Essential Shipyard University · Outdrives & Outboards
Gear Ratio Tool
The decal is gone. It usually is. By the time a drive is being replaced, the ratio decal has weathered off, been painted over, or the drive has been swapped at some point in its life and never re-tagged. You still have to know the ratio — it decides which drive or gearset you order, and it decides propping. This tool determines the ratio from what's physically in front of you.
Outdrives and outboards. MerCruiser Alpha One and Bravo One / Two / Three; Volvo Penta SX, DPS and the legacy Duoprop and Aquamatic drives; and outboard gearcases across the brands. Both have a lower unit you can turn and count.
Not inboard shaft drive — that ratio is the marine gearbox's, and it's stamped on the box's own plate. Look, don't measure.
Prop off, in forward gear. Turn at the input, stop on a whole number of prop shaft revolutions, and read the fraction at the input. With the engine still in the boat — which is most of the time — the input is the crankshaft pulley. Setup notes below.
Count teeth on the drive (pinion) and driven gear of every reduction stage. The overall ratio is all stages multiplied together.
How to count turns — the field method
Take the propeller off, and make it impossible for the engine to start.
Prop off. You are turning a drivetrain by hand and watching a shaft go round. There is no reason for a set of blades to be on the end of it while you do that — a prop you're rotating by hand is a laceration and pinch hazard, and it's the one part of this job that can hurt you badly for no gain. The prop is usually off at replacement time anyway.
Ignition dead. Key out, kill-switch lanyard pulled, battery disconnected, helm tagged so nobody touches it while you're at the drive. An engine that fires while you're at the gearcase is a life-changing injury. Nobody near the key, nobody in the cockpit, and say out loud what you're doing before you start.
While the prop is off anywayCheck behind the hub for wound fishing line, which cuts the prop shaft seal and is a common route for water into a gearcase. And check prop shaft play against the manual's figures — on Alpha One Gen II, side-to-side under .003 in. (0.08 mm) and rotational .005 in. (0.127 mm) or less.Tier 1 · pub. 90-806534970 Out-of-spec play doesn't change the ratio, but it changes what the job is.
Pick the setup you're actually in. The math is the same in all three — input revolutions divided by prop-shaft revolutions.
Situation
Turn this, count that
Notes
Sterndrive, engine in place (the usual case)
Turn at the crankshaft pulley — socket and breaker bar on the crank bolt. Watch the bare prop shaft and stop on a whole revolution. Read the fraction at the crank.
With the prop off there's no lever at the shaft, so the crank is the end you drive from — and it's the end with the leverage anyway.
Sterndrive, drive off the boat
Turn the U-joint yoke, count prop shaft revolutions.
Cleanest of the lot — no engine, no compression, both ends in easy reach. If the drive is coming off anyway, count it here.
Outboard, on the transom
Turn the flywheel, count bare prop shaft revolutions.
Crank to driveshaft is 1:1, same as a sterndrive, so flywheel turns are gearcase input turns.
Outboard, lower unit off
Turn the driveshaft, count prop shaft revolutions.
If it's already down for a water pump, take the count on the bench.
With the engine in place you're measuring crank-to-prop — which is the number that actually matters. On a sterndrive the crankshaft drives the U-joint shaft 1:1, and on an outboard the crank drives the driveshaft 1:1 — so crank (or flywheel) turns and gearcase input turns are the same count. Turning the engine and counting the prop gives you the ratio directly. Crank-to-prop is the total reduction, and total reduction is what governs propping and WOT rpm — so with the engine aboard this is the more useful measurement, not a compromise.
Put the drive in FORWARD gear. In neutral the clutch dog is disengaged and the prop shaft isn't coupled — you'll turn the input and nothing happens at the other end.
Mark both ends. A paint mark or tape flag on the bare prop shaft and on the crank pulley (or yoke / flywheel / driveshaft), each against a fixed reference on the housing or block. A bare shaft gives a far sharper read than a blade passing a point.
Take up the backlash in the direction you're about to turn, and call that zero. Start and finish at the same take-up point or you bake the backlash into the answer.
Turn at the input, and watch the output. Count the prop shaft's revolutions and stop on a whole one — ten is the number to use. Smoothly, one direction, no reversing mid-count.
Read the fraction at the input against its mark. The input has turned more times than the output, which is exactly where the resolution comes from.
Enter both numbers above. The tool converts to a ratio and tells you whether you turned enough revolutions to separate it from the next ratio in the family.
Do it twice. Two counts that agree is a result. Two that disagree is a miscount — better found now than after the parts order.
Turning against compression — the count-wrecker. With the plugs in, the engine will fight you over each compression stroke and then spring back when you ease off. If the drivetrain rocks backward mid-count, your count is corrupted and you won't necessarily notice. Keep steady pressure, never let it kick back, and if it does — start over. This is the single biggest reason the plugs-out setup is worth the extra few minutes.
Plugs out means a compression test. That's the standard. If you're pulling spark plugs on a gas engine to spin it freely — or injectors on a diesel — you run a compression test while you're in there. The access is already paid for, the cylinder is open, and the tool is in your hand. Record the readings per cylinder against engine hours. A ratio count and a compression baseline out of one setup is a good hour's work.
Diesels are a different problem. Diesel compression is far too high to turn comfortably by hand, and pulling injectors is not the casual job that pulling plugs is. Use a decompression lever if the engine has one; otherwise plan on doing the count with the drive off, or turn at the crank with proper leverage and accept that it's slow. Do not improvise a way to spin a diesel over — and never bump the starter to move it, with anyone's hands anywhere near the prop.
Turn counting doesn't care how many gearsets are inside. This is the reason to reach for it first. Turning the output and counting the input measures the whole drivetrain at once — one gearbox or three stages, it makes no difference to the number you get. Tooth counting is exact, but it only gives you the drivetrain ratio if you count every stage: on a compound drive with an upper and a lower gearset, counting just the upper produces a plausible-looking number that is simply wrong. When you don't know what's inside, count turns.
Why ten turns and not one. Ratios inside a family sit close together — 1.50, 1.65, 1.81. Stop on one prop-shaft revolution and misread the input by a tenth of a turn, and your ratio is off by 0.10 — most of the way to the next ratio. Take it to ten prop revolutions and that same tenth-of-a-turn misread becomes 0.01. The error divides by the number of turns you take, which is the entire reason this method works. The tool computes your uncertainty from the turn count you enter and says plainly whether it's tight enough.
Identifying it without turning anything
Try these first — they're faster when they survive:
Method
Where
Confidence
Housing decal
Port side of the drive, reads as a ratio in brackets, e.g. "(1.50R)", alongside a seal numberTier 1
Good — when it's still legible. Usually it isn't.
Letter stamped on the U-joint yoke
Stamped on the universal joint splined yoke, e.g. "F"Tier 1
Survives far better than a decal. You need the family's letter-to-ratio table to read it — ours is not yet confirmed (see below).
Serial number lookup
OEM parts catalog, by drive serial
Most reliable when the serial plate survives. Do this before counting anything.
Count the gears
Drive and driven gears in the drive shaft housingTier 1
Exact. Requires the drive apart.
The case where the decal and the stamp both lie. Mercury's own manual states that if a drive has been altered for high-altitude service, neither the decal nor the yoke stamp is valid, and the ratio must be determined by counting teeth on the drive and driven gears.Tier 1 · Bravo Series Service Manual, pub. 90-863160, §3A p.3A-10 The same logic applies to any drive whose gearset has been changed at some point in its life and never re-tagged — which, on a boat old enough to need a drive, is not unusual. If the decal reading and your turn count disagree, believe the turn count.
Letter codes are not confirmed. Reported Bravo Three yoke codes — F = 1.50, C = 1.65, G = 1.81 — come from technician forums, not a factory table.Tier 4 Do not order a gearset on the strength of a letter alone. Use the letter as a cross-check against a turn count, and confirm against the Mercury parts catalog for that serial.
Duoprop and counter-rotating drives — take care. A dual-prop drive has two counter-rotating output shafts, and the tooth counts feeding them are not necessarily identical. Count on one shaft, note which shaft you counted on, and treat the result as that shaft's ratio until you've confirmed how the family expresses it. We have not verified how each duoprop family publishes its ratio — flag it rather than assume.
Why the ratio has to be right
Ratio, propeller pitch and hull type are one decision, not three. Fit a drive with a different ratio and the boat will not reach its wide-open-throttle rpm range — over-revving or lugging, either of which loads the whole drivetrain wrong for the rest of its life and hands the customer a boat that doesn't perform like the one they had. Ratio is also selected for the new package, not simply copied off the old one, on any repower where the engine changed.
Record the ratio you determined, the method you used, and the serial number on the ticket. The next tech should not have to do this twice.
How the math works. Turn count: ratio = input turns ÷ prop-shaft turns. Uncertainty = your reading error on the input (taken here as ±0.1 turn) divided by the number of prop turns. Tooth count: ratio = driven teeth ÷ drive teeth, multiplied across stages for a two-stage drive.
Ratio tables. Bravo Two's five ratios are Tier 1 from Mercury pub. 90-863160 §3A p.3A-10. Every other family's table on this page is unconfirmed and marked accordingly — they are there to tell you what a plausible answer looks like, not to be ordered from. Confirm against the OEM parts catalog for the drive's serial before ordering a drive or a gearset. Determining a ratio and ordering a part are two different decisions.
