offset key for stretched chain - PeachParts Mercedes-Benz Forum

goldveg · #1 03-13-2014, 09:45 PM

Hello all. I have a 617 Mercedes 300sd 1983 turbo diesel. I have recently checked the timing chain.
I checked it with the two timing marks.
I am not happy.

The car currently starts and idles ok.
But, it is extremely lethargic at its acceleration.

The timing mark at the cam matches.
The mark at the outside pulleys is on 15 degrees BEFORE the OT timing.
I am confused by this.
I thought that chain stretch would make it read in degrees AFTER the OT TDC timing mark.

Can someone please tell me why it is reading like this?? Is it because the chain skipped a tooth?
How can I changed the chain back?
I see that you can place a 4,6,8, or 10 degree key in so that it retards or advances the timing.

Is this my best option?
Or should I replace the chain altogether?
The side of the chain tensioner is tight.
The other side is on the loose side.

Again thanks, goldveg
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whunter · #2 03-14-2014, 01:08 AM

Timing Chains Don't Last Forever
Timing Chains Don't Last Forever

Diesel Timing Chain Links Thread
Diesel Timing Chain Thread - PeachParts Mercedes ShopForum

Diesel Injection Pump Timing Device threads
Diesel Injection Pump Timing Device threads - PeachParts Mercedes ShopForum

Diesel Injection Pump TIMING thread
Diesel Injection Pump TIMING thread

Diesel Harmonic Balancer threads
Diesel Harmonic Balancer threads

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goldveg · #3 03-14-2014, 07:20 AM

Hello and thanks again. I realize that many people ask questions on here. I know that it is easy to provide me with threads that have a broad base of information. I have spent hours reading threads and links in threads. But, I have a more specific question. Why am I getting a reading of 15 degrees before TDC? I would logically think that all chain stretch leads to 5 - 15 degrees AFTER TDC. Please save me some time. Thanks again, Kev

Stretch · #4 03-14-2014, 08:02 AM

Quote:

Originally Posted by goldveg

Hello and thanks again. I realize that many people ask questions on here. I know that it is easy to provide me with threads that have a broad base of information. I have spent hours reading threads and links in threads. But, I have a more specific question. Why am I getting a reading of 15 degrees before TDC? I would logically think that all chain stretch leads to 5 - 15 degrees AFTER TDC. Please save me some time. Thanks again, Kev

The alignment of the notch with the graduations at the crank is a notoriously poor method of assessing the chain elongation. It is very hit and miss and not very reliable.

The 2mm lift method is the approved method of doing it. See this DIY in the wiki

PeachPartsWiki: Measuring Timing Chain Stretch

There's the offset key install DIY here too

PeachPartsWiki: Camshaft Woodruff (offset) Key install

A well respected member suggested that if no clock gauge is available you can lean your chin on say the front of the engine or the front of the engine bay and estimate the drop of the valve spring by placing a ruler behind. This of course is not as accurate as a clock gauge but because the off set keys only come in set sizes the error in your measurement is likely to be within the sizes that are available.

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Help for doing the job

1) Remove the glow plugs to help spin the engine - no sense in fighting compression
2) Only do this measurement with the chain tensioner fitted
3) Crank the engine ONLY in a clockwise direction - seen from you standing in front of the engine looking at it
4) Use a 27mm deep socket on the crankshaft bolt do not turn from camshaft cog - I don't like to read about people turning from the power steering pump either so don't tell me if you do that!!!
5) IMPORTANT - remove bolt for camshaft cog to check for an existing offset key before you order a new one
6) Make this measurement more than once - make sure you get repeatable results before changing anything

whunter · #5 03-14-2014, 10:13 AM

Quote:

Originally Posted by goldveg

Hello and thanks again. I realize that many people ask questions on here. I know that it is easy to provide me with threads that have a broad base of information. I have spent hours reading threads and links in threads. But, I have a more specific question. Why am I getting a reading of 15 degrees before TDC? I would logically think that all chain stretch leads to 5 - 15 degrees AFTER TDC. Please save me some time. Thanks again, Kev

If the reading is correct ???

* Previous owner installed a new chain, and lost timing.
* The cam sprocket was removed and re-installed backward.
* The harmonic dowel's are broken = shifted position.
* The timing chain tensioner is damaged.

At this late date, Offset keys are not a wise choice.

Timing Chains Don't Last Forever
Timing Chains Don't Last Forever

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Stretch · #6 03-14-2014, 12:07 PM

Quote:

Originally Posted by whunter

...
* The cam sprocket was removed and re-installed backward.
...

Is this possible? I thought mine had a notch that aligned with the woodruff key position on the camshaft.

whunter · #7 03-14-2014, 03:09 PM

Quote:

Originally Posted by Stretch

Is this possible? I thought mine had a notch that aligned with the woodruff key position on the camshaft.

It is possible, and even fairly common for beginners on the OM616 - 617..

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Diesel911 · #8 03-14-2014, 09:42 AM

Maybe something happend to the pointer and bent or moved it?

This was mine at 198K Miles.
The pic on the Right is a split picture with the Camshaft Gear and on the other side is the Crankshaft Dampner.

cho · #9 03-14-2014, 02:43 PM

Quote:

Originally Posted by Diesel911

Maybe something happend to the pointer and bent or moved it?

This was mine at 198K Miles.
The pic on the Right is a split picture with the Camshaft Gear and on the other side is the Crankshaft Dampner.

11-5=6

.... did you go with 4 key?

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Diesel911 · #10 03-14-2014, 03:20 PM

Quote:

Originally Posted by cho

11-5=6

.... did you go with 4 key?

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Doing this sort of check is not the same as doing the 2mm Method so you would not be using the 2mm Method Specs.

I have not checked it recently but it is on about 2.5 degrees after OT (Top Dead Center) and the closest Offset Key is for 4 degrees.

All the Number in the pic above Zero are after top dead center. So in My case the Pointer does not quite point to 2.5 ATDC but I think that is due to the Camera Angle. So I am not yet a candidate for an Offset Woodruff Key.
If it was 4 or more degrees ATDC then I would be a candidate for the Offset Key. I would also do the 2mm Method go be sure.

But here is the Bugger. I have also not checked to see if some Previous Owner installed an Offset Key.

cho · #11 03-14-2014, 03:32 PM

Quote:

Originally Posted by Diesel911

Doing this sort of check is not the same as doing the 2mm Method so you would not be using the 2mm Method Specs.

that is valuable info for me at least

as I always thought that same rule
apply for ballpark and 2mm.... read the balancer and subtracts from 11 (for my engine) to get real stretch.

thanks mate!

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Diesel911 · #12 03-14-2014, 10:35 PM

Quote:

Originally Posted by cho

that is valuable info for me at least

as I always thought that same rule
apply for ballpark and 2mm.... read the balancer and subtracts from 11 (for my engine) to get real stretch.

thanks mate!

.

I actually don't know as I have not done the 2mm Method or installed an Offset Key to see what it actually does.
I just try to understand as best I can how things work.

But, in general when the offset Key Moves the Camshaft Gear rotation of the Camshaft Gear 1 degree changes 2 degrees on the Crankshaft.

I guess I will have to pull out the Manual and take a better read of the Instructions on the 2mm Method.

Stretch · #13 03-14-2014, 12:05 PM

By my calculations the engine would be in trouble if the camshaft notch aligned and the crankshaft was at 15 degrees before TDC - the intake valve is meant to open at 9 degrees after TDC - with a bit of elongation after 20,000 km chapter 05-215 in the FSM reckons this valve will open at about 11 degrees after TDC...

...seems to me the inlet valve would be open just as the piston was at TDC

Diesel911 · #14 03-14-2014, 11:06 PM

I still do not get part or the 2mm Method.
Since the Dial Indicator was set at Zero I am not getting how this happens: "Dial Gauge has gone back by 2mm (valve lift) to 1mm.

I will likely have to read someoned DIY to understand it.

In the Offset Key Selection Chart it looka like if you did the Camshaft Alignment you could use that to select an Offset Key. However, the camshaft Alignment is not a precision as the 2mm Method.

And, then there is the Chart on the first page of the PDF. It has a New Chain and used chain if I remember with 20,000 miles on it specs. I don't recall the instructions saying which of the 2 to use to selecte the offset Key.

People have suggested that the 20,000 Miles is sort of the expected stretch; sort of a break in stretch that was calculated on at the Factory. So you would think that if you were installing a Offset woodruff Key on an Used Chain you would use the Used Chain you would use the Used Chain specs.

So for Me there is a few unknowns.

Stretch · #15 03-15-2014, 07:01 AM

Quote:

Originally Posted by Diesel911

I still do not get part or the 2mm Method.
Since the Dial Indicator was set at Zero I am not getting how this happens: "Dial Gauge has gone back by 2mm (valve lift) to 1mm.

I will likely have to read someoned DIY to understand it.

In the Offset Key Selection Chart it looka like if you did the Camshaft Alignment you could use that to select an Offset Key. However, the camshaft Alignment is not a precision as the 2mm Method.

And, then there is the Chart on the first page of the PDF. It has a New Chain and used chain if I remember with 20,000 miles on it specs. I don't recall the instructions saying which of the 2 to use to selecte the offset Key.

People have suggested that the 20,000 Miles is sort of the expected stretch; sort of a break in stretch that was calculated on at the Factory. So you would think that if you were installing a Offset woodruff Key on an Used Chain you would use the Used Chain you would use the Used Chain specs.

So for Me there is a few unknowns.

The procedure is this

You turn the crank until the cam lobe on the intake valve (2nd one in!) over #1 cylinder is pointing upwards

This means two things

a) the valve is closed
b) you are close to the TDC before the intake stroke

The clearance between the rocker arm and the cap on the valve stem needs to be removed. So you set the valve clearance to zero - if you like no feeler gauge fits.

You now set up your DTI (clock gauge) by putting the probe on the valve spring retainer

You push the DTI downwards so that you have a 3mm pre-load.

You need this pre-load because although this method is called the 2mm "lift" method - the valve will drop downwards into the cylinder {but the valve "lifts" from the valve seat}

If you don't have any pre-load, the needle on the DTI stays in the same position

With the 3mm pre-load set you continue to rotate the crank in its normal direction of rotation until the DTI shows a drop of 2mm - so it will be reading 1mm

This is the point at which you read off the graduations at the crank

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Please note this is NOT the elongation

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You now have to find the camshaft code on the back of the camshaft (near to cylinder #5) and compare the measurement you have made with the data in the FSM