Mercury Quicksilver OEM Thermostat Cover Gasket 8503181 | V6 200–300 HP EFI DFI OptiMax Pro XS SportJet
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Mercury Quicksilver OEM Thermostat Cover Gasket — Part 8503181
- Part Number: 8503181
- Full Catalog Number: 27-8503181
- Brand: Mercury Marine / Quicksilver (Genuine OEM)
- Official Product Type: Cooling System — Thermostat Cover Gasket
- System: Cooling — thermostat cover-to-cylinder-head sealing surface
- Supersedes: 850318 / 27-850318 (earlier Mercury reference)
- Qty Required: Many V6 applications require 2 gaskets per powerhead (port and starboard thermostats) — confirm quantity against your engine diagram before ordering
- GTIN / UPC: 745061422780
- Weight: approx. 0.03 kg
What It Does
The 8503181 is the genuine OEM thermostat cover gasket used on select high-horsepower Mercury and Mariner V6 outboards and Mercury SportJet powerheads. It seals the joint between the thermostat cover and the cylinder-head cooling passage, keeping cooling water contained around the thermostat assembly. Mercury parts diagrams identify 27-8503181 directly beneath the thermostat and thermostat cover on numerous 2.5L and 3.0L V6 EFI, DFI, OptiMax, and Pro XS applications. Because many of these engines have a thermostat on both cylinder banks, two gaskets are required for a complete thermostat service on most applicable powerheads — confirm the quantity in your engine diagram before ordering.
Compatible Applications
Confirm fitment by engine serial number and cylinder-block or powerhead diagram before ordering. Mercury used several thermostat and cover configurations across the 2.5L and 3.0L V6 families.
Mercury / Mariner Outboards
- 200 HP 2.5L V6
- 200 HP 3.0L EFI
- 200 HP 3.0L DFI / OptiMax
- 200 Pro XS 3.0L DFI
- 225 HP 2.5L V6
- 225 HP 3.0L EFI
- 225 HP 3.0L DFI / OptiMax
- 225 Pro XS 3.0L DFI
- 225 Sport XS / 225 XS
- 250 HP 3.0L EFI
- 250 HP 3.0L DFI
- 250 Pro XS 3.0L DFI
- 250 Sport XS / 250 XS
- 300X 3.0L EFI Pro Max
- Selected 300 HP / 3.2L XS applications
Mercury SportJet
- 250 DFI Jet Drive
- 250 DFI M2 Jet Drive
- Selected 3.0L DFI SportJet powerheads
Not sure it fits? Send us your Mercury or Mariner engine serial number before ordering.
Related Products
- Mercury Quicksilver Thermostat 130°F — 8M0057307 — thermostat used with this gasket in applicable V6 cooling assemblies
Assembly Context — What Goes Around This Gasket
On many 3.0L V6 applications, Mercury diagrams show the following components at the thermostat cover location:
- 8M0057307 — Thermostat 54°C / 130°F
- 8503181 — Thermostat cover gasket (this part)
- 8M0039357 — Later thermostat cover
- 887601002 — Port thermostat cover (selected applications)
- 887602002 — Starboard thermostat cover (selected applications)
- 82130025 — M6 x 25 thermostat-cover screw
Inspect the thermostat and thermostat cover for corrosion while the assembly is apart. If the thermostat is being replaced, replace this gasket at the same time — reusing a compressed gasket is a common cause of cooling water seepage after reassembly.
Part Numbers & Supersession
- 8503181 — Current Mercury Marine OEM reference
- 27-8503181 — Full Mercury gasket catalog format
- 850318 / 27-850318 — Earlier Mercury reference; superseded by 8503181
When to Replace
Replace the thermostat cover gasket whenever the thermostat cover is removed, and especially when the old gasket is:
- Crushed or permanently compressed from previous service
- Torn during thermostat or cover removal
- Hardened from heat and age
- Showing evidence of cooling-water seepage at the thermostat-cover joint
- Damaged by corrosion around the sealing surface
A thermostat service is the right time to replace this gasket rather than attempting to reuse the original compressed seal.
Service Interval
No fixed replacement interval — replace whenever the thermostat cover is removed. Inspect at each cooling system service and replace if the gasket shows any sign of compression set, cracking, or seepage. Annual inspection is recommended on outboards operated in saltwater or warm-water environments where thermostat cycling is frequent.
Installation Notes
- Confirm the required quantity (port and starboard) from the engine diagram before disassembly — most V6 applications need two gaskets
- Clean the thermostat cover and cylinder-head mating surfaces thoroughly and remove all old gasket material before installing the new gasket
- Inspect the thermostat cover for corrosion or distortion around the sealing face
- Inspect the thermostat while the cover is removed and replace if in doubt
- Follow the applicable Mercury service manual for cover fastener torque and tightening sequence
Technical FAQ
Q: How many 8503181 gaskets does my engine need?
Most applicable 3.0L V6 engines have a thermostat on both the port and starboard cylinder banks, each with its own cover and gasket. That means two 8503181 gaskets are required for a complete thermostat service. Confirm the quantity against the cylinder-block or powerhead diagram for your specific engine serial number before ordering.
Q: What superseded 8503181?
8503181 (27-8503181) is the current Mercury OEM reference, superseding the earlier 850318 / 27-850318. If your older parts list shows the earlier number, order 8503181 as the current replacement.
Q: Is 8503181 used on MerCruiser sterndrive engines?
Verified applications for 8503181 are Mercury and Mariner outboards (V6 EFI/DFI/OptiMax/Pro XS) and Mercury SportJet powerheads. Confirm any MerCruiser sterndrive application against the specific serial-number-based parts diagram before ordering.
Q: Can I reuse the old gasket if it looks intact?
Reusing a thermostat cover gasket that has been compressed in service is not recommended. A gasket that appears undamaged may not reseal correctly after removal, and a minor seep at the thermostat cover on a high-performance outboard can affect cooling efficiency over time.
Parts Desk Insights
The most common issue with this part is customers ordering one gasket when their engine needs two. On V6 Mercury outboards there is usually a thermostat on both cylinder banks, which means both the port and starboard covers need a fresh gasket for a complete service. If the engine is already apart and only one gasket is on hand, that means waiting for a second one before the powerhead can be buttoned up — check the diagram for quantity before the job starts. The other thing to watch is the supersession: older parts lists may still show 850318, but 8503181 is the correct current number to order. If a customer comes in with the old number, 8503181 is the right replacement.
Shipping information
In-stock and OEM parts: ship same or next business day.
Locally warehoused items: ship within 1-3 business days.
Special-order items: timeline confirmed after order.
Returns & warranty
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