Mercury MerCruiser Gearcase Shim Set 15-34800A1 | Speedmaster / SSM / Alpha One SS

Mercury MerCruiser Gearcase Shim Set — Part No. 15-34800A1

This is the genuine Mercury Marine OEM precision shim set listed under part number 15-34800A1 (base number 34800A1). Used during gearcase rebuilds on select MerCruiser performance, racing, and legacy drive applications, this shim set establishes the correct gear depth, bearing preload, and internal gearcase alignment where the Mercury service procedure specifically calls for this number. Precision shimming is not optional on these drives — it directly affects gear tooth contact, bearing load, and long-term gearcase reliability.

Part Details

• Part Number: 15-34800A1
• Base Number: 34800A1
• Type: Precision Gearcase Shim Set
• Package Dimensions: 7.50 in. L x 5.00 in. W x 0.25 in. H
• Listed Weight: 0.05 lb (23 g)
• GTIN / UPC: 745061137585
• Type: Genuine Mercury Marine OEM shim set

Compatible Applications

• Mercury Speedmaster / Super Speedmaster gearcase — where service procedure specifies 15-34800A1
• MerCruiser SSM II / SSM III / SSM IV / SSM V — where service procedure specifies
• MerCruiser Alpha One SS — where service procedure specifies
• MerCruiser III drive configurations — select applications per service procedure
• Mercury Racing Dry Sump Six / M8 / SSM Six / SSM Seven / NXT1 — where gearcase service procedure calls for this shim set

Confirm fitment by drive model, serial number, gearcase type, gear ratio, bearing location, and exact Mercury service procedure before ordering. Shim selection is determined by measured gear depth, bearing preload, and gear contact pattern — not by outside diameter or appearance alone.

Alternate & Superseded Part Numbers

• 15-34800A1 — current Mercury/MerCruiser formatted number
• 34800A1 — base part number

System & Diagram Position

This shim set is used in the internal gearcase assembly — specifically at bearing cup, gear depth, and preload positions as called out in the Mercury service manual for the applicable drive. The exact shim location(s) and thickness selection must be determined using the correct measuring tools and setup procedure for the specific gearcase being rebuilt. Refer to the Mercury service manual for the drive model and serial number being serviced.

When to Replace

Use this shim set during gearcase rebuilds, bearing cup replacement, gearset replacement, or any internal drive service where the original shim stack is missing, damaged, corroded, distorted, or no longer producing the correct preload and gear contact. Shims are also required when replacing bearings, gears, shafts, or housings that affect internal gear alignment.

Installation Tips

• Measure and record the original shim stack at each location before disassembly whenever possible
• Keep shims organized by position — do not mix shims from different bearing or gear locations
• Final shim thickness must be selected using the correct Mercury service manual, precision measuring tools, and the gearcase setup procedure for the specific drive
• Do not estimate shim thickness — incorrect shimming causes gear whine, overheating, bearing failure, poor tooth contact, and accelerated wear
• Verify gear contact pattern and bearing preload before closing the gearcase

Technical FAQ

Q: Can I reuse the original shims if they look undamaged?
A: Possibly, if they are flat, uncorroded, and the correct thickness for the measured setup. However, any time bearings, gears, or housings are replaced, the shim stack should be re-measured and verified — not assumed to be correct from the previous build.

Q: How do I know which shim thickness I need?
A: Shim thickness is determined by measuring gear depth and bearing preload with the correct Mercury service tools and following the setup procedure in the service manual for the specific drive. There is no shortcut — the measurement process is required for a correct setup.

Q: Is this shim set compatible with aftermarket gears or bearings?
A: Shim compatibility depends on the dimensions of the specific parts being installed. If aftermarket components are used, verify that their dimensions match OEM specifications before applying OEM shim selection procedures.

Parts Desk Insights

Best use case: This shim set is the right call for a technician doing a full gearcase rebuild on a Speedmaster, SSM, or Alpha One SS drive where the Mercury service procedure specifically calls for 15-34800A1. It’s a precision setup part — not a general-purpose washer — and it belongs in the hands of someone working from the correct service manual with the proper measuring tools. On performance and racing gearcases, even small shim changes affect gear tooth contact and bearing load, so the setup process matters as much as the part itself.

Common mistakes: The most frequent error is selecting shim thickness by eye or by matching the original stack without re-measuring after new components are installed. Any time bearings, gears, or housings change, the shim stack must be re-verified. The second common mistake is ordering this shim set without confirming the exact service procedure calls for 15-34800A1 — similar-looking shim sets exist for different gearcase positions and drive models, and ordering the wrong one delays the rebuild.