Sierra 18-3650 Complete Thermostat Housing Kit, Replaces 3377341, for MerCruiser 120/140 153/181 CID L4 Engines

The Sierra 18-3650 is a complete thermostat housing kit engineered as a direct replacement for MerCruiser 120 and 140 model sterndrive engines built around the 153 and 181 cubic inch L4 blocks. Rather than a bare thermostat, this is a full housing kit, meaning the components needed to replace a worn or corroded thermostat housing assembly are included in one part number, simplifying the repair instead of requiring multiple separate purchases. It is intended for engines within the 1968-1989 production span covered by the applicable MerCruiser 120/140 L4 engine families, including the later 2.5L 153 cid L4 variant.

This kit directly replaces the OEM thermostat housing components originally used across several engine year ranges, and cross-references to numerous factory MerCruiser part numbers so it can be identified and ordered using either the Sierra number or the original equipment number. For the 1968-1972 120/153 L4 application, the kit corresponds to engines fitted with a cast iron exhaust manifold. Installation follows standard thermostat housing service: drain the cooling system, remove the old housing and thermostat, install the new gasket and housing from the kit, and refill and bleed the cooling system per the engine service manual.

Specifications

• Brand: Sierra
• Manufacturer Part Number: 18-3650
• Product Type: Complete thermostat housing kit
• Category: Mercruiser Thermostats & Gaskets
• Weight: 0.13 pounds
• UPC: 030999236920
• OEM Cross-Reference: 2324347, 3377340, 3377341, B531853, B531854, C853255, 2508261, B450800

Fitment

• MerCruiser 120, 153 cid L4, 1968-1972 (cast iron exhaust manifold engines)
• MerCruiser 120, 153 cid L4, 1972-1986
• MerCruiser 2.5L, 153 cid L4, 1987-1989
• MerCruiser 140, 181 cid L4, 1968-1986

Replaces (OEM)

• 2324347
• 3377340
• 3377341
• B531853
• B531854
• C853255
• 2508261
• B450800

Frequently Asked Questions

What does the Sierra 18-3650 kit include? The Sierra 18-3650 is a complete thermostat housing kit, meaning it supplies the housing assembly components needed to replace a worn or leaking factory thermostat housing in a single part number rather than requiring separate individual pieces.

Which engines does the 18-3650 thermostat housing kit fit? It fits MerCruiser 120 and 140 sterndrive engines built on the 153 cid and 181 cid L4 blocks, covering the 1968-1972, 1972-1986, and 1968-1986 model ranges depending on engine variant, as well as the 2.5L 153 cid L4 engine used from 1987-1989.

Does the 18-3650 kit work on the 1968-1972 MerCruiser 120? Yes, for the 1968-1972 MerCruiser 120 with the 153 cid L4 engine, the 18-3650 kit applies to engines fitted with a cast iron exhaust manifold.

What OEM part numbers does the Sierra 18-3650 replace? The 18-3650 cross-references to OEM numbers 2324347, 3377340, 3377341, B531853, B531854, C853255, 2508261, and B450800, covering the various MerCruiser 120/140 L4 engine years it fits.

Is the 18-3650 the same part as the Sierra 18-3551? No. The 18-3551 is a separate Sierra part number used for the MerCruiser 140 six-cylinder application, while 18-3650 is the complete housing kit for the 120/140 L4 (153/181 cid) applications listed.

How much does the Sierra 18-3650 thermostat kit weigh? The Sierra 18-3650 thermostat housing kit weighs 0.13 pounds.

Will this kit fit a 2.5L MerCruiser engine? Yes, the 18-3650 kit fits the MerCruiser 2.5L 153 cid L4 engine for the 1987-1989 model years.

Can I identify this part by its UPC? Yes, the Sierra 18-3650 thermostat housing kit carries UPC 030999236920.

Is the 18-3650 an aftermarket or genuine OEM part? The 18-3650 is a Sierra-branded replacement kit designed to be a direct equivalent for the original MerCruiser thermostat housing components it cross-references, rather than a factory MerCruiser-branded part.

What is the difference between the 120 and 140 applications for this kit? Both the MerCruiser 120 (153 cid L4) and 140 (181 cid L4) engines use the Sierra 18-3650 kit, but each engine family has its own set of OEM cross-reference numbers listed for the specific model years it covers.