As 617 powered M-Bs are acquired by new owners the question of valve
adjustment arises increasingly. This modest instruction attempts to
cover the basics of a DIY valve adjustment. Old hands may have
suggestions that differ and those are encouraged to enrich the archives.
Until the mid 1950s all auto engines used mechanically adjusted valves;
thereafter hydraulic valve compensation was adopted for most US engines.
(I bet many "valve and carbon" jobs were done for neglected valve adjus-
tments.) Thereafter only Detroit's high performance engines and imports
retained mechanical valve adjustments.
Mercedes-Benz cars were designed for top performance with the cost being
more maintenance intensive until the 1980s. Better car design and
increased competition reduced maintenance and its costs. But that leaves
the need for DIY owners to know how to maintain these vintage four and
five cylinder diesels, all of which should have their valves adjusted
every 15,000 miles.
The valves in an engine look like mushrooms that are raised from and
lowered into their seats. The contact area between the valve and its
seat in the cylinder head is quite small and, though hardened, wears
over the miles; the wear allows the valve to recede into its seat.
That means the valve moves closer and closer to its cam lobe until
the cam prevents the valve from fully closing and sealing. To prevent
this and to ensure its complete closure and seal the engineers put a
safety gap between the cam and valve. The adjustment is to restore the
correct safety gap between the cam and the rocker arm and hence its valve.
Adjusting valves is not hard physical work but is rather a fastidious
task that requires some time (2 or 3 hours for the first time) so have
the time available and be in a patient mood for the job. You need a good
drop light, a new valve cover gasket, two thin 14 mm straight open ended
wrenches or better, the Hazet 14 mm offset valve adjusting wrenches,
and a blade feeler gauge. Doubled latex gloves are a helpful option.
You need to be organized and methodical. There are ten valves to be
checked (8 in a 240D); typically only a few will need to be adjusted.
The intake valves' gap should be set to .004 INCH and the exhaust
valves' gap should be set to .014 INCH; both specifications are for a
COLD turbo diesel engine (60 - 70 degrees F.) that, ideally, has stood
overnight without having been started.
YOUR engine's specification may be different, check before starting
Remove the throttle linkage from the top of the valve cover. The sockets
are pried off their pins with a flat bladed screw driver. Just get the
linkage out of the way and let it hang. Remove the valve cover, also
known as the cam box. It fits tightly but lifts off with some vertical
lifting and wiggling. Set it somewhere clean, upside down.
Before you is the camshaft and ten very black valve springs and rockers.
It's now time to stop and make a check off list for these valves all
look the same after you've checked a few and you don't want to omit any,
or work on the same one twice!
Make a layout sketch like this (from the RIGHT side): I=intake,
>> REAR #5 #4 #3 #2 #1 FRONT
>> I E E I I E E I I E
>> Look at the right side of the engine, at the ports attached to the
head,some are Intake ports, the others are Exhaust ports. Except at #1
and #5 one port serves two cylinders. The Exhaust ports are usually
slightly rusty from heat. Confirm the above diagram against your engine.
Remember, the turbo's Intake valves' gap should be .004 INCH and the
Exhaust valves' gap .014 INCH. That's why a guide and check off list
is needed to avoid confusion between the specs, not omit any valves or
duplicate your work.
I'm right-handed so I stand on the car's right and reach to the left;
I also remove my engine's air cleaner for better access to its valves.
OK, it's time to begin. Look at the cam from the front of the car and
find a lobe that's pointed at 1:00 o'clock - that puts its lobe about
180 degrees opposite its rocker arm. (First pick one that's easily ac-
cessed so you can concentrate on the valve, rather than the struggle to
Determine if that valve is an I or a E and slip the appropriate feeler
gauge blade between the cam and its rocker arm. It should be snug and
take a bit of a tug to move it through the gap. It's OK if the blade
slides through the gap with resistance; if so, mark it "OK" on your
check off list.
The valve is too tight if you can't get the gauge into the gap and
needs to be loosened. Conversely, if the gauge is really loose the
gap should be reduced. Bear in mind that most adjustments are only about
.002 to .004 INCH or about one eighth of a turn of the wrench so start
with modest expectations - of making a fine adjustment.
When in doubt, looser is better than tighter, but too loose = tapping.
Below the rocker arm and above the valve spring are two nuts. The TOP or
cap nut adjusts the valve's gap, the LOWER nut is the lock nut. These
are firmly snug, not very tight. Hold the cap nut and loosen the lock
nut, then turn the cap nut tighter or looser as needed, check the gap
and, if OK, hold the cap nut and while you snug the lock nut. Check the
gap again. You can turn the valve spring with your fingers (wear double
latex gloves) if needed to get the wrench onto the nut. That's one
valve done so X it off your check list. (I note which valves I adjust
so if, there's a problem, I only need to check those, not all.) Turn
the engine by touching it with the starter and look for a cam lobe
that's opposite its rocker arm.
Officially one is supposed to turn the engine by placing a 27 mm socket
onto the crankshaft pulley's nut. Sometimes you can also turn the engine
by putting a wrench onto the power steering pulley bolt but be sure to
turn it the CORRECT direction or rotation. Use a ratchet to ensure
>> NEVER turn the engine backwards and
>> NEVER try to turn the engine from its CAM.
While the cover is off you can also check the timing chain's stretch.
Between the chain sprocket and the front most camshaft support or
tower is a thrust collar; it has a notch. On the driver's side of that
front most cam tower, is a horizontal mark cut into its exterior at the
mid point of the camshaft. Turn the engine until the collar's notch
and the groove are exactly aligned. That puts the camshaft at TDC (top
dead center). Now read the degree scale on the crankshaft balancer.
( 00 degrees is TDC on the crankshaft's scale.) (You'll need to clean
the dirt off the crankshaft's balancer and its reference pointer to
read the angle.)
The timing chain should be replaced if the crankshaft reading is 5
degrees or more ahead of the cam (now at TDC) for that indicates the
chain's worn and stretched and the greater these are the greater are the
odds of the chain breaking. When it breaks the valves and pistons
collide and another engine will be needed for repairing the resultant
mess is much more costly than a used engine. Chains last a long time
but a new chain can be installed DIY if necessary.
After all the valves have been checked and adjusted as needed it's time
to install a new gasket onto the valve cover, install the cover and
torque its nuts to 11 ft lbs. Reassemble the throttle linkage and
lubricate each socket with a few drops of ATF and snap them together and
check that the linkage moves freely.
Job complete. Inventory your tools and mark the date and mileage on your
check off chart so you'll know when it's next due.
I hope the job creates an appreciation of the precision and reliability
that "old mechanical systems" can achieve. No computers, no lasers, just
some personal attention. Afterward you know how it works and what you've
>> Thomas Lambach
>> '80 300SD
>> This is a NG post from an owner who did an adjustment:
>> Start to finish about 1.5 hours. Most frustrating
>> problem was putting the valve cover back on the
>> engine. I should have paid closer attention as
>> I removed it. The gasket kept getting knocked off
>> of the cover..... finally got it on. Two of the valves
>> were correct all others tight. The idle is much
>> Only major problem was breaking two vacuum lines.
>> 20 year old plastic that lives under the hood tends
>> to turn to glass. I will be replacing all the lines soon.
>> If anyone is thinking about adjusting your valves
>> but is not sure you can do it..... buy some wrenches
>> feeler gauges and go for it. It is really a simple job.
>> By the way, there is no way I can get a socket on
>> the power steering pump to crank the engine, hose
>> in the way. Turning the engine with the crank was
>> not a problem, ratchet and deep 27mm socket worked
>> fine. I put a piece of masking tape on the inner fender
>> and marked 1 through 10, (for the valves). Under each
>> number I wrote the gap setting. Then checked the cam
>> lobes to see which was closest to position for gap setting.
>> Cranked the engine to position and adjusted valve, marked
>> off the valve on the tape and cranked to get the next
>> lobe to position...etc. Only had to crank the cam over one
>> time. I did keep an eye on the timing mark on the back
>> of the cam sprocket, and when it lined up with the tower
>> mark checked the chain stretch.