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81 300TDT WAGON , 82 300DT SEDAN, 84 300TDT WAGON(retired),
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Discussion Starter #1
We all know the clearance specification from the Mercedes.
When I opened the engines on my two cars in 2010, I found the clearance on both engines are less than Mercedes specification.
What caused the clearance become smaller, instead of increasing?
How Mercedes calculated the specification? Is the specification for perfect engine only?
If the engine is old and with low compression rate, can I increase the clearance when adjusting?
what is the consequence if I increase the clearance higher than Mercedes spec?
If I increase the clearance for intake valve, it will suck more air, will it increase compression rate?
 

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money pits of various forms
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We all know the clearance specification from the Mercedes.
When I opened the engines on my two cars in 2010, I found the clearance on both engines are less than Mercedes specification.
What caused the clearance become smaller, instead of increasing?
How Mercedes calculated the specification? Is the specification for perfect engine only?
If the engine is old and with low compression rate, can I increase the clearance when adjusting?
what is the consequence if I increase the clearance higher than Mercedes spec?
If I increase the clearance for intake valve, it will suck more air, will it increase compression rate?
The spec is based on timing. More clearance shorter duration.Keep it in spec. If the engine is old its probably a good idea to replace the roto caps.
 

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1984 300D
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It might be nice to know what Engines and Chassis you are speaking of.

If you live in the US on top of the Cross Member in front of the Radiator on the Driver side is an emissions sticker that has the Valve Adjustment Clearances on them.

The clearances get tighter mainly due to Wear on the Valve Faces and their seats that causes the Valves to move up and close the clearances.

You will not get more air be increasing the Clearances. If you increase the Clearances the Valves will open late and close sooner. So in short the Valves would not be opened as long.
Tight Valve clearances decrease compression.
They Valve Clearances are the same for new and old Engines.
Only Mercedes knows why the picked the Valve Clearances they use. I would assume it was done after some research on the subject.

The Engine was designed to work best at the Factory recommended Valve Clearances. I have not read anywhere that changing; not using the Factory recommended Clearances makes anything better.
 

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Slightly increasing the valve clearance on the intake valves can improve effective compression, it is also something that is recommended by MB for very cold climates. There might be some drawback, less performance at full load probably.
 

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Slightly increasing the valve clearance on the intake valves can improve effective compression, it is also something that is recommended by MB for very cold climates. There might be some drawback, less performance at full load probably.
The extra Valve Clearance for extreme cold weather has to do with the fact that in the extreme cold the length of the Valve Stem is shorter; but when the Engine heats up to the normal operating temp the Valve Stem is going to return to the normal length it would be at that temp.

I believe if your Clearances are tighter than the Factory Specs increasing the clearance to the Factory specs will improve compression (quite of few forum members have said they had better cold starting after they did a Valve Adjustment).
But, I do not believe increasing the clearance beyond the Factory Specs will improve compression. At least I have never read that in 3 years reading info on Forums or in my Diesel Tradeschool classes.
 

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If the valve stem decreases in length in cold weather, so does the engine head. And the recommended increase in valve clearance (0.05 mm) is prescribed for cold and hot engine. If your argument was true, it would only be prescribed for a cold engine in a -20 degrees C climate.

Valve clearance increases when the engine is hot, so the compensation-for-a-shorter-valve stem argument doesn't hold true.
 

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81 300TDT WAGON , 82 300DT SEDAN, 84 300TDT WAGON(retired),
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Discussion Starter #7
Both engines are om617.952. One is sedan another is wagon.
Last winter, I can not start my sedan when temp reached -3 without block heater. It was not adjusted in the past 6 years told by PO.
After adjustment, I can start the engine at -11 without block heater plugged.
Clearly the valve adjustment , in my case , actually increased the clearance. After adjustment, it sucked more air, improved the compression, made engine easier to be started.

Slightly increasing the valve clearance on the intake valves can improve effective compression, it is also something that is recommended by MB for very cold climates. There might be some drawback, less performance at full load probably.
More air means more power, why it is a performance drawback?

],
"The clearances get tighter mainly due to Wear on the Valve Faces and their seats that causes the Valves to move up and close the clearances.
"
according to your response, to increase the clearance is to push down the valve stem, without change the valve and head, how to compensate the worn between valve face and head after increase the clearance?
 

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If you increase the clearance, the valve will open later and close sooner, the valve will be open for a shorter period of time. That means that there is less time to suck in air, maybe resulting in less air in the cylinder. You will not notice that at idle and partial load, but you might notice it at full load: there will not be enough air to burn all the diesel.
 

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Discussion Starter #9
If you increase the clearance, the valve will open later and close sooner, the valve will be open for a shorter period of time. That means that there is less time to suck in air, maybe resulting in less air in the cylinder. You will not notice that at idle and partial load, but you might notice it at full load: there will not be enough air to burn all the diesel.
You are saying the clearance has impact to timing?
I am lost completely.
By looking at the following pictures.

To increase the clearance, actually is moving the valve stem down a little bit, right?

If you are saying less air get sucked, how to explain the compression is increased after valve adjustment?

:confused:
 

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No, if you adjust the valve clearance, you adjust the nut on top of the valve stem, the valve stem stays put.

If the valve closes earlier, compression starts earlier, that means more compression. I would think there would be a drawback if you increased the valve clearance for the intake valve, otherwise the valve clearance could always be greater, not just when it is really cold (< 20 degrees C). It all depends whether less air is sucked in when the valve clearance is increased.
 

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1984 300D (235,000 miles), 2001 E430 (78,100 miles)
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Loose vs Tight

Of course, you'd prefer to have the valve clearances in spec. Having said that, though, it is better to be on the loose (larger clearance) than tight (smaller clearance) side of that. With tight valves, they are closed (seated) for a shorter time during the cycle (and may not be closed as tightly), which can lead to valve seat erosion & "burnt valves".
 

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If the valve stem decreases in length in cold weather, so does the engine head. And the recommended increase in valve clearance (0.05 mm) is prescribed for cold and hot engine. If your argument was true, it would only be prescribed for a cold engine in a -20 degrees C climate.

Valve clearance increases when the engine is hot, so the compensation-for-a-shorter-valve stem argument doesn't hold true.
Rather than having a dispute about it I will let you do the research on it if you are interested in finding out.
None of what I said is a theory I made up on my own.
 

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I would think tight clearances simply stop the valves from fully seating. simple as that. increase clearances you won't be sucking in more air, the overlap duration would lessen though which would mean better performance low end rpm range but it would also hinder the high rpm range as at high rpm's the engine depends on overlap to actually suck in more air (much like a 2 stroke using the exhaust to pull in air faster.) This is also the reason with naturally aspirated cars if you remove the exhaust entirely you lose torque. Some back pressure is needed to keep the flow moving through the engine properly.
 
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