Outboard Lower Unit Explained: What Is Inside, How It Comes Off and When to Replace the Whole Unit

The lower unit is the gearcase at the bottom of an outboard: it turns the rotation of the vertical driveshaft into thrust at the propeller. Suzuki's service manuals describe a bevel gear reduction, a gear-type reversing system (forward, neutral and reverse on most models) and, on most models, a rubber hub in the propeller that protects the driveline from impact; the early DF2.5 and the early DF4/DF5 use a shear pin. The cooling water pump sits on top of it.

If your gear oil came out milky or carrying metal, or the gearcase took a hit, there are two routes: have a shop open the unit, replace what is damaged and re-shim the gears, or fit a complete Suzuki lower unit, which comes fully assembled, is special order and cannot be returned. This page covers Suzuki four-strokes from the DF2.5 to the DF350A.

What is inside a lower unit

From the parts lists in the DF9.9B/DF15A/DF20A and DF70A/DF80A/DF90A/DF100B service manuals:

• Driveshaft. Runs down from the power unit. Its top engages the crankshaft on the DF9.9B/15A/20A and a driven gear shaft on the DF70A–DF100B.
• Pinion, forward and reverse gears. The pinion is the bevel gear at the bottom of the driveshaft. It drives the forward gear, which has its own bearing in the gearcase, and the reverse gear, which sits in the propeller shaft bearing housing.
• Clutch dog shifter. A sliding collar on the propeller shaft between the gears. It locks one gear to the shaft, or neither in neutral.
• Shift rod and shift cam. The shift rod joins the engine's clutch rod and carries a stepped cam, which on the DF9.9B/15A/20A works a push rod and push pin inside the propeller shaft against a return spring. The DF70A–DF100B uses a shifter yoke and connector pin.
• Propeller shaft bearing housing. Held in the gearcase by two bolts; it carries the two propeller shaft oil seals.
• Water pump. Its impeller is keyed to the driveshaft; a water tube carries the water up to the power unit.
• Anode. Bolted to the gearcase. The DF70A–DF100B gearcase also carries the trim tab.

What changes as the motors get bigger

The manuals for the larger motors and a few special versions add these.

• Two-stage reduction. From the DF70A/DF80A/DF90A/DF100B up, a crankshaft drive gear turns a driven gear under the powerhead, and the lower unit gears are the second reduction. The top of the driveshaft engages that driven gear shaft; on the DF8A through DF60A it engages the crankshaft.
• Counter rotation. On the DF115AZ/DF140AZ, DF115BZ/DF140BZ, DF150AZ/DF175AZ/DF200AZ, DF200Z/DF225Z/DF250Z and DF300Z the propeller shaft turns counterclockwise in forward. The DF100A to DF200A manuals call this the left hand rotation unit. Inside, the forward and reverse gears trade places, so it is a separate assembly, shimmed in its own order.
• Suzuki Selective Rotation. On the AP models one lower unit serves for regular or counter rotation without being changed. The ECM (engine control module) reads which rotation select connector is fitted: blue for regular and white for counter on the DF150AP/DF175AP/DF200AP, blue and green on the DF300AP (the DF250AP pages name the two connectors without giving colors). Main switch OFF and a one-minute wait come before any swap.
• Electronic shift. The DF300, the AP models, the DF250ATSS and the DF300B/DF325A/DF350A shift with an actuator on the engine. On the DF300, the AP models and the DF250ATSS the main relay or shift actuator fuse comes out before the clutch link rod does. After a lower unit is replaced, the manual calls for a shift position sensor voltage check (2007 DF300 and DF250ATSS) or shift learning (2008 DF300). The DF250S supplement for serial 340001 and later prints the same voltage check, although its lower unit steps name no shift actuator or clutch link rod.
• Dual propellers. The DF300B/DF325A/DF350A manual calls its gearcase the Dual PROP Lower Unit: front and rear propellers on outer and inner propeller shafts, contra gears, a separate reverse gear set and a two-piece driveshaft. Both propellers must have the same number of blades, diameter and pitch, or the lower unit can be damaged.
• High-thrust DF50AV/DF60AV. For these models the manual says the lower unit has been changed: a larger gearcase holding 1,050 ml of gear oil against 610 ml, a 12:29 gear set and a bigger propeller with a different propeller bush. The DF50A/DF60A propeller bush cannot be used on it; the manual says the DF50AV/DF60AV use the same bush as the DF70A/80A/90A.
• Serial-number changes. From serial 210001 the DF8A/DF9.9A gears went from spiral bevel to straight bevel, with a different clutch dog shifter and forward gear thrust washer. The DF25A/DF30A AQ has its own clutch dog shifter; the manual says never to use the A/AR/AT one on it. The DF70A/DF80A/DF90A/DF100B manual (from 2023/12) and the DF100C/DF115B/DF140B manual (from 2024/06) each add a revised propeller shaft assembly from a set serial number, saying which marked side of the clutch dog shifter faces the reverse gear. That is why we ask for the serial number.

Gear ratios by family:

The DF2.5 has no reverse gear: its manual lists the transmission as forward and neutral and shows only a pinion and a forward gear. The DF4/DF5/DF6 row is the earlier family of that name, not the current DF4A/DF5A/DF6A. On the counter-rotation DF115AZ and DF140AZ the propeller shaft turns counterclockwise in forward, and that lower unit is listed as its own assembly; the counter rotation and AP selective rotation units carry the same ratios as the regular units. Oil quantities are in our gear oil capacity chart.

How the lower unit comes off and goes back on

Suzuki warns that removing or installing a lower unit without proper precautions can result in severe personal injury. The DF9.9B and DF70A procedures start by disconnecting the battery cable and removing the emergency stop switch lock plate.

DF9.9B, DF15A and DF20A

Removal:

1. Loosen the clutch rod lock nut.
2. Unscrew the connector to separate the clutch rod from the shift rod.
3. Remove the four bolts and separate the gearcase from the driveshaft housing.

Installation:

1. Insert the two dowel pins.
2. Apply water resistant grease to the driveshaft splines and a light coating of Suzuki silicone seal to the gearcase and driveshaft housing mating surfaces.
3. Slide the lower unit into place so the top of the driveshaft engages the crankshaft and the water tube locates in the water pump case outlet. Apply silicone seal to the retaining bolts and tighten them to 17 N·m (12.3 lbf-ft).
4. Reconnect the shift linkage. The connector is a long right-hand-thread nut, not a turnbuckle: screw it fully onto the clutch rod and the lower nut fully onto the shift rod. Holding the shift cam, clutch lever and shift lever at neutral, screw the connector onto the shift rod until it contacts the lower nut, then tighten the nut against it.
5. Shift from neutral to forward and reverse and check that both gears start to engage at an equal angle from neutral.

DF70A, DF80A, DF90A and DF100B

Removal:

1. Shift to neutral. Remove the bolt and the trim tab if necessary.
2. Remove the five bolts and separate the gearcase from the driveshaft housing, disconnecting the speedometer pickup tube before the gearcase comes away completely.

Installation:

1. Make sure the shift rod is at neutral.
2. Insert the two dowel pins.
3. Apply water resistant grease to the driveshaft and shift rod splines and a light coating of Suzuki silicone seal to the mating surfaces.
4. Set the clutch control lever at neutral and slide the lower unit into place so the top of the driveshaft engages the driven gear shaft and the water tube locates in the water pump case outlet. The clutch rod may need a slight turn right or left for the splines to align.
5. Apply silicone seal to the five gearcase bolts and tighten them to 55 N·m (40.0 lbf-ft).
6. Shift the clutch control lever from neutral to forward and reverse to check gear engagement.

DF150A, DF175A and DF200A

Removal:

1. Disconnect the battery cable, remove the emergency stop switch lock plate and remove the STBD lower side cover.
2. AP models: remove the main relay or the shift actuator fuse first, to avoid shift actuator damage, then the clip, washer and clutch link rod from the clutch control lever.
3. Shift to neutral and remove the bolt and trim tab.
4. Remove the seven gearcase bolts and separate the gearcase from the driveshaft housing.

Installation:

1. If the clutch rod was removed, grease its splines and install it, aligned with the splines in the clutch shaft.
2. Turn the shift rod right or left to neutral.
3. Insert the two dowel pins, grease the driveshaft and shift rod splines and coat the mating surfaces lightly with silicone seal.
4. With the clutch control lever at neutral, slide the lower unit into place so the driveshaft engages the driven gear shaft and the water tube locates in the water pump case outlet.
5. Apply silicone seal to the seven bolts and tighten them: 10 mm to 55 N·m (40.0 lbf-ft), 12 mm to 83 N·m (60.0 lbf-ft).
6. Check gear engagement in forward and reverse and install the side cover; on AP models, connect the clutch link rod with its washer and clip.

DF300B, DF325A and DF350A

Removal (serial 340001 and later):

1. Disconnect the battery cable and remove the emergency stop switch lock plate.
2. Make sure the clutch is in neutral.
3. Remove the screw and the driveshaft housing cap.
4. Remove the eight bolts and separate the gearcase from the driveshaft housing; remove the driveshaft housing seal if necessary.

Installation:

1. Confirm the shift actuator arm is in neutral: connect the battery cables, turn the main switch from OFF to ON and back to OFF, wait about one minute, check the arm, then disconnect the battery cables.
2. Install the driveshaft housing seal and turn the shift rod right or left to neutral.
3. Insert the two dowel pins, grease the driveshaft and shift rod splines and coat the mating surfaces lightly with silicone seal.
4. Slide the lower unit into place so the driveshaft engages the driven gear shaft, the water tube locates in the water pump case outlet and the clutch rod engages the shift rod.
5. Apply silicone seal to the eight bolts and tighten them: 10 mm to 55 N·m (41 lbf-ft), 12 mm to 85 N·m (63 lbf-ft).
6. Install the driveshaft housing cap and tighten its screw to 5.0 N·m (3.7 lbf-ft).

Gearcase fasteners for every family

These are the torques printed in the procedures; the torque tables print 54 N·m for the DF100A/DF115A/DF140A and DF100C/DF115B/DF140B bolt, and 50 N·m (37 lbf-ft) in 2022/08 for the DF300B/DF325A/DF350A 10 mm bolt. The DF300, DF250AP/DF300AP row is from the supplement for serial 240001 and later; the 2007 DF300 base manual prints the same sequence with 40.0 lb-ft and 60.0 lb-ft.

From the DF8A up, every manual fits two dowel pins and puts water resistant grease on the driveshaft splines and silicone seal on the mating surfaces and gearcase bolts. The DF8A/DF9.9A, DF25A/DF30A and DF40A/DF50A/DF60A shift linkage is reconnected as in the DF9.9B steps.

What differs on the DF2.5 and the DF4/DF5/DF6

Both manuals start by disconnecting the spark plug cap. On the DF2.5 the power unit comes off first (four engine mounting bolts), the shift rod is taken out of the clutch rod arm and two bolts release the gearcase; they go back with silicone seal at 8 N·m (6.0 lb-ft). On the DF4/DF5/DF6 the gear oil is drained first, the rod connecting plate bolt is loosened about one turn, and two nuts (L transom) or two bolts (S transom) release the gearcase. They go back with silicone seal at 17 N·m (12.5 lb-ft), with the water tube seated in the water pump outlet hole and the shift rod through the rod connecting plate. The clutch is then set with the lever in neutral and the push rod exactly on the neutral position of the shift cam, and the plate bolt is tightened to 11 N·m (8.0 lb-ft).

What a rebuild involves

On the DF9.9B/DF15A/DF20A the order is: lower unit off, propeller and water pump off, gear oil drained. Two bolts release the propeller shaft bearing housing, which is pulled with the propeller shaft using a sliding hammer and a propeller shaft remover. The driveshaft, its oil seal housing and the shift rod lift out. That frees the pinion gear with its thrust washers, thrust bearing and back-up shim, then the forward gear with its thrust washer and shim. Last, the propeller shaft is stripped of the reverse gear and shim, push rod and push pin, dog pin, clutch dog shifter and return spring.

The DF70A–DF100B differs in one way that matters: its pinion gear is held by a pinion nut, tightened to 120 N·m (86.8 lbf-ft). Suzuki recommends the original nut for shimming and a new one for final assembly. The DF9.9B/15A/20A has no pinion nut; the driveshaft is lowered through the pinion. The DF25A/DF30A has one too, at 18 N·m (13.0 lbf-ft), and so does the DF40A/DF50A/DF60A: 45 N·m (33 lbf-ft) on the standard unit, 120 N·m (88.5 lbf-ft) on the DF50AV/DF60AV.

A counter rotation unit comes apart in a different order. On the regular DF100C/DF115B/DF140B unit the forward gear and its thrust bearing come out of the gearcase after the pinion and the reverse gear stays with the propeller shaft bearing housing; on the left hand rotation unit the reverse gear comes out after the pinion and the forward gear stays with the housing. The other counter rotation manuals describe the same swap.

What gets inspected

Parts are judged by eye: any component that is excessively worn, cracked, defective or damaged is replaced. The manuals name the gearcase (replace if cracked or damaged), gear teeth and engaging dogs, bearings (pitted, noisy or rough), shaft splines (worn or twisted), oil seals and O-rings (nicked, cut or worn) and the impeller (vanes cut, torn or worn). One of the few printed measurements is the DF9.9B/15A/20A clutch return spring: 70 mm (2.8 in.) free length, with a 67 mm (2.6 in.) service limit. A pinion bearing or oil seal is not reused once removed, and Suzuki recommends new seals, gaskets and O-rings on assembly.

Shims, backlash and thrust play

This step decides whether a rebuilt unit lasts. Suzuki requires shimming for gear contact and backlash to be checked or adjusted whenever a lower unit has been rebuilt or has had components replaced; if the original gears go back in, no shim or thrust washer is added or left out. The DF9.9B/15A/20A and DF70A–DF100B each have five shim and washer positions: pinion gear back-up shim, forward gear back-up shim, forward gear thrust washer, propeller shaft reverse thrust washer and reverse gear back-up shim. The DF8A/DF9.9A, DF25A/DF30A, DF40A/DF50A/DF60A and standard DF100A to DF250 units match; the DF300T has four.

The DF2.5, DF4/DF5/DF6, DF9.9B/15A/20A and DF70A–DF100B manuals all add a tooth contact check: the forward gear gets a light coat of Prussian blue, the driveshaft is rotated and the pattern is read off the gear. Too much backlash (or driveshaft thrust play on the DF70A–DF100B) means a thicker forward gear back-up shim, too little a thinner one. Top-side toe contact calls for a thinner forward gear shim and a slightly thicker pinion shim, bottom-side toe contact the opposite, and Suzuki warns that either extreme can chip the gears. Propeller shaft thrust play is measured last, with a dial gauge. Suzuki's notice: failure to correctly adjust the gear position will result in lower unit damage.

Rows that show driveshaft thrust play print no backlash figure. On the DF25A/DF30A and DF40A/DF50A/DF60A that play sets the forward gear back-up shim, the tooth contact pattern is checked, and the reverse gear back-up shim is then set by measuring the play again with the reverse gear and propeller shaft fitted.

A counter rotation unit is shimmed in the opposite order: the reverse gear and pinion first with the forward gear left out, then the forward gear. Its propeller shaft thrust play is adjusted only with a thrust shim, in place of the regular unit's reverse thrust washer.

The selective rotation units, the DF250W and the dual-prop unit are shimmed to zero propeller shaft thrust play. The thrust shim is the measured play plus 0.1 mm (0.004 in.) on the DF150AP/DF175AP/DF200AP and plus 0.2 mm on the DF250AP/DF300AP and DF250W. The dual-prop gearcase has seven shim positions, and its reverse pinion gear thrust play is 0.2–0.4 mm (0.008–0.016 in.).

The DF300T and DF300Z rows are printed only in the 2007 DF300 base manual. Four cells give millimeters only, because the printed inch figure does not match the millimeters: the DF100A/DF115A/DF140A reverse gear backlash, the DF150A/DF175A/DF200A thrust play, and the DF250W and DF250AP/DF300AP driveshaft thrust play.

Special tools and the pressure test

The DF9.9B/15A/20A procedure calls for a sliding hammer (09930-30104), a propeller shaft remover, a shift lock pin remover (09922-89810), a pinion bearing remover and installer, the gear adjusting gauge (09951-09530) and a gearcase oil leakage tester (09950-69512) with a hand air pump. The DF70A–DF100B adds a driveshaft holder for the pinion nut.

The other manuals call for the same gauge; the counter rotation, DF250AP/DF300AP and dual-prop gearcases add tools of their own.

The finished gearcase is pressure-tested through the oil level hole. The DF2.5, DF4/DF5/DF6, DF9.9B/15A/20A and DF70A–DF100B manuals print the same figures: 100 kPa (14.2 psi), steady for at least 5 minutes once stabilized, and never more than 110 kPa (15.6 psi), because excess pressure damages the oil seals. The DF2.5, DF4/DF5/DF6 and DF70A–DF100B manuals also call for a low initial pressure of 20 to 40 kPa (2.8 to 5.7 psi) first.

The nine other manuals print the same 100 kPa test and 110 kPa maximum, and all but the DF8A/DF9.9A manual print the 20 to 40 kPa first step. Most also repeat the hold of at least 5 minutes. Psi conversions vary slightly: the DF100C/DF115B/DF140B manual prints 14.5 psi and 16.0 psi. On the DF150A/DF175A/DF200A, the DF200/DF225/DF250 base models and the DF300B/DF325A/DF350A the tester goes in the oil drain hole.

Shimming a gearcase is shop work. It takes the gauge, a shim assortment, the leakage tester and experience reading a contact pattern. Suzuki's manual is written for authorized dealers and qualified service technicians, and says do-it-yourself mechanics without the proper tools and equipment may not be able to perform the services properly. If the motor is under warranty, check with your dealer first.

Signs the lower unit needs attention

The first clue is the gear oil, which Suzuki says to check at every drain for:

• water ingress: a white, milky appearance;
• overheating or burning: a black appearance and a charred smell;
• metal filings or deposits.

Milky oil means water is getting in. Suzuki warns that a used plug gasket may leak and let water into the gearcase, causing severe damage, so plug gaskets are not reused. Our guide to changing lower unit gear lube covers the drain and fill.

The second is the lower unit trouble chart in the DF9.9B/15A/20A and DF70A–DF100B manuals:

• Engine stalls when engaging gear: idling speed set too low, or propeller shaft bearings damaged, lacking lubrication or seized.
• Engaging gear has no effect on propulsion: clutch out of adjustment, driveshaft broken or splines damaged, chipped or worn dog clutches, or a propeller not secured correctly.
• Loss of power with a healthy engine: propeller bush slipping, or a bent or worn propeller.
• Engine shakes the boat: failed propeller bush, bent driveshaft or propeller shaft, or a damaged propeller.
• Clutch will not engage or disengage: seized shift rod, detached clutch shaft and clutch rod, seized or broken remote control cable, or a problem at the control box end.

Rebuild or replace

A rebuild fits when the damage is contained: seals that let water in, a rough bearing, a chipped clutch dog, a bent propeller shaft. Parts are available individually, including gear sets and pinion gears, lower unit bearings and seals and shims and spacers. The real work is the shimming, pattern check and pressure test, so the shop doing it matters more than the parts list.

A complete unit fits when the gearcase itself is cracked or damaged, which the manuals answer with one word (replace), or when gears, bearings and shafts are all damaged at once. Every complete Suzuki lower unit we list is a genuine Suzuki assembly that comes fully assembled with all internal gears, shafts and seals installed.

Before you order one:

• The part number is specific. A complete unit is one Suzuki part number per model, model year, color and shaft length. On Suzuki's June 2023 parts bulletin, for example, the 2014–2015 DF9.9B/DF15A/DF20A unit in Shadow Black is 50010-89826-0EP with a 15 in. shaft and 50020-89826-0EP with a 20 in. shaft; the last three characters are the color code. The full list is on our complete lower unit part number lookup.
• Rotation and version matter on the bigger motors. Regular or counter rotation, AP selective rotation, AS, AV high thrust and shaft length are each a different Suzuki part number; check the lower unit part number lookup.
• It is special order. We order the unit from Suzuki when you place the order. Once submitted, the order cannot be canceled or changed, and lower units are non-returnable and non-refundable.
• Confirm first. Send us the model code and serial number from the motor's ID plate and we will confirm the part number that fits. The number we order today can be newer than the one printed in the bulletin.

Routine care that keeps you out of the gearcase

Suzuki's gear oil interval is a first change after 20 hours (1 month), then every 100 hours. The calendar limit printed with it is 6 months in the DF4/DF5/DF6 manual and 12 months in the DF25A/DF30A, DF70A/DF80A/DF90A/DF100B, DF100A/DF115A/DF140A and DF100C/DF115B/DF140B manuals. The DF2.5, DF8A/DF9.9A and DF40A/DF50A/DF60A manuals printed 6 months and later changed it to 12 (from serial 410001 on the DF2.5 and serial 310001 on the other two). Go by the schedule in your own manual. Recheck the level 10 minutes after filling. The water pump is reached by removing the lower unit, so plan the impeller for the same visit: see the impeller interval guide and the impeller replacement how-to. Replace an anode once two-thirds of it has corroded away and never paint it (see the anode guide). With the propeller off, coat the propeller shaft splines with water resistant grease and check the propeller bush for slipping; propellers by model are in the propeller chart.

Parts for this job. Complete assemblies are in our Suzuki complete lower unit collection. For a rebuild, start with lower unit and gearcase parts and finish with fresh ECSTAR lower unit gear oil. Boat Max Online is an authorized Suzuki, Honda and Yamaha Marine dealer.

Sources: Suzuki service manuals 99500-97J01-01E (DF2.5), 99500-91J03-01E (DF4/DF5/DF6), 99500-99J00-01E (DF8A/DF9.9A), 99500-89L00-01E (DF9.9B/DF15A/DF20A), 99500-94L10-01E (DF25A/DF30A), 99500-88L11-01E (DF40A/DF50A/DF60A), 99500-87L10-01E (DF70A/DF80A/DF90A/DF100B), 99500-92J00-01E (DF100A/DF115A/DF140A), 99500-99L10-01E (DF100C/DF115B/DF140B), 99500-96L30-01E (DF150A/DF175A/DF200A), 99500-93J05-01E (DF200/DF225/DF250), 99500-98J02-01E (DF300, DF250AP/DF300AP) and 99500-98L10-01E (DF300B/DF325A/DF350A), with their bound-in supplements; Suzuki Marine USA parts bulletin PA23-13 (June 16, 2023); Boat Max Online catalog, October 2026. Figures are Suzuki's published values; confirm against the manual for your model year and serial number.