Yamaha 1.8L / 1.9L Free-Flow Rear Exhaust | Rev 1.0 — 2026-05-24

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Fitment — Yamaha 1.8L / 1.9L

Not for: 1.0L / 1.05L / 1.6L Yamaha engines, JetBlaster, standup hulls. Confirm engine displacement (1.8L or 1.9L).

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Before You Start — Decision Tree

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In the Box

• (1) GT40 mandrel-bent stainless rear exhaust tube
• (1) Through-hull stainless outlet flange
• (1) Stainless V-band clamp, 2.5" ID
• (1) High-temperature exhaust gasket, fire-rated
• (3 ft) High-temperature silicone water injection hose, 3/4" ID
• (2) #12 stainless hose clamps
• (4) M8 × 25 mm SHCS — mounting hardware
• (4) M8 stainless flat washers
• (1) Marine-grade exhaust sealant tube
• (1) Hardware bag — spare gasket, spare V-band
• (1) GT40 product registration card
• (1) This guide

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Tools Required

• 10 mm / 13 mm sockets
• T25 + T30 Torx drivers
• 5 mm + 6 mm hex keys
• 1/4" + 3/8" drive torque wrenches, 5–25 Nm range
• 2-3/4" hole saw + arbor (only if installing new through-hull outlet)
• Hose-clamp pliers
• Marine-grade exhaust sealant applicator
• Flashlight or headlamp
• Marine-grade copper anti-seize compound

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Safety

1. Battery disconnected for duration
2. Cold engine only — Yamaha 1.8L/1.9L exhaust retains heat for over an hour
3. Eye protection mandatory
4. Respirator mandatory if cutting through-hull (fiberglass dust)
5. No smoking / open flame
6. Two-person lift recommended for stainless tube positioning on FX Cruiser hulls

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Pre-Install Checklist

• [ ] Watercraft on stable trailer
• [ ] Engine 1.8L/1.9L confirmed
• [ ] Battery negative disconnected
• [ ] Factory exhaust routing photographed
• [ ] Sealant cure time scheduled (2 hours minimum before first start)

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Step-by-Step Install

Step 1 — Access the Engine Bay

Remove seat (two-person lift on FX Cruiser). Open engine bay top hatch. Disconnect battery negative.

Step 2 — Remove Factory Exhaust

Factory Yamaha rear exhaust connects from the waterbox outlet to the through-hull discharge fitting via a short tube.

1. Disconnect water injection line at the factory tube
2. Loosen the waterbox-to-rear-exhaust V-band clamp (13 mm)
3. Pull the factory exhaust tube free
4. Inspect the waterbox V-band groove + the through-hull approach for any damage

Step 3 — Inspect and Clean Mounting Surfaces

1. Wipe the waterbox V-band flange clean
2. Inspect for flatness and warping — surface must be uniform
3. Light scuff with 220-grit sandpaper if contaminated
4. Wipe with solvent, allow 60 seconds evaporation

Step 4 — Install Through-Hull Outlet (if new install)

Skip this step if a through-hull outlet is already in place.

1. Use the supplied transom template (or measure for symmetric placement on Yamaha hull)
2. Center-punch the cut location
3. Pilot drill, then 2-3/4" hole saw at 300-500 RPM
4. Deburr cut edge
5. Vacuum all fiberglass dust

Step 5 — Install GT40 Rear Exhaust Tube

1. Apply marine anti-seize to V-band clamp threads
2. Position stainless gasket on the waterbox outlet flange
3. Apply thin bead of exhaust sealant to outboard face of gasket
4. Position GT40 stainless tube against waterbox flange
5. Slide V-band clamp around joint, snug finger-tight
6. Rotate tube to align with through-hull exit — straight approach, no side load
7. Torque V-band to 22 Nm (16 ft-lb) in two passes

Step 6 — Install Through-Hull Outlet Flange

1. Apply marine sealant around the through-hull cut perimeter
2. Position the supplied outlet flange against the hull exterior
3. Thread 4 M8 × 25 mm SHCS through the flange + hull, with flat washers + lock nuts on interior side
4. Snug all 4 in a cross pattern
5. Wait 5 minutes for sealant skin-cure
6. Torque all 4 M8 to 8 Nm (71 in-lb) in star pattern — over-tightening causes hull distortion

Step 7 — Reconnect Cooling Water Injection

1. Cut supplied silicone hose to length
2. Slide #12 clamps onto each end
3. Push hose onto engine block water source + onto the new GT40 tube's water injection port
4. Tighten clamps to 3 Nm (27 in-lb)
5. Verify hose routing — minimum 25 mm clearance to hot exhaust components

Step 8 — Allow Sealant Cure

Minimum 2 hours cure time before engine start. 24 hours recommended for full strength. Saltwater exposure should not occur for 48 hours minimum.

Step 9 — First Start

After cure:

1. Walk engine bay — every fastener, every clamp
2. Reconnect battery negative — torque to 10 Nm
3. Connect cooling flush hose
4. Start engine on trailer — idle 60 seconds, listen for exhaust leaks
5. Verify water flow at through-hull outlet
6. Bring to 3000 RPM briefly — confirm new exhaust note (loud, free-flow)
7. Scan with Yamaha YDS — no new fault codes

If clean: install complete.

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Post-Install

Break-In

First water test: conservative throttle for first 5 minutes — gradual ramp to half throttle. Sealant final cure happens during this period.

The Yamaha 1.8L/1.9L exhaust note will be substantially louder than stock. SVHO trim shows the most dramatic change. Some hulls resonate at specific RPM (typically 4500-5500) — normal hull acoustics, not a fault.

Tuning

Mechanical-only. No tune required. Stage 2 tune yields full gain when paired with intake + intercooler + waterbox.

Service

• Inspect V-band joint every 25 hours
• Re-torque V-band at 10-hour service to 22 Nm
• Inspect through-hull seal annually for sealant migration
• Annual freshwater flush of the exterior tube for saltwater operation
• Replace silicone water injection hose every 3 seasons

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Troubleshooting

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Torque Reference Summary

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Warranty

90-day GT40 warranty per standard policy. Does not cover improper hull cut, premature engine start before cure, operation without proper water flow, or use outside fitment.

Claims: support@gt40marine.com with order, photos, failure description.

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Support

• Email: support@gt40marine.com
• Site: gt40marine.com
• Install help: include SKU, hull HIN, trim, model year, and engine displacement in correspondence

Built in the United States. Designed for buyers who want the best.

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This guide is © 2026 GT40 Marine. All rights reserved.