SeaSierra Thermostat Kit for OMC Cobra & Volvo Penta 4.3–5.8L — 160°F Small Volvo‑Style

SeaSierra 383307 Thermostat Kit is a direct-fit replacement thermostat assembly designed for OMC Cobra and Volvo Penta engine drives on 4.3L, 5.0L, 5.7L and 5.8L Ford- and GM‑based V6/V8 marine engines.

This kit supplies the small-size thermostat used by many OMC Cobra and Volvo Penta lower units, with a 160°F (71°C) temperature rating. It replaces several OEM and aftermarket part numbers, and is intended as a maintenance or repair component for cooling systems that require the smaller Volvo-style thermostat. Before installing, check your existing thermostat style and part number to ensure this kit matches your drive assembly.

Which engines and drives will accept this thermostat kit?

The SeaSierra 383307 fits most 4.3L, 5.0L, 5.7L and 5.8L engines built from Ford and General Motors V6 and V8 platforms when used with compatible OMC Cobra and Volvo Penta lower units. It is especially common on setups where the smaller Volvo thermostat was originally installed.

• Compatible with OMC Cobra and Volvo Penta engine drives
• Suitable for many Ford- and GM-based 4.3L–5.8L V6/V8 marine engines
• Designed as the smaller Volvo-style thermostat element

What temperature does this thermostat open at and why that matters?

This thermostat has a rated opening temperature of 160°F (71°C). That temperature helps the engine reach and maintain its designed operating range, balancing warm-up speed with cooling control to protect internal components from overheating under load.

• Rated at 160°F / 71°C for consistent thermal control
• Helps stabilize engine operating temperature under varying loads
• Manufactured to match common OEM thermostatic characteristics

Which part numbers does this SeaSierra kit replace, and how to confirm fit?

This kit can substitute for several OEM and aftermarket identifiers, but fit should be confirmed visually and by part number. The listing replaces numbers like 383307, 3856961, 778056, 307239, 312797 and Sierra 18-3677, though Volvo and OMC sometimes used different sizes.

• Replaces: 383307, 3856961, 778056, 307239, 312797, Sierra 18-3677
• Compare your existing thermostat dimensions and shape before ordering
• Contact SeaSierra or your mechanic if your drive previously used the larger Volvo thermostat

How should I check compatibility before installation?

Inspect the thermostat you are removing and compare its shape, overall diameter, and mounting style to this kit. Volvo used two different thermostat sizes in some years; this SeaSierra kit is the smaller version, so double-check to avoid ordering the wrong size.

• Remove and photograph your current thermostat for comparison
• Verify stamped part numbers or measurements against this kit
• Reach out to SeaSierra support with vehicle and drive details if unsure

Who this is for: This kit is intended for boat owners, marine technicians and DIY mechanics who need a reliable, small-size replacement thermostat for OMC Cobra or Volvo Penta lower units on 4.3L–5.8L Ford- or GM-derived V6/V8 engines. If you operate a vessel with the smaller Volvo thermostat style and require a 160°F (71°C) replacement or a direct substitute for the listed part numbers, this SeaSierra thermostat kit offers a cost-effective maintenance option. If your drive used the larger Volvo thermostat or you cannot confirm the existing part, consult your service manual or contact SeaSierra before purchasing to ensure correct fitment.