Sierra Marine Thermostat Temperature Chart

A marine engine thermostat controls coolant flow as the engine warms. When replacing one, the opening-temperature rating is only part of the identification: the original housing, seat, diameter, height and engine application also matter. This guide compares eight Sierra Marine thermostat listings so you can confirm the exact part number and whether the replacement includes a gasket.

Three checks before choosing a marine thermostat

Engine identificationFind the engine make, model and serial number, then consult the correct cooling-system parts diagram or Sierra application catalog.

Thermostat ratingRead the original rated opening temperature. A 140°F thermostat is not an approved substitute for a 143°F or 144°F unit without engine-manufacturer confirmation.

Seal and housingConfirm the seat, gasket and thermostat dimensions. One listing may include a gasket while another requires a separate part.

Sierra marine thermostat MPN, temperature and gasket chart

Temperatures and included-gasket details below are the specifications attached to these exact store products. They do not constitute an engine fitment chart.

Sierra MPN	Listed temperature	Gasket included?	Selection note
18-3549	120°F	No	Use the exact original part and engine application; identical temperature ratings do not confirm fit
18-3525	122°F	No	Use the exact original part and engine application; identical temperature ratings do not confirm fit
18-3535	130°F	No	Use the exact original part and engine application; identical temperature ratings do not confirm fit
18-3632	140°F	No	Use the exact original part and engine application; identical temperature ratings do not confirm fit
18-3543	143°F	No	Use the exact original part and engine application; identical temperature ratings do not confirm fit
18-3631	144°F	No	Use the exact original part and engine application; identical temperature ratings do not confirm fit
18-3554	160°F	No	Use the exact original part and engine application; identical temperature ratings do not confirm fit
18-3677	160°F	Yes	Use the exact original part and engine application; identical temperature ratings do not confirm fit

160°F is not a shared fitment: Both 18-3554 and 18-3677 are listed at 160°F, but they are separate manufacturer part numbers and the catalog says only 18-3677 includes a gasket. Match the actual engine and housing, not just the opening temperature.

What to verify when servicing the cooling system

Confirm the engine's full model and serial range, cooling system type and original thermostat manufacturer number.
Compare the original thermostat's diameter, seat design, installed height and valve configuration against the replacement drawing.
Check the correct rated temperature from the engine maker. Do not change to a hotter or cooler thermostat solely because it is available.
Confirm whether the housing needs a separate gasket or sealing component. For listings marked “gasket not included,” inspect the OEM parts list for the required seal.
Follow the engine manufacturer's draining, refill, bleeding and tightening procedures. A damaged housing or incorrectly installed thermostat can cause overheating.

Frequently asked questions

Can two marine thermostats with the same temperature be swapped? Not without verifying the complete engine application, housing and geometry. The temperature rating alone does not establish interchangeability.

Does Sierra 18-3677 come with a gasket? Its current store catalog specification identifies a gasket as included. The separate 18-3554 listing does not.

Do all Sierra thermostats in the chart include a seal? No. Only the listed 18-3677 is shown with a gasket in this eight-part selection. Confirm current product packaging on the specific listing.

Find Sierra Marine thermostats by exact MPN

Open the exact product number below to review availability, condition and package contents. The product photos are catalog images, not a physical-condition assessment of an individual item.