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How to Drill Jets using Numbered Drill Bits?

Numbered drill bit sizes range from 80 (smallest) to 1 (largest). The numbers on numbered drill bits relate to wire gauges, so larger numbers indicate smaller drills.


Mercury 2-stroke outboard carb jet sizes, on the other hand, are related to thousandths of an inch which matches typical jet sizes found in both main and air jets in WH, WMV, WMH, and other carburetor types.


A jet size primary passage bore in diameter indicates flow rate. Larger main jets (which carry fuel) are "fatter", while, larger air jets (which carry air) make those circuits "leaner".


While jets should be reamed or bored (not drilled) to ensure a consistent diameter, drilling jets can be an affordable way to make bigger jets to save or tune a motor that is too lean.


This numbered drill bit chart is correlated with typical outboard carburetor jet sizes. Numbered drill bit kits can be purchased at your favorite tool stores.


We've created this chart as a convenient reference with the most common numbered drill bits sizes (39 to 59) used to enlarge more common smaller jets to larger sizes when otherwise not available to purchase.


To drill a jet with numbered drill bits, you can try these steps:

  • Run the engine with the stock main jet, and determine if too lean

  • If lean, use a drill bit that's one size larger to drill out the jet

  • Run the engine again and take another read

  • Repeat until jetted properly

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