Sierra Piston Rings 18-3915,.030 Oversize, 3.217 Bore, Evinrude 50-70 HP 3-Cylinder, Replaces 391416, 434109

The Sierra 18-3915 piston ring set is built for the 3-cylinder, 50-70 HP outboard engine family with a 3.217 inch cylinder bore and a.030 inch oversize specification. Matching your engine's bore size and oversize is critical when rebuilding a powerhead, since installing the wrong ring size can lead to poor compression, oil consumption, or premature cylinder wear. This set is designed to be installed on the corresponding oversize piston during a top-end or full powerhead rebuild, restoring proper ring-to-cylinder-wall seal so the engine can rebuild compression and run efficiently.

Sierra manufactures replacement marine engine components to OEM specifications, and the 18-3915 is cross-referenced to the original equipment piston ring numbers 391416 and 434109, making it a direct fit for engines originally equipped with those parts. It is intended for use by rebuilders and marine technicians performing cylinder and piston service on qualifying 3-cylinder 50-70 HP outboards.

Specifications

• Brand: Sierra
• Manufacturer Part Number: 18-3915
• Product Name: Piston Rings
• Engine Application: 50-70 HP, 3-Cylinder
• Cylinder Bore Size: 3.217 inches
• Oversize:.030 inch
• OEM Cross Reference: 391416, 434109
• Weight: 0.07 pounds
• Package Dimensions: 0.63 x 3.06 x 3.06 inches
• UPC: 030999542809

Fitment

• 50-70 HP, 3-Cylinder outboard engines with a 3.217 inch cylinder bore,.030 inch oversize

Replaces (OEM)

• 391416
• 434109

Frequently Asked Questions

What engine does the Sierra 18-3915 piston ring set fit? It fits 3-cylinder, 50-70 HP outboard engines with a 3.217 inch cylinder bore and a.030 inch oversize specification.

What OEM part numbers does 18-3915 replace? The Sierra 18-3915 is a direct cross-reference to OEM piston ring numbers 391416 and 434109.

Is this a standard or oversize ring set? The 18-3915 is a.030 inch oversize ring set, matching a 3.217 inch cylinder bore rather than the standard bore diameter.

What are the package dimensions of the 18-3915 ring set? The Sierra 18-3915 piston rings measure 0.63 x 3.06 x 3.06 inches as packaged.

How much does the 18-3915 piston ring set weigh? It weighs 0.07 pounds.

When would I need to install oversize piston rings like the 18-3915? Oversize rings such as the 18-3915 are used after a cylinder has been bored out.030 inch over standard, typically during a powerhead rebuild to restore proper compression.

Do I need to replace the piston along with the 18-3915 rings? The 18-3915 ring set is sized to match a 3.217 inch bore,.030 oversize piston, so it should be installed on the corresponding oversize piston for that bore specification.

Is the Sierra 18-3915 a genuine OEM part or aftermarket? The 18-3915 is manufactured by Sierra as a replacement part built to OEM specifications, cross-referenced to original equipment numbers 391416 and 434109.

What is included when I order part number 18-3915? Ordering 18-3915 gets you the complete piston ring set for one 3.217 inch bore,.030 oversize cylinder on a 50-70 HP 3-cylinder outboard.

Will the 18-3915 rings fit a standard-bore cylinder? No. The 18-3915 is specifically sized for a 3.217 inch bore at.030 inch over standard, not a standard-bore cylinder.