NC250 Engine: Top End Disassembly
 

Removing the Rocker Cover, Camshaft, Cylinder Head, Cylinder, and Piston

Perform the following, preliminary steps.

Remove the fuel tank.
Drain the coolant.
Remove the radiator hoses from the cylinder head and water pump.
Remove both radiators from the frame.
Remove the exhaust header.
Remove the perimeter engine bracket.
Remove the entire engine from the frame of the motorcycle.

Set the crankshaft to TDC on the compression stroke, which will release pressure on the rockers and all four valves. Remove the two plug bolts from the cylinder head to verify the camshaft is in the proper position.

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Remove the automatic, cam chain tensioner (ACCT).

First remove the center bolt to release the spring pressure.
Remove the spring and O-ring.
Remove the two screws at the base of the ACCT.

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Caution:
After removing the ACCT, return the plunger to its initial position by pressing on the locking cam, and pushing the plunger to the base of the tensioner.

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Removing and Inspecting the Rocker Cover

Before removing the rocker cover, remove the valve covers.

Remove the bolts from the rocker cover. Take care to remove the numbered bolts crosswise, from the outside in, to properly release the tension on the cylinder head.

Note: Remove only the numbered bolts. Loosen the bolts in the numbered sequence from 1 to 8. Take note of the four copper washers.

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Note: The two large, 10 mm bolts are not removed. They are holding the rocker pins inside the rocker cover.

Remove the rocker cover from the cylinder head. It is not necessary to remove the two screws that lock the rocker pins, unless you need to replace either the rockers, or the rocker pins.

Note: If you remove the rocker pins, always verify their correct orientation, or the oil lubrication passages will be in the wrong position. Improper orientation of the rocker pins will starve the cylinder head of oil, and damage the engine.

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While the rocker cover is removed, check the axial and radial movement of the cam rollers on the rockers. If a cam roller is damaged, replace the rocker.

Also check the surface of the cap at the end of the tappet. If the cap is deformed, replace the tappet. This will allow the tappet to make better contact with the valve head, prolonging the life of the valve.

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Removing the Camshaft

Remove the rubber stopper (A).
Remove the bolt and washer (B).
Remove the retaining clip (C).

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Remove the bearing, and check for axial and radial movement. If the bearing is loose in either radial, or axial movement, replace the bearing.

Carefully note the correct orientation of the washer (A). The washer must be properly installed to provide enough space for the proper operation of the decompression mechanism in the camshaft.

Note: If the washer (A) is not properly installed, it might also cause noise in the engine.

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Remove the washer. Finally, remove the cam chain, and put a large zip tie around it, so you can retrieve the cam chain from the engine. Lower the cam chain into the timing hole, and remove the camshaft.

Removing the Cylinder Head

Remove the bolt (A), which retains the clip supporting the hose leading from the oil contaminant separator (OCS) to the air box. This bolt should always be removed first, before releasing the torque of the four cylinder nuts.

Next, remove the four cylinder nuts (B).

Note: Loosen the nuts incrementally, in a crisscross pattern, to properly release the tension from the engine.

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Remove the cylinder head.

Remove the cylinder head gasket. Remove the two dowel pins (A). Remove the cam chain guide (B).

Note: Always replace the cylinder head gasket.

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Removing the Cylinder

Remove the cylinder.

Remove the two guide pins (A). Remove the cylinder gasket.

Note: The cylinder has a Nikasil coating, and does not contain a sleeve. If the nikasil coating is in good condition, you can reuse the cylinder.

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Removing the Piston

The piston has a coating of molybdenum and graphite on the skirt, which lubricates contact with the cylinder. If the piston skirt is in good condition, you can reuse the piston.

Note the orientation of the piston. The arrow on the piston must face the exhaust header.

Stuff a rag into the hole surrounding the piston to prevent dropping a wristpin circlip into the engine. Remove one of the wristpin circlips. From the opposite side of the piston, push the piston pin out of the piston, and remove the piston from the connecting rod of the crankshaft.

Remove the piston rings from the piston.

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All of us are a long way from needing to do a top end rebuild of the engine. ;) However, I decided to post this thread so we will be ready when the job is required. :)

It is nearly impossible to add every little detail regarding the disassembly of the top end of the engine. Please feel free to add you comments to fill in the gaps. :)

So is this your engine Spud?

:thanks:

No, I borrowed the photographs from a document produced by AKT in Colombia. The document was discovered weeks ago by forum member Katflap. :tup: The AKT document also includes very nice photographs of the cooling system, transmission, clutch, et cetera. The winters here are long, so I will probably post several more tutorials in the future. ;)

... and, can we have links to said AKT's tutorials?

Yes, the links are provided in the RX3 megathread. Do a search for a post by forum member Katflap.

http://www.chinariders.net/showthrea...13440&page=306

In this video, the videographers remove the cylinder head from the engine in a Kayo T6 dirt bike.

https://www.youtube.com/watch?v=0M8SY2Z7RtY

Here is a link to the original document in Spanish. :)

https://drive.google.com/open?id=0Bz...VRJdi1xRmlHekk