How to adjust the valves in the M110 engine – PART 1

M110 Engine Valve adjustment – Part 1

I planned the valve adjustment job to be straightforward and easy, making it a delight to write an article about afterwards, but no! In the end I could not even adjust a single valve out of the 12 in total, and this is why I will divide the so called adventure into multiple parts and share my experience on the topic.

“Mother nature beats father engineer every time..”

Valve adjustments should be done every 15.000 km which means for most people every 2. or 3. year and is important for engine performance and fuel consumption. Neglected maintenance can even end up wearing your cam lobes prematurely if they get too tight!

A story of success

I got all the parts I needed, that is really only a valve cover gasket set. I let the car stay overnight for the engine to cool down. Then I start pulling out the spark plug wires, no need to mark them with cylinder number..haha! I unbolt the valve cover in a breeze, the nuts I throw somewhere neat.

Then I check the valve lobe timing, oh see that the first one is perfectly aligned already so I can start adjusting the first valve immediately. I find my valve feeler gauge easily from the toolbox and it’s not rusted together at all. I adjust the first valve and since my memory is faultless I can easily keep track of all the valves I have adjusted and the ones who are left. The crank is so easy to turn by hand and I don’t need to remove the radiator shroud or anything silly like that. I get everything done in under a less hour and bolt everything together with the new valve valve gasket and I’m now already on my way to the race track with 40 more HP.

A cake, but it's a lie

This is all however a big lie.

Sometimes wisdom words are painfully true: “Mother nature beats father engineer every time..”. The real struggle you can follow below.

bleak Reality

So before starting you should take care of a few things. In theory a valve adjustment is easy and requires little preparation and engine know how. You need a new valve cover replacement kit, since your old one is probably leaking oil and a new one is very cheap. The valve adjustment can be made either when the engine is warm (just after driving) or cold (20°C). I really recommend the latter since you will not burn your fingers and get better results since the engine is not cooling down slowly. Ideally it should be at ambient summer temperatures in the shade. You have to leave the car overnight to properly cool down after driving so this have to be planned in advance.

The tools you need is just really a spanner and some feeler gauges for the adjustment and a socket for turning the crank over, but as we will see here this is not the case for my case.

M110 engine with air filter off
Marked spark plug wires.

I start by removing the air filter housing to access the valve cover freely. Then I mark every spark plug wire with a number on a piece of tape. Usually the spark plug wires are different length, but it’s so much easier to plug them in afterwards. Then I take out the spark plugs.

Now I can start removing the valve cover bolts and place them somewhere safe. I note that the middle ones have a copper shim which I can keep, but the new valve cover gasket comes with replacement ones. It’s always nice to keep bolts, nuts and shims if they are in good order.

M110 engine with valve cover open

With the valve cover off is is extremely important to not drop anything into the engine like a nut or something, since this will cause catastrophic engine failure. Also keep dirt away and exercise hospital  level cleanliness while the engine is open. Cover it always when not working on it, like lunch break etc..

I then go in the the toolbox to find my gauge feeler blades and realize they have rusted together and end up throwing them. I go in my dad’s old toolbox and luckily find some that are in a bit better shape. With a cold engine the inlet valve clearing should be 0.10 mm and the exhaust 0.25 mm. I draw a diagram over the valves so I can mark them after every adjustment, it will be very hard to keep track of otherwise. Just try remembering 12 boolean positions for reference!

Valve gauge feeler tool

Of course none of the valve lobes are pointing  directly vertical for any of the valves after opening the cover, that would be too easy! So I go ahead and try to turn the crank, but come over a problem. The fan shroud is too tight to engine for me to get any socket tool and ratchet to the crank. Loosening just the fan shroud is not helping either since the fan is sitting tight to the shroud. Then I loosen the fan and I can just get enough room for the ratchet and finally being able to turn the crank.

I align the first cam lobe and is ready to check the valve clearing. I expect most of the valves to be to bee way too tight, and the first one is as expected. I grab my spanner to start adjusting, but the valve adjusting nut is not moving at all. In fact I’m starting to slightly round off the nut. I stop before I ruin it. Then I quickly check a few other valves, they are all tight as expected, but also here the valve adjusting nuts are super tight. I am totally stuck.

After searching the Internet for similar issues it seems this is a common scenario for these aging engines. The nut material is quite soft and can easily be rounded off when they are tight. The solution is to remove the rocker arms to access the whole nut so you can get a socket over it to exert more force.

Seems easy right? To remove the rocker arm you need to depress the valves somehow. They are under very high pressure from the valve springs and a special compressor tool is needed. There are many different DOHC valve spring compressor tools on the market, however the M110 engine have very tight space between the cams and the engine walls where the spring compressor have to fit.  And the valve is at an angle so the tool need to be just for this purpose. There is such a speciality tool for the M110 engine, but I can’t find it on a single place on the internet.  It turns out it is in fact out of production. Fuck!

I am stuck and have to abandon my job and put everything back together. This was a waste of time! (But in the back of my cynical mind I think it can be turned into an article at some point..)

Here is a diagram over the valve assembly to illustrate the point better.

Valve Configuration M110 engine

The valve compressor tool is like a leverage bar with a hook for the cam axle and a crow foot depressor at an angle that can slide between the engine wall and the cam axle down on the top of the  valve. It looks like this:

M110 DOHC valve compressor tool

how to solve the tool problem

I have considered some options below and ranked them from easier/cheaper towards more expensive options.

  • Obtain the tool

I have really tried to find this on the internet, but with no success. I think I will not be able to obtain it. Not sure if Mercedes has one , but I am scared of even asking, it’s probably super expensive.

  • Make the tool myself

I know how it looks like and roughly the sizes. I can easily obtain some steel, and a crow foot. I have a welder so I can make the tool. However I expect it to take some time to assemble. Definitely worth looking into.

  • Take the car it to a workshop

I can just take it to a workshop and tell them to adjust the valves, but they are going to run into the same problems like me and they might end up ruining the valve nuts unless I explicitly tell them the problem in great detail. Remember these days no cars require manual valve adjustment and only the more experienced mechanics have done this. If you want things done right, better do it yourself.

  • Contact Mercedes on how the fuck….

This is my last option and will consist of asking if they have such a tool, and if the tool is not for sale (or out of production), if they can at least adjust the valves if they can send a tool from Mercedes Classic in Germany to their workshop. Problem is that is going to be very expensive for a simple job. Not likely I will venture down this path. Also would turn into a dreadfully boring article for you DIY people!

More on this adventure when we return in Part 2 of the valve adjustment nightmare… soonish

(Not really, it’s damn winter and car is parked)

Cheers, Robs out!

AC Part 3: Changing the Receiver/Drier in W123

The AC receiver/drier should always  be replaced when servicing the AC system.

Luckily the receiver/drier is the most easily accessible and easiest component to change when servicing the AC system. It is located behind the front headlight in the engine compartment of the w123. If you were following my Part 2 of the AC system series, you might already have read that you always need to replace the receiver/drier when changing the compressor. The failure of the drier might have been the cause of failure of the compressor in the first place.

You can read about the drier/receiver function in AC Part1: Air Condition Systems 101


For preparation you need not to do much else than emptying the AC refrigerant at a professional workshop, get a new receiver/drier and have some basic tools ready.


  • New Receiver/Drier (it comes with the necessary gaskets)
  • New high pressure AC hoses if they are cracked/worn/old.


  • Socket tools
  • Spanners


If you haven’t emptied the AC refrigerant, take the car to a workshop and get them to empty and properly dispose your old refrigerant. AC refrigerants contains highly volatile greenhouse gases and needs to be disposed properly!

When back at your own garage. Start with opening the hood and disconnect the battery since you will be disconnecting some wires. Locate the receiver/drier behind the right headlight. It is the small canister with two hoses connected to it. Start by disconnecting the two high pressure AC hoses. Again this will mark the importance of emptying the refrigerant before, since we are dealing with high pressure systems and volatile gases. You might not be able to access both of them while the drier is connected to the chassis, then loosen the drier until you can access both hoses.

AC drier w123
Old AC Drier

As a tip the holes to the bolts going through the thin metal in the chassis might have started to rust. To avoid holes in the future you might want to get rid of the rust and paint over the area.

Disconnect the electrical wires connected to the receiver/drier, but remember how they were connected by marking the wires or taking pictures before. One is a pressure sensor for the compressor and another is a temperature sensor which controls the auxiliary fan.

AC drier
The Pressure sensor inlet has a protective cap which has to be removed.

The sensors do not come with the new receiver/drier, so these sensors have to be taken off the old one and put back on the new one before proceeding further. Be careful not to break off the brittle electrical connector pins, since they can be quite fragile. If you break it you need to find new replacements components since the AC system will not work without them. Also the pressure sensor will need a tight seal with the receiver/drier otherwise refrigerant will leak out. The pressure sensor has a small O-ring, replace the old one with a new O-ring that comes with the new receiver/drier as seen in the picture under.

AC drier pressure sensor gasket
The pressure sensor gasket

The temperature sensor sits at the top of the canister and can be removed with a 19mm spanner. It does not require any gasket. The pressure sensor can be removed with a 27mm spanner. Remember to replace the gasket before assembling it with the new receiver/drier.

AC drier Temperature sensor
Temperature sensor placement

After the the sensors are transferred you can install the new canister. Start by replacing the gaskets for the two hoses and tighten them onto the canister.

Replace the high pressure hose gaskets

Then finally secure the canister to the chassis with the two bolts to the chassis. If you have other AC components to change this is a good time to do it. For example you can replace the expansion valve which also is a common wear item on the AC system. Although the receiver/drier are more common to change.

At a workshop they will pressure test and fill up the the AC system and find any leaks if they are present.

AC drier w123
New AC drier installed

Cheers, Robs out!