Mercury MerCruiser Hex Screw 10-34500 | 5/16"-18 x 5/8" OEM Fastener

Mercury MerCruiser Hex Screw — Part No. 10-34500

This is the genuine Mercury Marine OEM hex screw listed under part number 10-34500 (base number 34500). A 5/16"-18 coarse-thread fastener at 5/8" in length, it is one of the most widely used screws across the Mercury and MerCruiser engine and sterndrive lineup — appearing in cylinder block assemblies, camshaft setups, accessory brackets, and drive components across a broad range of outboard and sterndrive applications. When the parts diagram calls for 34500 or 10-90285, this is the correct OEM replacement.

Part Details

• Part Number: 10-34500
• Base Number: 34500
• Supersedes / Alternate: 10-90285
• Thread Diameter: 0.312 in. (5/16 in.)
• Thread Pitch: 18 TPI (coarse thread, 5/16"-18)
• Length: 0.625 in. (5/8 in.)
• Type: Genuine Mercury Marine OEM hex screw

Compatible Applications

• MerCruiser 2.5L inline-4 — sterndrive and inboard applications, per diagram
• MerCruiser 3.0L inline-4 — sterndrive and inboard applications, per diagram
• MerCruiser 4.3L V6 — sterndrive and inboard applications, per diagram
• MerCruiser 5.0L V8 — sterndrive and inboard applications, per diagram
• MerCruiser 5.7L / 350 MAG V8 — sterndrive and inboard applications, per diagram
• MerCruiser Black Scorpion V8 — per diagram
• MerCruiser MX 6.2L V8 — per diagram
• MerCruiser 7.4L / 454 MAG V8 — sterndrive and inboard applications, per diagram
• Mercury / Mariner outboards — 65 HP Jet through V-150, V-175, and V-200 EFI platforms, per diagram

Always confirm fitment by engine or drive serial number and the correct Mercury parts diagram before ordering. Thread pitch and length must match the exact diagram callout for the assembly location.

Alternate & Superseded Part Numbers

• 10-34500 — current Mercury/MerCruiser formatted number
• 34500 — base part number
• 10-90285 — superseded / alternate reference

When to Replace

Replace this screw any time it shows signs of corrosion, thread damage, rounding, stretching, or deformation, or whenever it is removed from a critical fastened joint during engine or drive service. In block and camshaft assemblies, a compromised fastener can affect torque retention and joint integrity under heat and vibration cycles.

Installation Tips

• Confirm thread pitch (5/16"-18) and length (5/8") against the parts diagram before installation — visually similar screws in different lengths are common across the same engine
• Follow the Mercury service manual for the correct torque specification for the specific assembly location
• Check whether the service manual calls for thread-locking compound at this location before installing
• Do not substitute a hardware-store screw — OEM fasteners are specified for material grade and coating appropriate to the assembly
• In blind holes, confirm the replacement screw is not longer than the original — bottoming out before clamping is a common installation error

Technical FAQ

Q: Why does this screw cost more than a hardware-store equivalent?
A: OEM marine fasteners are specified for thread class, material grade, and corrosion resistance appropriate to the assembly. A hardware-store substitute may look identical but not meet the clamping and corrosion requirements for engine block, camshaft, or drive applications exposed to heat, vibration, and saltwater environments.

Q: Can I reuse this screw after removing it?
A: In non-critical bracket locations, reuse may be acceptable if the screw shows no corrosion, thread damage, or deformation. In block, camshaft, or torque-sensitive locations, the service manual may specify replacement. When in doubt, replace it — at this price point, it’s not worth the risk.

Q: My engine isn’t listed — will this fit?
A: This screw appears across a very wide range of Mercury and MerCruiser applications. If your engine serial number and diagram call for 34500 or 10-90285, this is the correct part. If you’re unsure, our parts team can help verify before you order.

Parts Desk Insights

Best use case: This screw is the right call when the parts diagram specifically calls for 34500 or 10-90285 in a block, camshaft, bracket, or drive assembly. Its wide application range across the Mercury and MerCruiser lineup makes it a practical item to keep in service stock if you work on multiple engines or platforms. It’s also a smart add-on for customers ordering gaskets, seals, or top-end rebuild kits — having the correct fasteners on hand before teardown saves a trip mid-job.

Common mistakes: The most frequent error is ordering by thread size alone. Mercury uses 5/16"-18 screws in multiple lengths across the same engine, and a screw that is 1/4" too long can bottom out in a blind hole before the component is properly clamped. Always match by part number and diagram position. The second common mistake is substituting a generic hardware-store screw — which may not meet the material or grade specification for the assembly — especially in block and camshaft locations where torque retention matters.