NC250 Engine: Top End Assembly

SpudRider · 11-20-2015, 05:05 AM

Installing the Piston Rings, Piston, Cylinder, Cylinder Head, Camshaft, and Rocker Cover

Installing the Piston Rings

Install new piston rings. The silver piston ring is the top ring. The darker piston ring is the second ring. Both piston rings should be installed with the writing facing the top of the piston. The oil scraper rings are installed in the bottom groove.

Space the gaps in the piston rings so they do not overlap.

SpudRider · 11-20-2015, 05:06 AM

Install the cylinder guide pins (A). Install a new cylinder base gasket.

Installing the Piston

Place a wristpin circlip into the left side of the piston.
Lubricate the piston rings and the skirts of the piston with motor oil.

Place the cylinder upside down on the workbench, and install the piston in the bottom of the cylinder by compressing the piston rings in succession. Make sure the mark on the top of the piston is facing the front (exhaust) side of the cylinder.

Installing the Cylinder

Pull the zip tie on the cam chain up through the timing hole in the cylinder, and also place the cam tension lever through the timing hole.

Lower the cylinder/piston over the cylinder bolts, and align the hole in the piston with the hole in the connecting rod.

Lubricate the piston pin with motor oil, and push the piston pin through the piston and the connecting rod.

Stuff a rag into the hole surrounding the connecting rod to prevent dropping the wristpin circlip into the engine.

Install the remaining wristpin circlip.

Remove the rag from the engine, and lower the cylinder until is is fully seated on the engine crankcase.

SpudRider · 11-20-2015, 05:09 AM

Installing the Cylinder Head

Replace the two dowel pins (A).
Install a new cylinder head gasket.
Install the cam chain guide (B).

Install the cylinder head.

Tighten the 4 nuts (B), incrementally, in a crisscross pattern. Final torque for each nut is 28 foot-pounds (50 Nm).

Install the bolt (A), which retains the clip supporting the hose leading from the contaminant separator to the air box. This bolt should be tightened to a torque of 9 foot-pounds (12 Nm).

Check, and make sure the crankshaft is positioned at the TDC mark on the flywheel.
If necessary, hold the cam chain and rotate the crankshaft to the proper position.

SpudRider · 11-20-2015, 05:12 AM

Installing the Camshaft

Remove the large zip tie from the cam chain.

Place the camshaft into the cylinder head, and attach the cam chain. Keep tension on the left (forward) side of the cam chain, and align the two timing marks on the cam sprocket so they face upward, and the dot on the cam sprocket is level with the top of the cylinder head.

Install the large washer (A) with the proper orientation.

Install the bearing. Install the bolt and washer (B), and the retaining clip [C]. Install the rubber plug (A).

Place the rocker cover on top of the cylinder head, and verify the two timing marks on the cam sprocket are properly aligned in the check holes.

Double check to verify the flywheel is still at TDC. If necessary, remove the rocker cover, rotate the flywheel, and/or move the timing chain on the sprocket to get the proper alignment.

Repeat the previous steps until the flywheel and cam sprocket are properly aligned.
After proper alignment is verified, remove the rocker cover.

SpudRider · 11-20-2015, 05:14 AM

Installing the Rocker Cover

Evenly apply a small amount of oil-resistant silicone to the mating surfaces of the rocker cover. I recommend Permatex Ultra Black RTV Silicone.

Install the rocker cover onto the cylinder head.

Install the 8 bolts in the cylinder head. Don't forget to install the 4 copper washers.
Tighten the bolts incrementally, in numbered sequence. The final torque of all the bolts is 9 foot-pounds (12 Nm).

SpudRider · 11-20-2015, 05:15 AM

Replace the Automatic Cam Chain Tensioner (ACCT)

Install the ACCT with the two screws at the base.
Install the spring and O-ring.
Install the center bolt to set the spring pressure.

SpudRider · 11-20-2015, 05:15 AM

Adjust Valve Lash

Adjust the valve lash, and replace the valve covers.

SpudRider · 11-20-2015, 05:16 AM

Reassemble the Motorcycle

Re-install the engine into the frame of the motorcycle.
Install the exhaust header.
Install the radiators.
Install the radiator hoses to the cylinder head and water pump.
Fill the radiators with coolant.

SpudRider · 11-20-2015, 05:47 AM

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 assembly of the top end of the engine. Please feel free to add you comments to fill in the gaps.

Weldangrind · 11-20-2015, 11:18 AM

Quote:

Originally Posted by SpudRider

Installing the Piston

Place a piston retaining ring into the left side of the piston.

Using needle nose pliers, install the remaining, piston retaining ring.

Some may refer to that as a wristpin circlip.

SpudRider · 11-20-2015, 01:33 PM

Quote:

Originally Posted by Weldangrind

Some may refer to that as a wristpin circlip.

Thank you, Weld. I'm sure that is the proper term.

I wrote two engine tutorials yesterday, and I didn't bother to check whether some of my homemade terms were accurate.

I will go back and edit the posts, inserting the proper term.

SpudRider · 11-26-2015, 03:59 AM

This video shows the installation of the cylinder head, and adjusting the valve lash.

ChopperCharles · 03-18-2021, 01:40 PM

Are you sure about only 28 foot pounds on the head bolts? That seems extremely low to me. Also, 50 Nm is 37 ft lbs, which sounds much more reasonable.

Charles.

franque · 03-18-2021, 02:12 PM

Spud hasn't been on in a while, you're not likely to get a response from him... Depending on the bolt diameter, that could be reasonable. Thankfully, there's no guessing needed, just give CSC a call!

ChopperCharles · 04-11-2021, 11:24 AM

Questions:

1. Are the top rocker cover and bottom cylinder head a mated pair? Or can I use my old rocker cover and followers with a new cylinder head? I'm asking, because the pins that the rockers slide on are seemingly not removable. They're a tight fit into a blind hole, and there's no way to push them out the way they went in.

2. What's the best sealant to use to bind the two halves together

3. How long should I wait for the sealant to cure before starting the motor?

Charles.

11-20-2015, 05:05 AM	#1
SpudRider Join Date: Jul 2008 Location: Idaho Posts: 25,054	NC250 Engine: Top End Assembly Installing the Piston Rings, Piston, Cylinder, Cylinder Head, Camshaft, and Rocker Cover Installing the Piston Rings Install new piston rings. The silver piston ring is the top ring. The darker piston ring is the second ring. Both piston rings should be installed with the writing facing the top of the piston. The oil scraper rings are installed in the bottom groove. Space the gaps in the piston rings so they do not overlap. __________________ Spud "Never argue with an idiot. He will drag you down to his level, and beat you with experience." Mark Twain 2015 Zongshen ZS250GY-3 (RX3) 2006 Zongshen ZS200GY-2 (Sierra 200) 2005 Honda XR650L 2004 Honda CRF250X 1998 Kawasaki KDX220 Mods made to my Zongshen ZS200GY-2: http://www.chinariders.net/showthread.php?t=6894 Last edited by SpudRider; 12-08-2015 at 02:24 PM.
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11-20-2015, 05:06 AM	#2
SpudRider Join Date: Jul 2008 Location: Idaho Posts: 25,054	Install the cylinder guide pins (A). Install a new cylinder base gasket. Installing the Piston Place a wristpin circlip into the left side of the piston. Lubricate the piston rings and the skirts of the piston with motor oil. Place the cylinder upside down on the workbench, and install the piston in the bottom of the cylinder by compressing the piston rings in succession. Make sure the mark on the top of the piston is facing the front (exhaust) side of the cylinder. Installing the Cylinder Pull the zip tie on the cam chain up through the timing hole in the cylinder, and also place the cam tension lever through the timing hole. Lower the cylinder/piston over the cylinder bolts, and align the hole in the piston with the hole in the connecting rod. Lubricate the piston pin with motor oil, and push the piston pin through the piston and the connecting rod. Stuff a rag into the hole surrounding the connecting rod to prevent dropping the wristpin circlip into the engine. Install the remaining wristpin circlip. Remove the rag from the engine, and lower the cylinder until is is fully seated on the engine crankcase. __________________ Spud "Never argue with an idiot. He will drag you down to his level, and beat you with experience." Mark Twain 2015 Zongshen ZS250GY-3 (RX3) 2006 Zongshen ZS200GY-2 (Sierra 200) 2005 Honda XR650L 2004 Honda CRF250X 1998 Kawasaki KDX220 Mods made to my Zongshen ZS200GY-2: http://www.chinariders.net/showthread.php?t=6894 Last edited by SpudRider; 01-14-2017 at 04:39 AM.
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11-20-2015, 05:09 AM	#3
SpudRider Join Date: Jul 2008 Location: Idaho Posts: 25,054	Installing the Cylinder Head Replace the two dowel pins (A). Install a new cylinder head gasket. Install the cam chain guide (B). Install the cylinder head. Tighten the 4 nuts (B), incrementally, in a crisscross pattern. Final torque for each nut is 28 foot-pounds (50 Nm). Install the bolt (A), which retains the clip supporting the hose leading from the contaminant separator to the air box. This bolt should be tightened to a torque of 9 foot-pounds (12 Nm). Check, and make sure the crankshaft is positioned at the TDC mark on the flywheel. If necessary, hold the cam chain and rotate the crankshaft to the proper position. __________________ Spud "Never argue with an idiot. He will drag you down to his level, and beat you with experience." Mark Twain 2015 Zongshen ZS250GY-3 (RX3) 2006 Zongshen ZS200GY-2 (Sierra 200) 2005 Honda XR650L 2004 Honda CRF250X 1998 Kawasaki KDX220 Mods made to my Zongshen ZS200GY-2: http://www.chinariders.net/showthread.php?t=6894 Last edited by SpudRider; 12-13-2016 at 02:28 PM.
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11-20-2015, 05:12 AM	#4
SpudRider Join Date: Jul 2008 Location: Idaho Posts: 25,054	Installing the Camshaft Remove the large zip tie from the cam chain. Place the camshaft into the cylinder head, and attach the cam chain. Keep tension on the left (forward) side of the cam chain, and align the two timing marks on the cam sprocket so they face upward, and the dot on the cam sprocket is level with the top of the cylinder head. Install the large washer (A) with the proper orientation. Install the bearing. Install the bolt and washer (B), and the retaining clip [C]. Install the rubber plug (A). Place the rocker cover on top of the cylinder head, and verify the two timing marks on the cam sprocket are properly aligned in the check holes. Double check to verify the flywheel is still at TDC. If necessary, remove the rocker cover, rotate the flywheel, and/or move the timing chain on the sprocket to get the proper alignment. Repeat the previous steps until the flywheel and cam sprocket are properly aligned. After proper alignment is verified, remove the rocker cover. __________________ Spud "Never argue with an idiot. He will drag you down to his level, and beat you with experience." Mark Twain 2015 Zongshen ZS250GY-3 (RX3) 2006 Zongshen ZS200GY-2 (Sierra 200) 2005 Honda XR650L 2004 Honda CRF250X 1998 Kawasaki KDX220 Mods made to my Zongshen ZS200GY-2: http://www.chinariders.net/showthread.php?t=6894 Last edited by SpudRider; 01-14-2017 at 04:42 AM.
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11-20-2015, 05:14 AM	#5
SpudRider Join Date: Jul 2008 Location: Idaho Posts: 25,054	Installing the Rocker Cover Evenly apply a small amount of oil-resistant silicone to the mating surfaces of the rocker cover. I recommend Permatex Ultra Black RTV Silicone. Install the rocker cover onto the cylinder head. Install the 8 bolts in the cylinder head. Don't forget to install the 4 copper washers. Tighten the bolts incrementally, in numbered sequence. The final torque of all the bolts is 9 foot-pounds (12 Nm). __________________ Spud "Never argue with an idiot. He will drag you down to his level, and beat you with experience." Mark Twain 2015 Zongshen ZS250GY-3 (RX3) 2006 Zongshen ZS200GY-2 (Sierra 200) 2005 Honda XR650L 2004 Honda CRF250X 1998 Kawasaki KDX220 Mods made to my Zongshen ZS200GY-2: http://www.chinariders.net/showthread.php?t=6894 Last edited by SpudRider; 01-14-2017 at 04:44 AM.
	0

11-20-2015, 05:15 AM	#6
SpudRider Join Date: Jul 2008 Location: Idaho Posts: 25,054	Replace the Automatic Cam Chain Tensioner (ACCT) Install the ACCT with the two screws at the base. Install the spring and O-ring. Install the center bolt to set the spring pressure. __________________ Spud "Never argue with an idiot. He will drag you down to his level, and beat you with experience." Mark Twain 2015 Zongshen ZS250GY-3 (RX3) 2006 Zongshen ZS200GY-2 (Sierra 200) 2005 Honda XR650L 2004 Honda CRF250X 1998 Kawasaki KDX220 Mods made to my Zongshen ZS200GY-2: http://www.chinariders.net/showthread.php?t=6894
	0

11-20-2015, 05:16 AM	#8
SpudRider Join Date: Jul 2008 Location: Idaho Posts: 25,054	Reassemble the Motorcycle Re-install the engine into the frame of the motorcycle. Install the exhaust header. Install the radiators. Install the radiator hoses to the cylinder head and water pump. Fill the radiators with coolant. __________________ Spud "Never argue with an idiot. He will drag you down to his level, and beat you with experience." Mark Twain 2015 Zongshen ZS250GY-3 (RX3) 2006 Zongshen ZS200GY-2 (Sierra 200) 2005 Honda XR650L 2004 Honda CRF250X 1998 Kawasaki KDX220 Mods made to my Zongshen ZS200GY-2: http://www.chinariders.net/showthread.php?t=6894 Last edited by SpudRider; 11-26-2016 at 05:03 AM.
	0

11-20-2015, 05:47 AM	#9
SpudRider Join Date: Jul 2008 Location: Idaho Posts: 25,054	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 assembly of the top end of the engine. Please feel free to add you comments to fill in the gaps. __________________ Spud "Never argue with an idiot. He will drag you down to his level, and beat you with experience." Mark Twain 2015 Zongshen ZS250GY-3 (RX3) 2006 Zongshen ZS200GY-2 (Sierra 200) 2005 Honda XR650L 2004 Honda CRF250X 1998 Kawasaki KDX220 Mods made to my Zongshen ZS200GY-2: http://www.chinariders.net/showthread.php?t=6894 Last edited by SpudRider; 11-20-2015 at 06:28 AM.
	0

11-26-2015, 03:59 AM	#12
SpudRider Join Date: Jul 2008 Location: Idaho Posts: 25,054	This video shows the installation of the cylinder head, and adjusting the valve lash. __________________ Spud "Never argue with an idiot. He will drag you down to his level, and beat you with experience." Mark Twain 2015 Zongshen ZS250GY-3 (RX3) 2006 Zongshen ZS200GY-2 (Sierra 200) 2005 Honda XR650L 2004 Honda CRF250X 1998 Kawasaki KDX220 Mods made to my Zongshen ZS200GY-2: http://www.chinariders.net/showthread.php?t=6894
	0

03-18-2021, 01:40 PM	#13
ChopperCharles Join Date: Feb 2018 Location: RDU, NC Posts: 677	Are you sure about only 28 foot pounds on the head bolts? That seems extremely low to me. Also, 50 Nm is 37 ft lbs, which sounds much more reasonable. Charles.
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03-18-2021, 02:12 PM	#14
franque Join Date: Nov 2017 Location: Marseille, France -> Conakry, Guinea Posts: 1,481	Spud hasn't been on in a while, you're not likely to get a response from him... Depending on the bolt diameter, that could be reasonable. Thankfully, there's no guessing needed, just give CSC a call!
	0

04-11-2021, 11:24 AM	#15
ChopperCharles Join Date: Feb 2018 Location: RDU, NC Posts: 677	Questions: 1. Are the top rocker cover and bottom cylinder head a mated pair? Or can I use my old rocker cover and followers with a new cylinder head? I'm asking, because the pins that the rockers slide on are seemingly not removable. They're a tight fit into a blind hole, and there's no way to push them out the way they went in. 2. What's the best sealant to use to bind the two halves together 3. How long should I wait for the sealant to cure before starting the motor? Charles.
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