Mercury MerCruiser Stainless Steel Washer 12-34931 | 0.406" x 0.734" x 0.063"
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Mercury MerCruiser Stainless Steel Washer — Part No. 12-34931
This is the genuine Mercury Marine OEM stainless steel washer listed under part number 12-34931 (base number 34931). A precision-dimensioned flat washer in marine-grade stainless steel, it is used where the Mercury or MerCruiser parts diagram specifically calls for this number — providing the correct bearing surface, load distribution, and corrosion resistance for the assembly location. When the diagram calls for 12-34931, a generic hardware-store washer is not an equivalent substitute.
Part Details
- Part Number: 12-34931
- Base Number: 34931
- Type: Flat Washer — Stainless Steel
- Inside Diameter (ID): 0.406 in.
- Outside Diameter (OD): 0.734 in.
- Thickness: 0.063 in.
- Material: Stainless steel (marine-grade corrosion resistance)
- Type: Genuine Mercury Marine OEM washer
Compatible Applications
- Use where the Mercury, MerCruiser, or Quicksilver parts diagram specifically calls for part number 12-34931 or 34931
- Confirm fitment by engine or drive serial number, assembly location, and current Mercury parts diagram before ordering
Washer fitment must be confirmed by part number and diagram position. Similar washers vary in inside diameter, outside diameter, thickness, and material — do not substitute by size alone.
Alternate & Superseded Part Numbers
- 12-34931 — current Mercury/MerCruiser formatted number
- 34931 — base part number
When to Replace
Replace this washer if it is corroded, deformed, cracked, missing, or no longer providing a flat bearing surface. Also replace any time the fastener assembly it belongs to is disassembled for service — especially in saltwater or high-vibration environments where washers can corrode or fatigue over time.
Installation Tips
- Confirm the washer is installed in the correct orientation and position as shown in the Mercury parts diagram
- Inspect the mating surface and fastener threads for corrosion or damage before reassembly
- Follow the Mercury service manual torque specification for the specific assembly location
- Do not substitute a washer of a different thickness — the 0.063" thickness is part of the designed fastener stack-up at this location
Technical FAQ
Q: Can I use a standard hardware-store stainless washer of the same size?
A: Not recommended for diagram-specified locations. OEM marine washers are dimensioned and graded for the specific fastener stack-up and clamping requirements of the assembly. A hardware-store washer may differ in thickness tolerance, material grade, or surface finish in ways that affect the joint.
Q: How do I find where this washer is used on my engine or drive?
A: Look up your engine or drive serial number in the Mercury parts diagram and confirm the diagram calls for 12-34931 at the specific assembly location you are servicing. If you’re unsure, our parts team can help verify before you order.
Parts Desk Insights
Best use case: This washer is the right call when the parts diagram specifically calls for 12-34931 and an OEM-dimensioned stainless replacement is needed. Its stainless steel construction makes it well-suited to the corrosive marine environment, and the precise 0.406" ID / 0.734" OD / 0.063" thickness means it fits the designed fastener stack-up correctly. It’s a low-cost part that’s easy to overlook and worth ordering ahead of any service that involves disassembling the assembly it belongs to.
Common issues: The most frequent problem is substituting a generic washer by approximate size, then discovering the thickness or OD doesn’t match the original stack-up. Even small dimensional differences can affect how the fastener clamps the assembly. Always order by part number from the diagram. The second common issue is not replacing corroded or deformed washers during service — a washer that looks serviceable may have lost its flatness or surface integrity after years in a marine environment.
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.