Sierra Piston Rings 18-3921 for Mercury Marine V-6 135-175 HP, 3.125 Inch Standard Bore, Replaces 39-93481A12

The Sierra 18-3921 is a piston ring set engineered for Mercury Marine V-6 outboards in the 135-175 HP range fitted with 3.125 inch standard-bore pistons. Sierra manufactures its piston rings to match OEM dimensions and tolerances, giving a factory-correct seal between the piston and cylinder wall for proper compression and oil control during a rebuild or piston replacement. This ring set is used with the 3.125 inch standard bore piston application on this engine family and applies to both the port and starboard cylinder banks, since the ring specification is identical for both sides at this bore size (only the piston part numbers differ by side, not the ring set).

These rings are a direct cross-reference to Mercury Marine OEM part number 39-93481A12, so they install using the same ring gap and groove specifications as the original equipment rings. They are sold as part of Sierra's line of Mercury outboard 2-ring inline piston components, intended for engine builders and mechanics performing a top-end rebuild, re-ring, or piston swap on a compatible V-6 outboard powerhead.

Specifications

• Brand: Sierra
• Manufacturer Part Number: 18-3921
• Category: Mercury Outboard 2 Ring Inline Pistons
• Bore Size: 3.125 inches, Standard
• Engine Fitment: Mercury Marine V-6, 135-175 HP
• OEM Cross Reference: 39-93481A12
• UPC: 030999621665
• Weight: 0.06 pounds
• Package Dimensions: 8 x 6 x 4 inches

Fitment

• Mercury Marine V-6, 135-175 HP, 3.125 inch bore, Standard size, port and starboard cylinders

Replaces (OEM)

• 39-93481A12

Frequently Asked Questions

What engines does the Sierra 18-3921 piston ring set fit? The Sierra 18-3921 fits Mercury Marine V-6 outboards rated 135-175 HP that use a 3.125 inch standard bore piston.

Is the 18-3921 for the port or starboard cylinder bank? The 18-3921 ring set applies to both the port and starboard cylinders on the 3.125 inch standard bore V-6 application, since the ring specification is the same for both sides; only the piston part numbers differ between port and starboard.

What OEM part number does the 18-3921 cross-reference? Sierra 18-3921 cross-references Mercury Marine OEM part number 39-93481A12.

What bore size are these piston rings sized for? This set is sized for a 3.125 inch standard bore, with no oversize (overbore) adjustment.

How much does the 18-3921 piston ring set weigh? The Sierra 18-3921 weighs 0.06 pounds.

Are Sierra 18-3921 rings a genuine OEM part or aftermarket? The 18-3921 is a Sierra-manufactured aftermarket part built to the same specification as Mercury Marine OEM number 39-93481A12.

Can I use the 18-3921 rings for a re-ring without replacing the pistons? Yes, the 18-3921 is a piston ring set intended for use with existing 3.125 inch standard bore pistons on the compatible Mercury V-6 135-175 HP application.

What category of engine components does the 18-3921 belong to? Sierra lists the 18-3921 under its Mercury Outboard 2 Ring Inline Pistons category.

Does the 18-3921 work with an overbore piston? No, the 18-3921 is specified for the 3.125 inch standard bore only; oversize applications use a different Sierra ring part number.

What is the UPC for the Sierra 18-3921 piston ring set? The UPC for the Sierra 18-3921 is 030999621665.