The Problem Every OX66 Owner Faces
You’ve got a year 2000 Yamaha OX66 250 that’s been running hard all season. Maybe it’s been sitting. Either way, the 100-hour service is due and you’re staring at an engine that hasn’t had its thermostats, impeller, fuel filters, or spark plugs touched in who knows how long. In Florida heat and saltwater, skipping that service isn’t just lazy — it’s expensive.
I recently did a complete 100-hour service on one of these and walked through every step on camera. Here’s the full breakdown so you can do it yourself.
Why It Matters
The OX66 is a bulletproof engine when it’s maintained. When it’s not, you’re looking at overheating from a blown impeller, corroded bolts that snap off in the thermostat housing, clogged fuel filters starving the engine, and fouled spark plugs killing performance. Skipping one service interval on a 25-year-old two-stroke is how you turn a manageable maintenance task into a major repair bill.
Products Mentioned
- Thermostats — Direct replacements for the OX66; two housings, four total bolts
- Thermostat housing gaskets — Seal the thermostat housings after reinstallation
- Spark plugs — Six total; require a 13/16″ socket
- Boat fuel filter — The large canister-style filter mounted on the engine
- Engine fuel filter — Secondary filter in the fuel delivery system
- Power trim and tilt fluid — Topped off through the 17mm fill plug
- Grease — Applied to all steering points, cylinder grease fittings, and fasteners throughout
Step-by-Step: Full 100-Hour Service on the Yamaha OX66 250
Step 1: Spark Plugs
There are six spark plugs on this engine. You need a 13/16″ socket to get them out. Straightforward swap — no tricks here, just don’t cross-thread them going back in.
Step 2: Thermostats
Each thermostat housing is held by two 10mm bolts — four bolts total across both housings. These are a serviceable part, meaning they’re designed to come out, but if they haven’t been touched in years they can be stuck hard enough to snap the bolt if you’re not careful.
I use a quarter-inch ratchet here instead of a three-eighths. You physically can’t apply as much torque with a quarter-inch, and that limits the risk of snapping the bolt. If you feel serious resistance, stop and work it back and forth before committing.
Once the housing is off, clean the mating surface before dropping the new thermostat in. And before reinstalling the bolts, put grease on the threads. You’ll thank yourself at the next service interval.
Step 3: Fuel Filters
There’s a 10mm bolt on top of the fuel filter bracket. Loosen that first to free up the bracket, then spin off the black retaining piece to drop the filter canister down and out.
Here’s the trick that most people get wrong with these: the red o-ring on the filter has to be completely dry before you reassemble. Get all the fuel off it.
Once it’s dry, slide the red ring in place, then use a torch to heat the filter housing. Watch the white plastic — as it heats up it turns clear. That’s your window. When it goes clear, open it up with a large flathead screwdriver. If there’s any fuel still on that red ring, it will catch fire. Don’t rush this step.
As the housing cools back down it hardens and locks in place. Also — don’t forget to transfer the o-ring from inside the old filter to the new one before installing. It’s easy to miss and it matters.
When tightening the filter back up, it doesn’t need to be torqued to 100 ft-lbs. Just snug and secure. Tighten the bracket bolt and you’re done.
Step 4: Grease Points
On any Yamaha, you want to check and grease all your steering points. There are fittings on both sides of the steering cylinder, additional points on the tilt mechanism, and grease fittings along the powerhead. Don’t skip these — seized steering components are a saltwater boating problem.
To access the steering cylinder grease fitting fully, turn the engine and tilt it up to move the cylinder out of the way.
Step 5: Power Tilt and Trim
Top off the power tilt and trim fluid. There’s a 17mm plug on the reservoir — crack it open, float it to full, reinstall the plug. Simple.
Step 6: Lower Unit Removal
Removing the lower unit on the OX66 takes six 14mm bolts — three on each side. There’s also a 12mm bolt at the anode and another 14mm tucked underneath it. Pull the cotter pin first, then use a 1-1/16″ socket on the prop nut.
Before you slide the lower unit back up into place, have one greased 14mm bolt staged and ready where you can reach it. Make sure the engine is in neutral — this is critical for lining up the shift shaft.
Slide the lower unit up, align the shift shaft and drive shaft simultaneously, and push it home. Once you feel it seat, get on the boat, shift it into gear, and confirm the shift shaft is actually engaged before you start threading bolts. Put your six 14mm bolts in on the sides, add the two 14mm bolts in the back, reinstall the handle, torque the prop nut, and don’t forget the cotter pin.
Pro Tips
Grease your bolts every time. Any bolt that’s going back into aluminum — thermostat housing, lower unit, anode — gets grease on the threads before reinstallation. Florida saltwater and bare steel threads in aluminum is a recipe for a broken bolt extractor job down the road.
Use a quarter-inch ratchet on stuck thermostat bolts. It’s not just technique — it’s a mechanical torque limiter. You can’t snap a bolt with a quarter-inch the way you can with a three-eighths. If the bolt isn’t moving, heat and penetrant before you put more muscle on it.
Don’t rush the fuel filter torch step. The window between “clear plastic, ready to open” and “red ring on fire” is small. Have the screwdriver ready before you apply heat, get all fuel residue off the o-ring, and work fast when it goes clear.
Final Verdict
This is absolutely a DIY job if you’re mechanically comfortable. None of these steps require specialty tools beyond a 1-1/16″ socket and a torch. The trickiest part is the fuel filter heating technique and keeping the lower unit shift shaft aligned on reinstall — both are manageable if you go slow.
Budget a half day for your first time through. Once you’ve done it once, it’s a 2–3 hour job.
FAQ
How often should I service a Yamaha OX66 250?
Every 100 hours or annually, whichever comes first. In Florida where engines run hard in hot weather, leaning toward the hour-based interval is smart.
What size socket do I need for Yamaha OX66 spark plugs?
13/16″ socket. There are six plugs total.
How do I know if my OX66 lower unit shift shaft is lined up correctly?
After sliding the lower unit home, get on the boat and physically shift the engine into gear before tightening any bolts. If it shifts cleanly, you’re aligned. If it won’t engage gear, the shift shaft isn’t seated — pull it back down and try again.
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