Vaderom (guest) View all posts in this topic  
Fri Apr 03, 2009 8:17 pm
Engine disassemble, piston and rings replacement and valve t        

Seeing as there were a number of posters asking questions about valve timing lately, and having to rebuild the engine on my scrapbike 250, I've decided to photograph the process and write an organized article about how it's done. I realize this is beyond the scope of the FAQ, so I'm posting it as an article. Whether to add this to the FAQ or not - it's up to you admins to decide.

In this writeup, we will take the ninjette engine top end apart, replace a piston and rings, put it back together and set the timing.

I?m sorry for the quality of the photos. They were taken using my cellphone camera and it has a tiny lens and didn?t like the lighting conditions in the room I was working in, so most pics came out blurry. I'm going to tear down another 250 engine next week to replace the head, and I promise an update with better photos and a more suitable resolution.

Disclaimer ? the author will take no responsibility for any damage the reader causes his bike, himself or others by following this guide. It?s written from personal experience and meant to aid people who want to do their own work on their bike, have some experience twisting wrenches and are prepared to take responsibility for their work. There is no intentionally misleading information in the guide, and any reported mistakes will be gladly fixed. A service manual and supplement are still required to perform the work detailed in this guide ? BUY THEM. Best 60$ spent on the bike if you intend to do any work on it yourself.

Ok, let's put on some gloves and get to work!

? A TIP for the entire process ? organize the parts, in order of disassembly, on a table or a piece of paper/cloth/towel, marking or noting if you feel like it. I arrange them so that the parts I disassemble first are furthest. This helps the assembly a great deal and keeps you from forgetting parts, or looking for them all over the place.

? NOTE about bolt/head sizes ? I?ll be noting bolt head sizes through the article. All sizes are metric, measured in mm. When talking about a bolt head, ill use M## to note the socket/wrench size needed. If talking about a thread, ill use ##mm to note the thread diameter.
? NOTE ? Cylinders are numbered #1 and #2 from left to right, according to the engine position in the frame, relative to the rider sitting on the bike. Therefore the #1 cylinder is the one closer to the alternator, and #2 is closer to the clutch. Also, any relative directions are assigned from the rider?s POV, assuming engine in the frame ? front is where the exhaust headers are, rear is carbs, left is alternator, right is clutch.

Tearing it down

Here is the engine, taken out of the frame and set on a DIY engine stand made from a few feet of aliminium angle profile and a few bolts (10$ and about 30 minutes of work). I used a wooden plank to get the engine horizontal (should have built the lower support as a part of the stand, but didnt have enough material, and decided it'll be ok)

First, we?ll remove the cam chain tensioner. I forgot to take a pic of that, it?s on the top right side of the engine, just behind the #2 cylinder (2 bolts, M8 head). Put the cam chain tensioner (CCT from now on) aside, noting that all the parts are present:
? Short wide spring
? Long thin spring
? Collar ball bearing
? Tensioner rod
? Tensioner body
? 2 bolts
? Holder bolt (it?s screwed into the top of the CCT body, a #2 Phillips head)

Now take off the valve cover (4 silver bolts with hats, M10 head)

Removing the valve cover, we see the valve train exposed.

Take off the two black rubber gaskets surrounding the spark plug holes (No 1 on pic)
? TIP ? shove a lint-free cloth or a paper towel into the cam chain space and jam it there so that anything you happen to drop into that opening doesn?t end up deep inside

Remove the 3 bolts holding the top cam chain guide (No 2 on pic, M10 head) ? CAREFUL NOT TO DROP THEM IN THE CAM CHAIN SPACE, ESPECIALLY THE SHORTER CENTER BOLT.

Remove the cam cap bolts (No 3 in pic, M10 head) and remove the caps. Caps need to be removed straight vertically upwards, they sit on locator pins. Keep the bolts in the caps (they?re not all the same length).

? NOTE ? I would keep the valve cover gasket on (black rubber piece surrounding the top of the head), if possible. Feel it before assembly ? if it?s hard/torn/looks worn ? probably worth replacing. Replacing the valve cover, don?t forget to put silicone sealant in the 4 half-circle recesses. No need to put sealant anywhere else.

Carefully lift the camshafts and take them out, removing the cam chain from the sprockets. Note which cam is the intake (rear of engine) and which is exhaust (front of engine). There are marks on the cams that tell you that, but just to be extra careful. You can drop the chain into the space under it.

Remove the rocker arms (8 bridge-like bits with a bolt and nut in them). Note the location of each rocker arm! (I arrange them on a paper towel in exactly the same arrangement as they were in the engine). Not only it?s good practice, but it will save you a valve adjustment when you put it all back. Also, see how the rocker arms sit on the valves (the bottom ?ball? part of the bolt is in the round recess and the front square notch in the rocker is on the valve top) ? that?s how they should go back on assembly.

Remove the 7 head bolts (M12 head). Note the side bolt is shorter than the rest and has to go where it came from. The rest, I arrange them so that I know which one went where, and I return them to their original location on assembly. Probably doesn?t matter, but I prefer it this way.

Remove the oil line banjo bolt (front of #2 cylinder, M10 head, I think) It should have 2 copper washers, above and below the line fitting. The book calls for replacing them on reassembly.

Remove the (annoying) bolt that holds the head to the cylinders from the outside (front of cam chain space, near #2 cylinder, from the outside, upside down. M10 head)

Remove the rear cam chain guide axis bolt (outside of #2 cylinder on the side, M12 head). Take the bolt and the axis out so that the rear cam chain guide rocks freely.

Carefully, pulling upwards and rocking back and forth slightly, remove the engine head. It might be stuck to the cylinders and when you pull up, both parts will be removed, it?s ok, just be careful. The cylinders are a little tight fit on the pistons, so they should come off with light resistance. If the engine head comes off alone, just remove the cylinders after it. The front cam chain guide should come out with the head. Check it for abnormal wear and replace if necessary.

In the pictures here we see the reason for this specific engine disassembly ? a holed piston! That won?t do! Let?s replace it.

Piston replacement

Open the alternator axis cap (the big round cap on the left side of the engine). You can see a bolt head in the center of the opening. It takes an M14 socket and you can rotate the engine with it.

? TIP - Put paper towels or lint-free cloth under the pistons so that you don?t drop anything into the crankcase!

Rotate the engine so that the piston you?re removing is as far up as possible

Remove the circlip from one side of the piston pin. The circlip is a piece of wire bent into ?almost a circle? and it holds the piston pit from moving out of the piston. I did it by using a small screwdriver to lift the piece of the circlip that sits over the groove in the piston pin post, and then pick it out with a small needle nose plier. CAREFUL ? the circlip is there under some pressure and has a tendency to jump out and disappear or worse ? hit you in the eye. I prefer removing the outside circlip first (the one further from the other piston) so that removing the pin is easier.

Remove the piston pin buy carefully tapping it out with a soft drift (a small piece of wood is good). I guess you can use a screwdriver too, as long as you take care not to slip it and scratch the pin. The pin might be stuck in there good, if the engine is old. In such case, a pin puller tool might be needed, or improvised with a long thin bolt and a couple of washers and nuts.

Take the piston off and remove the second circlip. Check the piston pin for wear signs. If worn/scored ? replace. The book says that the circlips must be replaced too, since removal weakens them. Probably a good idea?
Now to replace the rings.

Carefully expand the gap in the top ring (slightly!), pull one side of it out of the groove and remove it from the piston (it?s kind of hard to explain, but easy to figure out once you have the piston in your hand). If you?re putting new rings on then you can just tear the old ones off, breaking them. Same for the second ring and for the 3 piece oil return ring.

? NOTE ? rings are very hard and brittle and not nearly as flexible as they might seem and feel. You can skew them quite a bit and it would seem that you can bend them ? you CANT. When installing new rings, be VERY CAREFUL not to bend them or twist them. Use tons of feel, don?t push or pull on them too hard ? it?s ~40$ shipped for a new set of rings if you break even one.. Sucks. There are ring expander tools that can help you minimize the risk.

We?re done breaking it, now let?s build it back!


Take the new piston and rings. I install the oil return rings first, from the bottom, and the two compression rings from the top.

The oil return ring is composed of 2 thin rings and an expander ring

Take the first thin oil return ring, spread its gap slightly, and pull it on the piston from the bottom. Set it to be horizontal, and pull it up until it sits into its groove. Then do the same with the expander ring (the one that looks like a snake). Now ? the lower oil return ring. Make sure both rings are fully recessed into their grooves and not caught on one of the peaks of the expander rings. Obviously, the expander ring should be between the 2 oil return rings.

Now, put on the 2 compression rings, from the top of the piston this time. IIRC, they?re the same, so just put them on. If there are any markings on the rings, put them on so that the markings are facing up (I didn?t see any on the old rings though). Stagger the rings so that the gaps are not aligned, top ring gap should be at the front of the piston, and second ring gap at the rear. same for oil ring gaps (the manual has the specific setting for the ring staggering so you should follow it)

Now do the same for the second piston, if needed.

Put one circlip into the circlip groove in the piston. I did it by putting it halfway in and pushing it inside with my thumbs, You can use a piece of wood or, very very carefully, a small screwdriver, to compress the circlip slightly.

Look at the piston; it should have an arrow mark on it. Put the piston on the rod, the arrow towards the front of the engine (exhaust) and slide the piston pin into position.

Put the second circlip in (don?t drop them into the cylinders or let them jump out and fly). Repeat for the second piston.

Clean the mating surfaces of the cylinder base, the cylinders? top and bottom, and the engine head bottom until they?re clean and smooth. Any roughness or remains of the old gaskets should be removed.

Put new gaskets on the cylinder base, and put the cylinders on (the protruding part of the cylinder sleeve goes on the bottom, obviously). The cylinders are tapered in the bottom side so that they compress the rings while you push them on. It?s easier if one piston is sticking out more than the other so that you push the first piston in, and then it helps you keep the cylinders straight. Make sure the two locator pins on the corners of the cylinders are there.

Put a new gasket on the cylinders? top side and put the cylinder head on. Should have 2 locator pins too, IIRC.

? NOTE ? do yourself a favor and buy a small (0-20 NM) torque wrench. It will help you not twist the butter-soft head and camshaft cap threads. They require very little torque and break easily. But if you?re still cheap and want to do it by hand, then thread them in until they sit, and give them another 1/6 of a turn, roughly for 12 NM or ?-1/3 turn for 25 NM. Don?t cry to me if you strip one ? I told you to buy a torque wrench.

Put back the M12 cylinder head bolts in and torque them to 25 NM.

Put the front cylinder head bolt in (the one that?s upside down) and torque it to 12 NM.

Put the oil line banjo bolt back (don?t forget 2 copper washers, one between the bolt head and the line fitting, and one between the line fitting and the engine head). Torque it to 20 NM.

Pull the rear cam chain guide top back and put its bolt and axis back so that the axis goes through the hole in the chain guide. Torque it to 25 NM.

Pull the cam chain up and put the front cam chain guide back in.

Put back the rocker arms, preferably as they were. Notice the round bottom part of the bolt goes into the round crater near each valve, and the square recess on the other side sits square on the valve top.

Timing the valves

Open the view window on the alternator cover (small round slotted cap on top front of the alternator cover). Notice the small square-ish notch on the inner side of the window opening.

Put a ratchet with a M14 socket on the alternator bolt and rotate the engine clockwise with one hand, while keeping the cam chain stretched and rotating with the other hand (so that it doesnt jam in the bottom sprocket and stop the engine from rotating), while looking at the alternator rotor, until you see a notch with the mark ?2T? on the rotor (it might be hard to see, depending on the lighting conditions. A small flashlight is uber-helpful here).

Make sure the 2T notch is aligned with the notch on the window inner side. This means #2 cylinder is at TDC (top dead center). You can make sure it?s there by putting a long screwdriver or a stick in the #2 cyl. Spark plug hole and watch it rise and lower as the you rotate the engine. Also, you can look into the spark plug hole and see the piston at the top on the cylinder.

With the #2 cylidner at TDC, we put the camshafts back.
Look at the camshaft sprockets on the outside, where the bolt heads are. You can see that on one of the camshafts there is an EX mark right next one the bolt heads. That?s the exhaust camshaft. The other one has IN next to one of the bolt heads ? that?s the intake camshaft.

On the inner side of the cam sprockets you can see 2 opposing sets of notches, marked with IN and EX.
Put the camshafts back, EX camshaft on the front of the engine, IN camshaft on the back.

Rotate the camshafts so that the exhaust camshaft has it?s EX notch aligned so that it continues the line of the engine head.

Same for the intake camshaft, it?s IN notch should continue the line of the engine head.

Making sure the camshafts sit well in their bearings, put the cam chain on the sprockets, pulling the tension (front of the engine) side of the chain taut. Try not to rotate the camshafts out of alignment. We need all the slack of the cam chain to be located at the rear of the engine, so that the CCT will be able to compensate for it.

With the chain on, count the rivets between the EX and IN alignment notches, starting from the rivet closest and right above one of the notches, to the rivet closest and right above the other notch. We need 33 rivets. If you get more or less, you misaligned the notches.

Due to cam chain wear and subsequent elongation, the notches might be a bit off the exact imaginary line of the engine head, that?s ok, but not too much off, and you have to have 33 rivets between the marks.

All this while make sure the #2 piston is still at TDC, by checking the timing mark on the alternator rotor and looking down the spark plug hole.

After aligning the cams, put the camshaft bearing caps back on, in their exact positions. You can see numbers on the cam caps, they go like this : 1 to the exhaust left, 2 is exhaust center, 3 is intake left, 4 is intake center, the long side cap is on the right. Make sure all locator pins are in place (each of the caps has one).

The cam caps have a specific order of tightening - tighten the center caps first(2 and 4), front bolt on each and then rear bolt, then 1 and 3 then the side cap (where the sprockets are). Torque the cap bolts to 12 NM.

Put the top valve guide back on and torque it?s bolts to 12 NM.

Now we?ll assemble the CCT.
Take the CCT body, and unscrew the Phillips screw out a few mm.

Take the tensioner rod. Put the long spring into it, and put the rod, spring first into the body. Note the inside of the body is a round opening with a flat side, see that the rod doesn?t get caught on it. Push the rod in all the way, compressing the spring, and while holding it compressed, screw the Phillips screw back in, so that it holds the rod in place and the spring compressed.

? CAREFUL!!! Don?t look ?into? the rod while doing this, and don?t aim the tensioned rod at anything fragile. Try to keep your finger pushing on the rod as much as possible, because the bolt doesn?t hold it there all that well, and I nearly lost an eye (!!!) when the rod jumped out of the tensioner body and into my face.

Put the collar bearing on the rod, and the large spring on it.

Put the tensioner back on the engine and screw it?s bolts back in while pressing it down. Manual calls for loctite on the threads. Tighten them tight, but don?t go overboard, ~25 NM should do it (no specific torque rating in manual)

With the tensioner secure to the engine, unscrew the Phillips head screw a few turns. You should hear a loud 'click' sound and see the rear of the cam chain pull taut as the tensioner presses on the rear cam chain guide, making it pull the chain tight.

LOOK at the valve train. Is there anything obviously wrong? Check the cams again, could they be mixed up? (EX on intake, IN on exhaust, and yes, I did it once). Check the notches are still aligned with the top of the cylinder head and that #2 piston is still at TDC. Take the top valve guide off and count the rivets between the timing notches again and put it back on. Turn the engine over a few times, slowly. try to see hear and feel if the pistons hit the valves. Look at the rotating cams, are the valves moving fine?Reread the manual and make sure it?s all right. TERMINAL ENGINE DAMAGE might occur if you try to run it wrong.

Screw the alternator and view window caps back on. Check the O-ring on the alternator cap, if it?s old and hard and dry or missing, replace it.

Hurrah! we have ourselves a timed engine!

Now, adjust the valves per the valve adjustment procedure, and you can put the engine back on the bike. Good riding!

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