How to change the chain tensioner, camshaft sprockets, and timing chain on an MB SM651 without dismantling the engine

[ThomasWhited]

Hello everyone, I have written this instruction guide for myself and I wanted to share it with you.

I will start working on this tomorrow; I figured it could help someone in a similar situation, and I figured the folks here know more than I do about these engines, so they could make edits and repost it as a better guide. Once I finish, I will inform you if I managed to damage my engine or not.

For context, I'm not a mechanic; I'm an electrician. However, I have spent the last two years fixing my Mercedes C250 estate, so take that information as you will.

Like many others, this issue began with chain rattle upon startup. Looking back, I regret not replacing the chain tensioner immediately; doing so would have likely resolved my problem or prevented it from escalating to the point where I needed to replace the chain and potentially damaged the plastic guides (which I have not yet examined). However, here we are.

So, I figured I'd change the sprockets and chain at the same time. But, I couldn't find any videos on YouTube that showed how to do all three of those things without disassembling the engine, which isn't an option when working on the grass outside my apartment. So, I compiled a list of everything I needed to know about camshafts, valves, pistons, and timing, and here it is...
Please inform me if you find any problems with this guide; doing so might save me a lot of trouble and money.

Step One: Get Everything Ready

Fill the new tensioner with clean oil and gently prime it by pressing down on the inner mechanism with wide-opening grips a few times. Once some air has been purged and replaced with oil, the tensioner should become slightly more difficult to press; stop at this point.

To make room for the engine, take off the plastic undercarriage, the front engine covers, the serpentine belt, the main cooling fan assembly, and any other necessary parts. Get a good look at where the front crankshaft pulley is located, and then reach up from above. Take the rocker cover and any other necessary components off the engine's top, and clean the area where the gasket would normally be located.

To prevent engine damage, line the oil return channels and the cavity of the rear timing cover with clean, lint-free shop rags. This prevents any tools, chain links, or loose bolts from falling into the oil pan by acting as a safety net.

Choose a time: Manually turn the engine clockwise until the OT mark (TDC) aligns with the pointer needle cast onto the front engine casing. A specialised socket can be used for the front crankshaft bolts.

To lock the camshaft, use the OM651 Camshaft Locking Tool to align all of the laser etched marks on the camshaft. Position its female-specific triangle openings such that they touch the cams' wedge profiles. To lock the camshafts into true TDC, bolt the tool down and push it straight down so it's flush with the head.

To release any tension on the simplex chain, unscrew and remove the hydraulic timing chain tensioner from its mounting on the engine block.


Second Stage: Removing the Camshaft and Changing the Sprockets

To loosen the bolts on the center cam gear, you need a special tool (a 90° e14 with a ½" drive, ideal for confined areas). Keep in mind that you are working with left-hand threads (loosen by turning clockwise).

Take note of the specific locations of the camshaft bearing covers; you'll need to remove them one turn at a time to free them. Mark the left and right sides of the camshafts; then, lift them out of the machine and release the chain sprockets. This will ensure that all of the valves are closed. Pull the chain slack up and use mole grips on each side to hold it above the block.

For the purpose of changing sprockets, undo the e14 center bolts to remove the old sprocket from the camshafts. Insert the new sprockets, leaving the bolts half a turn undone to allow them to spin freely for the time being.


Third Stage: Introducing the New Chain

Use the 4 mm diameter mandrel pins to press out the rivets on one link of the old chain using a chain-splitting tool. Then, link the old chain to the new. (KEEP THE CHAIN, PIN, AND ANYTHING ELSE INSIDE THE ENGINE THE ENTIRE TIME) Remember that the new simplex chain will feed in from the right/clockwise side, so use a temporary connecting link to securely join its open end to the old chain's open end.

To roll the chain through, have two helpers stand on each side of the engine bay. With the old chain pulled diagonally up and out to the tensioner side, feed the new chain straight down. At all times, keep a lot of tension on the chain to prevent it from jumping a tooth. Slowly and carefully, use the specialised tool to manually rotate the crankshaft clockwise (keep track of the revolutions).

Be sure to stop turning the new chain the second it loops around and the two open ends meet at the top of the head; this will serve as your timing check and secure the chain. Hold the chain taut and reapply the mole grips to prevent the slack from falling into the engine (use a rag to protect the new chain from the grips) until you've confirmed that the front pulley is resting precisely on the OT TDC mark. If you can, use a bright torch to look inside the timing cover and make sure the chain links fit snugly into the lower gear's teeth.


Step 4: Positioning the Camshafts

Before reinstalling the camshafts, check that the crankshaft is at the OT mark TDC, that all the lower bearing caps are in the right places, that the rocker arms and lifters are perfectly straight so that they won't jam when compressed, and finally, lubricate everything with new engine oil. Carefully wipe down and lubricate each camshaft with fresh oil. Then, one by one, place the camshafts in the engine, making sure that they align with the TDC markings (the timing marks pointing upwards and th

To torque the camshaft, follow this sequence: starting at the center and working your way outward (from cylinders 2 and 3 to 1 and 4), ensuring that all upper bearing caps are fitted in their original orientation as noted earlier. Make sure to follow the same sequence from this point onwards and install each bolt until it is finger tight. Second, tighten the bolts by turning them a full turn each. Third, apply 5 Nm of torque to each bolt. Turn the bolts' torque to 9 Nm. If you encounter abrupt, strong mechanical resistance, you must immediately stop turning. Because of this, you should double-check the lifters and rocker arms.

After you've clamped down the camshaft locking timing tool and disconnected the split link holding the old chain to the new one, rivet the chain closed. Before you use the rivet-pressing tool to force the permanent new master link rivet home, insert the new master link (which comes with the chain) into both ends of the new chain. Be careful not to overtighten the rivets; instead, press them home little by little. Before you press, check that the chain can still move freely and that the new rivets are the same spacing as the ones on either side when crimped.

After making sure the crankshaft is still at over-the-center (OT TDC) and that the camshaft timing tool is in place and properly timed, you can set the tensioner by removing the mole grips and turning the chain slack toward the exhaust/turbo side of the tensioner. To tension the simplex chain and remove any extra slack, thread the new tensioner onto the engine block and tighten it to the specified torque of 80Nm.

After you've made sure the new chain fits snugly onto the camshaft sprockets, use the 90° E14 tool to partially torque them. Before you torque the left-hand thread cam gear bolts to 55 Nm, make sure the camshaft locking tool is seated correctly.


Step 5: Last tweaks to the torque settings and alignment

To check the timing, take the locking tool off the camshafts, grease them and spin the crankshaft clockwise two complete revolutions by hand. Return it to the overtime mark. The locking tool for the camshaft must return to those triangle wedges with ease and cleanliness. Go back to the previous step and repeat the process if it binds; this indicates that your timing is slightly off. Loosen the bolts that hold the cam gear.

Once you're happy with the timing, fasten the camshaft locking tool. Before you torque the bolts on the cam gear, make sure an assistant fastens the crankshaft socket and breaker bar to the crankshaft. Brace the crankshaft in an anticlockwise direction so it doesn't turn clockwise and stays precisely at the 0⁰ OT mark. Finally, torque the bolts on the camshaft, which are left-hand threaded, down to 55Nm + 90 (use a paint marker to make sure the camshaft bolts are fully 90 degrees).

For the last timing check, take the camshaft locking tool out and spin the crankshaft two complete revolutions clockwise by hand. Return it to the overtime mark. The camshaft bridge tool needs to return to those triangle wedges with ease and cleanliness. Binding indicates that the timing is off by a small amount, so you'll need to get new bolts for the camshaft gear stretch and reset the timing.


Important details to finish the task

Cover song for rock opera

Specification for Torque: 9 Nm
The rule of sequence is as follows: begin at the very center bolts (between cylinders 2 and 3), and, in a neat, alternating spiral pattern, move outward toward the front (Cylinder 1) and rear (Cylinder 4) of the engine.

Methods for Cleaning Injectors

Put the sealing plug into the cylinder's opening to seal it.
After you've let the carbon cleaner or brake cleaner spray soak in the hole, step two is to scrub the vertical walls with the 18 mm brass-coated brush.
Suck out all the loose fluid and wet soot using a syringe or a narrow hoover attachment.
For step four, polish the aluminium flat by running the silicone-nylon brush over the bottom seat.
Use a bright torch to check for black pit marks or lines across the seat. If you find any, then lightly lubricate a 17 mm 19 mm flat cutter and smooth out the surface.
Sixth, use the retrieval tool to remove the sealing plug again.

The clamping claws of fuel injectors

Level 1: 15 NM
Step 2: Make a right angle cut
Third Stage: A Right Turn
(To ensure you accurately complete the full 180-degree total stretch rotation, use a paint marker on the bolt head.)

Gas pipelines

Step 1: After hand-tightening each flare nut, torque them to a baseline of 7 Nm using an open-ended torque flare nut socket.
Step 2 (Final Torque): At the injector end and the common rail end, tighten the line nuts to 33 Nm.

Prime with Low Pressure and Bleeding

Hold the key in the second position for 10 seconds each time; this will turn on the dash lights but not the crank. Repeat this process five or six times. As a result, the electric fuel pump located in the fuel tank is compelled to force fuel through the filter and into the common rail.
A Trick with Flare Nuts: On the injector end, keep the metal fuel line flare nuts slightly loose, about half a turn open. For only three or four seconds, have a helper turn the engine. Stop cranking and torque the nuts to 33 Nm as soon as you see clean, bubble-free fuel leaking out.
Last but not least, when you crank the engine again, it should start right away.

[ThomasWhited]

Potentially useful part numbers

Event sequence: A000993350264
Model number: A6510500864/A651050080064, chain tensioner
Gear for camshaft: A6510520000
Bolts for camshaft gears: A0029905103 Bolts for injectors: A0019902607.
Insertion gasket: A6110170060
No. A0000780580 for the injector leak off seal

[ThomasWhited]

Here are the issues I've encountered thus far:

I didn't start until Tuesday midday because of family stuff.

1. The bolts on the rocker cover are located at the rear. To loosen them, you'll need a small 1/4 extension, such as a 50mm (I lost my 138 bacho socket set years ago), or 75mm (which is too big), or a flexi (now I'm thinking about it), but I didn't have a flexi when it was 32⁰c, so I just struggled.

2. Upon examination of the camshafts, it appeared that the timing wasn't quite right to align the intake side precisely. The crankshaft had passed the OT indicator, but the exhaust cam was slightly behind. Additionally, I was unable to secure the tool without further rotating the crankshaft, which I attribute to the stretched chain. Rest assured, there were no engine codes before I began.

3. I purchased a Neilsen CT5430 OM651 timing tool kit, but the camshaft locking tool didn't fit properly on the engine top. It would rock when clamped down, so I ground off a little material to make it fit. Additionally, there's a small gap of about 0.5 mm where it seats against the cams, so it's not a snug fit. I'm going to try using some thick, high-quality printer paper on either side to fix it, but I'm not sure if it will work. In any case, I wouldn't recommend this tool.

4. Bolts for the camshaft bearing caps (A000000005885), I bent one during removal and decided to replace all of them except for the two with the strange cups on top; there are a total of 18 bolts; I suggest acquiring these before you begin so you don't waste time; they're inexpensive. A long reach t30 bit isn't long enough to reach the bolts with the caps.

5. After the old chain was split and I was ready to attach the new chain to it i realised I only had a duplex split link in my chain splitting tool kit (I also had other much longer ones ive been told would be for quad chains) it come with 3 & 4 mil bits and pieces but no single split link i attempted to use the duplex link and leave the slack towards the front of the engine and use electrical tape to fill the space so that the 2 chains wouldn't slide around on the link and miss align but it got stuck i brought it back out put the slack to the rear it got stuck did the same with the chains centered it got stuck so i took measurements of a solid outer link and got a split link from amazon which was the closest match I could find that would be with me in 2 days it was a 06B link this did in fact work but I could only fit the side with the 2 pins attached and the spring clip because it was a about a mil too narrow to get the plate on with the spring clip I did try to pull it apart and twist it to see if it would pop off but it held strong and decided to send it around the engine I recommend buying a febi a bilstein split link before starting the job although the 06B link did work fine.

6. Be cautious to enlist trustworthy individuals to assist you in rolling the chain so you don't experience the same delays I did (again). The chain can be easily rolled through with only four crankshaft rotations, so even a partner could be of assistance. Additionally, I used 1mm earth cable stripped from 1.5mm t&e to keep the chain taut after splitting, and a 2.5 core striped from 2.5 t+e to make a handle for the last link of the old chain, making it easier to hold onto.

7. Always use gloves when working with the cameras! I didn't give much thought to protecting the cams from fingerprints because I didn't think they would be sitting in a box for days. However, the weather has been unbearably hot, reaching 37⁰C in the UK, and I've been sweating buckets—salty sweat, of course—and my fingerprints have rusted onto the camshafts. Fortunately, there are only a few light marks on the last roller of each cam, and a few on the inbetween bits that haven't come into contact with anything. I've tried to rub them with clean oil and a lint-free rag, but the marks are still visible. I hope you won't be as unlucky as I am.

8. The sprocket stretch bolts with the number A0029905103 are incorrect; you should not purchase them because they are too large to fit in the cams that you require, which have the number A000000005884. You should obtain these before beginning the project, as they will likely have to be ordered in from Milton Keynes or somewhere. Unfortunately, this means that the project will be delayed even further into next week.

[maturo99]

https://www.youtube.com/watch?v=veEdhoiEZOQ

[ThomasWhited]

Yes, I did use this video as an example of what I mean when I say "chain," but it only covers the chain and doesn't cover other aspects, such as priming the chain tensioner or re-timing the engine after skipping a tooth. Despite this, it's still a good video, and I hope that anyone trying to do the same thing will come across this thread while researching so they don't have to go through what I did.

[ThomasWhited]

Hey everyone, I completely neglected to give you all an update on how things turned out. So, here are a few tweaks from my original plan:

1. I made a small loop and tied it like a noose. Then, once the desired tension was applied, I bent it back over. I didn't trust the molegrips, so I used copper wire (1.5mm cpc from 2.5mm twin and earth) to keep the chain taut after rolling it in.

2. The chain had to be joined and crimped before the cams could be reinserted because there wasn't enough space against the fire wall to fit the chain tool. I had to press the link several times and kept overtightening it. 3. Crimping the pins was a pain because I didn't have the official MB die sizes in my kit. After many frustrating hours, I finally succeeded in getting them crimped securely while the chain could still move freely, but it took me two evenings and one morning to get it right.

3. The camshaft sprocket bolts were a real pain. Trust me, you don't want to open a can of worms by attempting to use a breaker bar on the crankshaft pulley. My wife helped, but someone stronger would have been better. The force required could have snapped, destroyed or stripped the threads of the bolted down timing tool.

4. I used the paper as mentioned before, but it compressed under the force. Ideally, I would have had access to some steel shims or tolerance gauges to eliminate the play on each side, but I thought a small amount of allowance would be fine. As a result, I was unable to keep the marks perfectly center during the finishing process, although I came very close.

5. Cleaning the injector holes was a pain, but thankfully that was all I needed to do. I used twisted up kitchen roll to twist and push, and then I carefully used an L-shaped long reach pick (I had a set of these from when I battled with plugs in hard-to-reach areas last winter), hooked into the kitchen roll, compressed it down while it was spinning, and then hooked it out. Caution: after X turns, the pointed end will rip through the kitchen roll, scratching it. If you're not careful, you'll feel it happen, so stop immediately. Hopefully, you won't end up with a small scratch like I did—extra MB of lube on that one).

6. I didn't bleed the common rail when I removed the feed line from the gearbox and clutch. I thought it would be fine since my new fuel filters weren't primed with diesel. It took a few primes and turns, but it started up without any issues.

It was then that I went for a drive; everything was fine. A week later, I took it for a two-hour and forty-minute round trip; the engine and chain felt fine up until I decided to give it a little rag, reaching 125 mph before going into a soft limp; I could still rev three thousand and beyond without the eml light. obviously I completely shit myself but turns out its just an exhaust gas temp sensor (bank 1) the one on the exhaust manifold its self it disappears when cold and it only reappears once up to high temps (driving like a **** if you like), I also gained a tappy noise not long after that but turns out thats the vacuum pump on the way out i know this because it only happens on start up and whenever I release the break pedal before disappearing again, always something to fix on this ****ing car it's a love/hate relationship but I suppose I do drive it harshly and its not built for that everyone is telling me to get rid but o do love my manual c250 estate and they're hard to come by anyway hope you enjoyed the journey more then I did she's done a couple thousand miles since the chain and all is well if you have any questions I'd probably wont reply for weeks and weeks but ill try, hope all of this information helps at least 1 person and thank you for taking the time to read this thread

[ThomasWhited]

Number of the sprocket stretch bolt:
(A0029905103) Om651 2013+ (A000000005884) Om651 2012- (including the 2012 facelift model, which is the one I own)