18-3555 Thermostat Kit 160° for Mercruiser | JLM Marine

Thermostat Kit 160° for Mercruiser – Replaces 18-3555

This 160°F (71°C) thermostat kit is engineered for Mercruiser 3.7L 4-cylinder marine engines, covering models 170/470 from 1980–1989 and Mercury-built 4-cylinder non-Chevy engines (except 470, 1976–1982). Designed as a direct aftermarket replacement, it ensures reliable temperature regulation for optimal engine performance.

Direct Replacements: Mercury 99155A1, 99155T2; Sierra 18-3555; and additional cross-references listed below. This complete kit includes the thermostat, rubber ring, and gasket for a proper installation.

• Operating Temperature: 160°F / 71°C
• Complete Kit: Includes thermostat, rubber ring, and gasket
• Wide Compatibility: Fits multiple Mercruiser 3.7L and Mercury-built 4-cylinder engine series

Part Number Cross-Reference

Use this table to confirm equivalent part numbers across OEM and aftermarket brands.

Compatible Engine Models

Confirm your engine model/year in the table below. For Mercury owners, here's a guide to help you identify your engine's serial/model: Mercury Serial Number Guide. You can also review exploded diagrams to verify part numbers: Mercury Parts Diagrams & Catalog.

Technical Specifications

Package Contents

This complete thermostat kit includes:

• Thermostat: 99155A1 × 1
• Ring Rubber: × 1
• Gasket: 27-33918 × 1

Installation Guide

1. Preparation: Allow the engine to cool completely before starting. Drain the coolant into a clean container for reuse or proper disposal.
2. Access the Thermostat Housing: Locate the thermostat housing on your Mercruiser engine. Remove the hose clamps and disconnect any coolant hoses attached to the housing.
3. Remove Old Thermostat: Unbolt the thermostat housing cover (typically 2–4 bolts). Carefully lift out the old thermostat and rubber ring.
4. Clean Mating Surfaces: Scrape off the old gasket material from both the housing and cover using a gasket scraper. Ensure both surfaces are clean and dry.
5. Install New Gasket: Position the new gasket (27-33918) onto the housing, aligning all bolt holes.
6. Install New Thermostat: Place the new thermostat (99155A1) into the housing with the correct orientation (temperature-sensing element typically faces toward the engine). Install the new rubber ring around the thermostat.
7. Reassemble Housing: Position the thermostat housing cover back onto the housing. Hand-tighten the bolts first, then torque to manufacturer specifications in a crosswise pattern.
8. Reconnect Hoses: Reattach all coolant hoses and secure with hose clamps.
9. Refill Coolant: Add fresh coolant or the reused coolant back into the system. Fill to the proper level per your engine manual.
10. Bleed Air: Start the engine and allow it to reach operating temperature. Check for leaks around the thermostat housing and bleed any trapped air from the cooling system.
11. Final Check: Monitor the temperature gauge to confirm the thermostat is opening at approximately 160°F. Top off coolant as needed.
12. Dispose Properly: Dispose of old coolant and parts according to local environmental regulations.

Troubleshooting & FAQ

Q: How do I confirm this thermostat fits my Mercruiser engine?
A: Check the compatibility table above for your engine model and year. Cross-reference your current thermostat part number (99155A1, 99155T2, 18-3555, etc.) with our list. If your engine is a Mercruiser 3.7L 4-cylinder (170/470, 1980–1989) or Mercury-built 4-cylinder non-Chevy (except 470, 1976–1982), this kit is designed to fit.

Q: What are the symptoms of a failing thermostat?
A: Common signs include engine overheating, running too cool (below normal operating temperature), erratic temperature gauge readings, or poor cabin heat. If coolant is leaking from the thermostat housing, inspect the gasket and rubber ring for wear.

Q: How often should I replace my thermostat?
A: Thermostats should be inspected annually and typically replaced every 3–5 years or per your engine manufacturer's maintenance schedule. Replace immediately if you notice overheating, temperature fluctuations, or visible corrosion/damage.

Q: Can I use a different temperature thermostat in my engine?
A: Always use the thermostat temperature specified by your engine manufacturer. This kit is rated at 160°F (71°C) for specific Mercruiser applications. Installing a different temperature rating can cause overheating, poor combustion, or engine damage.

Q: Do I need to replace the gasket and rubber ring every time?
A: Yes, always replace the gasket and rubber ring when installing a new thermostat. Reusing old gaskets can lead to coolant leaks and improper sealing, which may cause overheating or engine damage.

Q: Is this an OEM or aftermarket part?
A: This listing is an aftermarket replacement part (not an OEM component). We build to OEM specifications using comparable-grade materials. Production is done in an ISO9002-certified facility, and each batch is inspected for consistent performance.

Q: How fast do you ship and how much does it cost?
A: We offer fast global shipping—typically 5–15 days worldwide—and standard shipping is free. Expedited options are available at checkout depending on your location.

Q: What is your warranty and return policy?
A: This product includes a 1-year warranty against manufacturing defects under normal use. If something goes wrong, contact our support team with your order details. Returns are accepted within our standard return window (unused & in original condition); please see our policy page or reach out for instructions.

Important Notes

• Verify that the product images and specifications match your original part. If discrepancies exist—even if your model is listed—do not proceed with the purchase.
• This product is an aftermarket replacement part and is not an OEM component.
• Professional installation is recommended to ensure safety and optimal performance.

Why Choose JLM MARINE?

• Quality Assured: As a trusted manufacturer for over 20 years, we supply top marine brands globally.
• Fast Global Shipping: Free delivery worldwide in 5–15 days.
• Direct from Manufacturer: Competitive pricing without compromising quality.