Yamaha F255 Outboard Rebuild Part 4: Bottom End 

After stripping down our Yamaha F225TXRC outboard motor, it’s time to rebuild it. 

This guide is a companion to our Yamaha F225TXRC Outboard Rebuild Part 4 video, which shows you how to reassemble the bottom end of the motor. The steps covered in this walkthrough include:

• Sizing the crankshaft bearings
• Installing the crankshaft
• Sizing the connecting rod bearings
• Measuring and adjusting the piston rings
• Installing the pistons

Watch the Yamaha F225TXRC Outboard Rebuild Part 4 video above and follow the steps provided below to rebuild an F225 outboard. Use our Yamaha F225TXRC exploded parts diagrams for additional reference.

Tools and Parts Needed – Yamaha F225 Outboard Rebuild

Tools

• Torque wrench
• Piston ring compressor or piston ring sleeve
• Piston ring pliers

Parts

• Pistons
• Piston ring sets
• Piston pins
• Piston pin clips
• Connecting rod bolts

NOTE: Aside from the special tools listed above, you’ll need a well-stocked toolbox that includes ratchets, a socket set, Allen set, wrenches, screwdrivers, pliers, etc. 

Sizing Yamaha F255TXRC Crankshaft Bearings

Step 1. Note the four numbers stamped into the bottom of the engine block, which correspond to the crankshaft journal bearings. Also note the numbers stamped into the crankshaft. The top four numbers correspond to the crankshaft journal bearings, the lower six numbers correspond to the connecting rod bearings. Compare the journal bearing numbers from the engine block and crankshaft against the Yamaha bearing size chart to determine the bearing sizes you’ll need.

Step 2. Install the journal bearings and thrust washers into the inner crankcase half, and lay the crankshaft in place. Don’t rotate the crankshaft. Install the journal bearings into the outer crankcase half and put a piece of plastic gauge across each bearing. Lay the outer crankcase half over the inner half, then oil the crankcase bolts and torque them in the sequence shown in the diagram below.

1. Torque bolts 1-8 to 18 foot-pounds

2. Torque bolts 9-16 to 29 foot-pounds

3. Torque bolts 17-29 to 10 foot-pounds

4. Torque bolts 1-8 by an additional 90-degrees

5. Torque bolts 9-16 by an additional 90-degrees

6. Torque bolts 17-29 to 20 foot-pounds

Step 3. Loosen and remove the crankcase bolts in the reverse order, and remove the outer crankcase half. Measure the plastic gauge to be sure the bearings are correctly sized. Next, remove the crankshaft and clean off all the plastic gauge from the crankcase and crankshaft.

Watch the clip below to see the crankshaft bearing segment of our Yamaha F225TXRC bottom end rebuild video.

Installing Yamaha F255TXRC Crankshaft

Step 1. Oil the main crankshaft oil seal and install it onto the crankshaft. Smear assembly lube onto the crankshaft bearings, and install the crankshaft into the inner crankcase half. Apply a thin bead of sealant around the inner crankcase mating surface, then flatten the sealant with your finger and install the O-rings.

Step 2. Apply assembly lube to the bearings in the outer crankcase, and install the outer crankcase onto the inner crankcase. Install the crankcase bolts in the sequence and to the same torque specs as before:

1. Torque bolts 1-8 to 18 foot-pounds

2. Torque bolts 9-16 to 29 foot-pounds

3. Torque bolts 17-29 to 10 foot-pounds

4. Torque bolts 1-8 by an additional 90-degrees

5. Torque bolts 9-16 by an additional 90-degrees

6. Torque bolts 17-29 to 20 foot-pounds

Step 3. Grease the oil pump seals, then install the oil pump onto the crankshaft and torque the four mounting bolts to 7 foot-pounds.

Watch the clip below to see the crankshaft installation segment of our Yamaha F225TXRC bottom end rebuild video.

Sizing Yamaha F255TXRC Connecting Rod Bearings

Step 1. Remove the old pistons from the connecting rods, then apply assembly lube to the new piston pins and install the new pistons onto the connecting rods.

NOTE: Pay attention to the orientation of the connecting rods in relation to the old pistons so the new pistons are installed the same way.

Step 2. Compare the connecting rod bearing numbers you took against the Yamaha bearing size chart to determine the bearing sizes you’ll need.

Step 3. Install the bearings into the connecting rods, then lay plastic gauge on each bearing, lightly oil the piston, and use the old connecting rod bolts to install the piston into the cylinder and connecting rod onto the crankshaft. 

NOTE: Connecting rod bolts are one-use only, so save the new bolts for the final install. 

Torque the connecting rod bolts as follows, and don’t let the crankshaft move or it will smear the plastic gauge and spoil the reading:

1. Torque the connecting rod bolts to 17 foot-pounds

2. Torque the connecting rod bolts to 35 foot-pounds

3. Torque the connecting rod bolts an additional 90-degrees 

Step 4. Remove the connecting rods, and measure the plastic gauge to be sure the bearings are correctly sized. Clean off all the plastic gauge from the connecting rods and crankshaft.

Watch the clip below to see the connecting rod bearings segment of our Yamaha F225TXRC bottom end rebuild video.

Measuring and Adjusting Yamaha F255TXRC Piston Rings

Step 1. Use the piston to set each of the piston rings an inch into the cylinder and measure the ring gap. The gap should be between 0.15mm and 0.3mm.

Step 2. Lightly oil the piston rings. Make sure the R marks on each are facing up and install the piston rings onto the piston with piston ring pliers in the following order and orientation:

1. Central oil ring gap at approximately 7 o’clock

2. Bottom oil ring gap at approximately 5 o’clock

3. Top oil ring gap at approximately 11 o’clock

4. Lower piston ring gap at approximately 1 o’clock

5. Upper piston ring gap at approximately 7 o’clock

Watch the clip below to see the piston ring segment of our Yamaha F225TXRC bottom end rebuild video.

Installing Yamaha F255TXRC Pistons

Step 1. Remove and discard the old connecting rod bolts, install the new bolts, then apply assembly lube to the connecting rod bearings and apply a film of oil to the cylinder bores.

Step 2. Use a piston ring compressor or a piston ring sleeve and install the piston into the cylinder. Make sure the mark on the connecting rod end cap is aligned with the mark on the crankcase, and torque the new connecting rod bolts to the torque specs listed below:

1. Central oil ring gap at approximately 7 o’clock

2. Bottom oil ring gap at approximately 5 o’clock

3. Top oil ring gap at approximately 11 o’clock

4. Lower piston ring gap at approximately 1 o’clock

5. Upper piston ring gap at approximately 7 o’clock

Step 3. Repeat the last four steps for the remaining five pistons.

Watch the video below to see the piston installation segment of our Yamaha F225TXRC bottom end rebuild video.

Completing Yamaha F255TXRC Bottom End Rebuild

Step 1. Apply threadlocker to the baffle plate bolts, then install the baffle plate and torque the bolts to 9 foot-pounds.

Step 2. Install a new crankcase cover gasket and the crankcase cover. Tighten the bolts in the numbered sequence stamped on the crankcase cover, and torque them as follows:

1. Torque the crankcase cover bolts to 10 foot-pounds

2. Torque the crankcase cover bolts to 20 foot-pounds

Step 3. Install the oil filter housing, the oil filler tube and the thermostat, and torque the bolts to 8 foot-pounds. Install the cooling cover and gasket. Make sure the adhesive side of the gasket is facing the engine block. Tighten the bolts in the numbered sequence stamped on the cooling cover, and torque them to 8.7 foot-pounds. Install the timing belt drive gear and torque the bolts to 5 foot-pounds.

Watch the clip below to see the bottom end rebuild completion segment of our Yamaha F225TXRC bottom end rebuild video.

The next step in our project is to rebuild the top end of the outboard. Watch the Yamaha F225 Powerhead Rebuild Part 5 video below to see how to get it done.