How to Rebuild a Yamaha F255TXRC Lower Unit

Welcome to our complete rebuild of the lower unit on a Yamaha F225 outboard. It’s unlikely you’ll ever have to undertake such a thorough overhaul, but if you need to make repairs to your Yamaha outboard’s lower unit, knowing how to rebuild it will help.  

Watch our Yamaha F225TXRC Lower Unit Rebuild video above and follow the steps below to learn how it’s done. Use our Yamaha F225TXRC outboard diagrams for additional reference.

Tools Needed – Yamaha F225 Outboard Lower Unit Rebuild

• Yamaha shim gauge
• Puller tool
• Digital caliper
• Bearing driver set

• Slide hammer puller
• Depth gauge with magnetic base
• Press

NOTE: Besides the common tools found in your toolbox, you may need some of the specialty tools listed above, depending on what kind of repair you’re doing. The OEM Yamaha F225TXRC parts needed will also vary depending upon the repair work you’re doing. 

Part 1: Sizing Yamaha F255TXRC Lower Unit Bearing Shims

Step 1. Note the numbers stamped on the lower unit housing beneath the trim tab. These are the deviation numbers in millimeters as measured by Yamaha, which enable you to accurately shim and position the forward, reverse and pinion gears.

Step 2. Place the forward bearing on a perfectly flat surface. Use a Yamaha shim gauge and feeler gauges to measure the gap between the bearing and the tool. Next, use the deviation numbers and the feeler gauge numbers to determine the correct shim size for the forward bearing.

Step 3. Install the shim or shims and the forward bearing race into the lower unit.

Step 4. Press the forward gear into the forward bearing, and press the reverse bearing into the bearing housing.

Step 5. Place the reverse bearing and bearing housing on a perfectly flat surface. Using a Yamaha shim gauge and feeler gauges, measure the gap between the bearing and the tool. Use the deviation numbers and the feeler gauge numbers to determine the correct shim size for the reverse bearing.

Watch the clip below to see the lower unit bearings sizing segment of our Yamaha F225TXRC lower unit rebuild video.

Part 2: Installing Yamaha F255TXRC Lower Unit Bearings

Step 1. Press the shim or shims and the reverse gear into the reverse bearing in the bearing housing. Next, press the needle bearing into the other end of the bearing housing.

Step 2. Use a length of threaded rod, a couple of nuts and a socket piece to pull the driveshaft needle bearing into the lower unit housing.

Step 3. Install the bearing, spacer, a shim and the bearing housing onto the driveshaft.

Step 4. Install the forward gear into the lower unit.

Step 5. Install the driveshaft into the lower unit, then install the pinion gear, washer and nut onto the driveshaft. Tighten the bearing housing on the driveshaft, then torque the driveshaft to 103 foot-pounds.

Watch the clip below to see the lower unit bearings installation segment of our Yamaha F225TXRC lower unit rebuild video.

Part 3: Assembling Yamaha F255TXRC Shift Assembly & Prop Shaft

Step 1. Assemble the shift assembly as shown in the diagram below and install the assembly into the prop shaft.

Step 2. Install the shifter and the clutch dog onto the prop shaft. Next, install the cross pin and cross pin ring to lock the shift assembly in place.

Step 3. Install two new oil seals and two new O-rings into the bearing housing.

Step 4. Install the prop shaft into the lower unit housing, then install the bearing housing over the prop shaft.

Step 5. Assemble the shift shaft, and install it into the lower unit housing. Then, install the water seal and the shift shaft plate.

Watch the clip below to see the shift and prop shaft assembly segment of our Yamaha F225TXRC lower unit rebuild video.

Part 4: Sizing Yamaha F255TXRC Driveshaft Bearing Shims

Step 1. Attach a flat plate to the top of the lower unit housing, and attach a tab to the driveshaft with a clip.

Step 2. Measure the gap for the forward gear backlash by putting a puller on the prop shaft, and tighten it to the point the shaft can no longer rotate. Next, invert the lower unit and use a depth gauge to measure the backlash.

Step 3. Measure the gap for the reverse gear backlash by putting a propeller on the prop shaft, and tightening it to the point the shaft can no longer rotate. Next, invert the lower unit and use a depth gauge to measure the backlash.

Step 4. Take the backlash numbers and use the equation in the image below to calculate the shim size for the driveshaft.

Watch the clip below to see the driveshaft bearing shim sizing segment of our Yamaha F225TXRC lower unit rebuild video.

Part 5: Installing Yamaha F255TXRC Driveshaft Bearing Shims

Step 1. Remove the driveshaft from the lower unit, and remove the bearing housing from the driveshaft.

Step 2. Add a shim to the existing shim to achieve the shim size calculated in the previous section, or remove the existing shim and use a different shim or shims to achieve the correct shim size.

Step 3. Reinstall the bearing housing onto the driveshaft, then reinstall the driveshaft into the lower unit and torque it to 103 foot-pounds.

Step 4. Re-measure the forward and reverse gear backlash using the method in the last section. If the backlash measurements aren’t within the Yamaha tolerances, you’ll have to change the forward and/or the reverse bearing shim accordingly.

Watch the clip below to see the driveshaft bearing shim installation segment of our Yamaha F225TXRC lower unit rebuild video.

All that remains now is to do a pressure test on the lower unit and refill the lower unit with fluid. Watch the video below to see how to pressure test a Yamaha F225 outboard lower unit.