Yamaha Blaster Top-End Replacement Guide
Follow these professional Yamaha Blaster top-end replacement tips from Ken OConnor Racing. Learn how to properly install the piston, rings, wrist-pin clips, cylinder and head while avoiding common assembly mistakes.
Replacing the top end on a Yamaha YFS200 Blaster requires careful cleaning, correct piston orientation and close attention to the piston rings and wrist-pin clips. A small mistake during assembly can cause serious engine damage, so never force parts together or rush the job.
1. Prepare the Cylinder and Gasket Surfaces
Make sure the cylinder, cylinder head and crankcase gasket surfaces are clean, flat and free of old gasket material. Do not gouge the aluminum while cleaning the surfaces.
Always use new gaskets. Never reuse an old base gasket or head gasket, and do not substitute silicone for the correct gaskets.
2. Protect the Crankcase
Stuff a clean, lint-free shop rag into the crankcase opening around the connecting rod. This prevents dirt, piston-pin clips or other small parts from falling into the bottom end.
3. Thoroughly Clean the Cylinder
Even a freshly bored or honed cylinder must be cleaned before installation. Wash the cylinder thoroughly with hot, soapy water and a clean brush. Immediately apply oil or WD-40 to the cylinder wall to keep it from developing surface rust.
Wipe the bore with a clean, lint free industrial paper towel. Continue cleaning until the towel comes out completely clean. Wipe in the same direction as the cylinder’s hone marks.
4. Install the First Wrist-Pin Clip
Install one new wrist-pin clip into the piston before placing the piston on the connecting rod.
Make sure the clip is completely seated in its groove. Position the opening of the clip straight up or straight down—not toward the clip-removal notch. An improperly installed wrist-pin clip can come loose and cause severe cylinder and piston damage.
Always use new wrist-pin clips. Never reuse an old clip.
5. Lubricate the Parts
Apply a light coating of quality two-stroke oil to the following parts:
- Wrist-pin bearing
- Wrist pin
- Piston pin bores
- Connecting-rod small end
- Piston skirt
- Cylinder bore
A small amount of two-stroke oil can also be applied to the connecting-rod big-end bearing before assembly. Use the same high-quality two-stroke oil that will be used in the engine.
6. Install the Piston
Place the new wrist-pin bearing into the connecting rod. Position the piston over the rod with the arrow on top of the piston pointing toward the exhaust port.
Slide the wrist pin through the piston, bearing and connecting rod. The pin should slide into place with hand pressure. Do not hammer it.
Install the second new wrist-pin clip and confirm that it is fully seated in its groove.
7. Double-Check Both Wrist-Pin Clips
Carefully inspect both clips before continuing. Each clip must be completely seated around its entire groove, with its opening facing straight up or down.
This is one of the most important checks during a Yamaha Blaster top-end installation.
8. Check the Piston-Ring End Gap
Place each piston ring squarely inside the cylinder and measure the ring end gap with a feeler gauge.
A commonly used minimum guideline is approximately .004 inch of ring gap for every 1 inch of cylinder bore, but the correct specification can vary by piston and ring manufacturer.
Always follow the clearance specifications supplied with your piston kit. Never assume that new rings have the correct end gap without checking them.
9. Install the Piston Rings
Install the rings with their identification marks—commonly “N,” “NA” or another manufacturer’s marking—facing upward.
Carefully align each ring end with its locating pin in the piston’s ring groove. These pins prevent the rings from rotating into a cylinder port.
On a Yamaha Blaster piston, the ring locating pins should face the intake side of the engine. If they face the exhaust port, the piston is installed backward.
Do not expand the rings any farther than necessary during installation.
10. Lubricate the Piston and Cylinder
Apply a light, even coating of two-stroke oil to the piston skirt, piston rings and cylinder bore. Excessive oil is unnecessary, but the parts should not be assembled dry.
11. Install the Base Gasket
Remove all oil, dirt and gasket residue from the cylinder and crankcase mating surfaces. Install a new Yamaha Blaster base gasket in the correct orientation.
Before lowering the cylinder into place, remove the shop rag from the crankcase and confirm that nothing has fallen into the engine. Be sure the cylinder locating pins have been installed.
12. Lower the Cylinder Over the Piston
Use your fingers to gently compress the piston rings while carefully lowering the cylinder over the piston.
Make sure each ring remains aligned with its locating pin. Do not twist, rock or force the cylinder over the rings. If the cylinder stops or hangs up, lift it slightly and recheck the ring alignment.
Forcing the cylinder can break a ring, damage the piston or score the cylinder.
Once the cylinder is seated, install and tighten the cylinder nuts evenly to the factory Yamaha torque specification of 18 Ft Lbs.
Rotate the crankshaft and confirm smooth operation.
13. Install the Cylinder Head
Install a new head gasket with the arrow pointing toward the exhaust and position the cylinder head correctly. Tighten the head nuts gradually in a crisscross pattern, working in several small steps until the factory Yamaha Blaster torque specification of 20 ft lbs. is reached.
Reinstall the intake gasket, intake, and reed cage. This area can be problematic. I always use case bond sealer (not silicone) on both sides of the intake gasket and I like to use an additional gasket between the reeds and the intake.
14. Leak-Test and Recheck the Engine
Before starting the engine, perform a two-stroke leak-down test. This confirms that the base gasket, head gasket, intake, crank seals and other sealing surfaces are airtight.
Air leaks can create a dangerously lean condition and quickly damage a new piston and cylinder.
Be sure the inside of the exhaust pipe is free of debris. Coat the exhaust gasket with high heat ultra copper silicone and install the exhaust system. Install the spark plug and any other components that were removed.
After the first complete warm-up and cool-down cycle, check the cylinder-head torque again with the engine completely cold.
ENTER THE BLASTER STORE
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