Mercury MerCruiser Quicksilver Screw 10-55300 (5/16"-18 × 5.250")
Free returns on all eligible orders
You have 30 days to request a return. All sale items are final sale.
Mercury Quicksilver Screw 10-55300 – 5/16”-18 × 5.250”
This is a genuine OEM Mercury Marine / Quicksilver structural screw used in MerCruiser engine and sterndrive assemblies. At 5.250 inches long with a 5/16”-18 UNC thread, this is a long-reach fastener suited to deep-stack assemblies such as exhaust manifolds, risers, engine mounts, or gimbal housing positions where standard-length screws cannot reach. Always confirm the correct diagram position using your engine model and serial number before ordering.
Part Details
- Part Number: 10-55300 (also referenced as 55300)
- Brand: Mercury Marine / Quicksilver
- Official Product Type: Screw / Structural Fastener
- Thread: 5/16”-18 UNC (0.312” diameter, 18 threads per inch)
- Length: 5.250 inches
- System: Engine or drive assembly fastening — confirm diagram position for your application
- Condition: New OEM
Compatible Applications
- MerCruiser engine assemblies requiring a 5/16”-18 × 5.250” screw — specific position varies by model and serial number range
- Commonly used in deep-stack fastening positions: exhaust manifolds, risers, engine mounts, or housing assemblies
- Verify fitment using the Mercury parts diagram, engine model, and serial number before ordering
- Match thread pitch, diameter, and length exactly — do not substitute a shorter or longer screw in a structural position
Dimensions
- Thread diameter: 0.312 in (5/16”)
- Thread pitch: 18 TPI (UNC coarse thread)
- Length: 5.250 in (133.4 mm)
- Approximate weight: 80 g (0.08 kg)
Installation Notes
- Verify thread specification and length against the parts diagram before installation
- Inspect threads in the mating component — chase or repair threads if necessary before installing a new screw
- Follow Mercury service manual torque specifications — long screws in deep assemblies are prone to over-tightening
- Apply thread sealant or anti-seize only where specified by the Mercury service manual
- Replace any screw showing corrosion, thread damage, or stretch deformation
Service Interval
Inspect fasteners whenever the assembly is disassembled for service. Replace any screw that shows corrosion, thread damage, or deformation. Mercury recommends replacing structural fasteners disturbed during major disassembly rather than reusing them in critical load-bearing positions.
Parts Desk Insights
Best use case: Replacement of a corroded or damaged long-reach structural screw in a deep-stack engine or drive assembly. The 5.250” length is specific — this screw is used where a standard 1” or 2” fastener cannot reach the mating thread, so having the correct length on hand before disassembly avoids delays mid-job.
Common mistakes: Substituting a shorter screw because it threads in — a screw that doesn’t engage the full thread depth in a structural position will loosen under vibration and can cause assembly failure. Also confirm the thread is UNC (coarse, 18 TPI) and not UNF (fine thread): both 5/16” diameters exist in Mercury assemblies and the threads are not interchangeable. Always torque to the Mercury service manual specification, not a generic fastener chart.
Related Parts
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
Returns & Satisfaction Guarantee
We stand firmly behind the quality of every product we sell. Your confidence matters to us — and we back it with a straightforward 30-Day Satisfaction Guarantee.
If you are not fully satisfied, you may request a return within 30 days of delivery. All returns must receive pre-approval prior to being sent back. Unauthorized returns will not be accepted.
To qualify for a return:
Item must be in new, unused condition
Product must be in its original packaging
All parts, components, hardware, manuals, and accessories must be included
Packaging must be undamaged and suitable for resale
Important:
Electronics are strictly inspected. No returns on open-box electronics. Exceptions may apply only with prior written approval. Any electronic unit that has been installed, powered, activated, or altered may not be eligible for return.
We are serious about product integrity — and equally serious about taking care of our customers. If there is ever a product issue, contact us immediately so we can make it right.
For return authorization, please contact our support team before shipping anything back.