Hi,

I rolled in a new timing chain (IWI) for my 1984 300SD with 180k miles. The timing was 16 degrees improved to 12.5 degrees with new chain. As per FSM, A worn chain should have 11 degrees while a new chain should have 9 degrees. All this was clarified using the timing cam mark and 2mm dial indicator method.

I have an offset key on the way to correct for 4 degrees. The question is, is normal sometimes that some engines may have timing off even with a new chain?

Btw- top chain guide, tensioner and tesioner rail were also replaced with chain swap.

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Theseus

Instragram: doss_project
1984 300SD - 930 Blue, OM 617
Murfreesboro, TN

Theseus owned a ship in Greek mythology, and when a part of the ship needed replacing, it was replaced. Eventually, everything was replaced. Is it the same ship? (I own the car of Theseus. )

A few thoughts...

How many degrees is one tooth, is it off by a tooth?

Are the gear teeth very worn, pointy like a shark fin or are the points symmetrical?

Any chance the key in there is an offset key?

Please post your findings and good luck!!!

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

According to this:

OM617 cam timing spec

Being off one tooth on the crank would equate to 18 degrees off.
You having 12.5 degrees with a 4 degree offset key puts you at 16.5 without the offset key, or with the old chain 16 degrees which would be 20 total... which is oddly close to 18 degrees.

I'd be reeeeaaallll suspicious of that. But I'm certainly no expert, just a boat mechanic that drives and maintains my lovely w123s.

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1982 300D (w123, "Grey Car")
1982 300D (w123, "Blue Car")
2001 Ford F150 "Clifford" (The Big Red Truck)
1997 Dodge Ram 2500 12V Cummins
1996 Dodge Ram 2500 12V Cummins
Previous Vehicles:
1995 E300D, 1980 300SD, 1992 Buick Century, 2005 Saturn Ion

I thought about the tooth skip option. I don't have the offset key installed. In the current measurement, the 4 degree offset key hasn't been installed, but it is on its way. I would be at either -5.5 or +30.5 degrees if I moved the chain down or up a tooth (18 degrees) starting at 12.5. The cam mark is off slightly from the indicator measurement, but it's close. This cam sprocket looks to be in good shape.

Attached Thumbnails

   

__________________
Theseus

Instragram: doss_project
1984 300SD - 930 Blue, OM 617
Murfreesboro, TN

Theseus owned a ship in Greek mythology, and when a part of the ship needed replacing, it was replaced. Eventually, everything was replaced. Is it the same ship? (I own the car of Theseus. )

Might want to verify top dead center for the crank pointer. Clear straw and liquid in the combustion chamber. I have not given much thought to this. Wife is agitating for us to leave.

^^^ Yep...Confirm the pointer is accurate.

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

Camshaft geometry is different than crankshaft geometry. The cam rotates at 1/2 the speed of the crank.

It takes TWO revolutions of the crank for a complete cycle. Suck-piston is down, squeeze-piston goes up, BANG-piston is forced down, blow-piston goes up to expel.

The cam rotates ONCE during the cycle.

Consequently, any degree changes at the cam will only have half the effect on the crank.

An 18 degree change at the cam (one tooth) equates to a 9 degree change at the crank.

I think you are off a tooth on the cam gear.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

Thanks for the suggestion on the two turns per camshaft turn. I'll look into it as I thought it was odd that the marks on the cam did not exactly line up with the compression TDC mark when I did another quick check.

__________________
Theseus

Instragram: doss_project
1984 300SD - 930 Blue, OM 617
Murfreesboro, TN

Theseus owned a ship in Greek mythology, and when a part of the ship needed replacing, it was replaced. Eventually, everything was replaced. Is it the same ship? (I own the car of Theseus. )

If one rotation of the cam equals two rotations at the crankshaft, would not 18º at the cam equal 36º at the crank?

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Well, I am going to figure it out today. The way I am visualizing it (before morning coffee kicks in), its like if I was on my small gear for a bicycle pedal crank and high gear at the wheel, i.e. more pedal cranks per wheel crank. So 18 degrees at cam should be 9 at crank. So not sure that is the fix but it is something to consider in my toolbox.

While this engine passed many other associated checks, it still has some rocking and some nailing. I noticed an obvious failure point immediately when I noticed the severe dent on the chain tensioner. The spring was so worn down that when I pulled it out, it broke in half. So there is possibility it may have skipped a tooth at some point of its operation. (see photos) Also, not sure if related, this engine has the valve cover with the "ridge" which apprently indicates factory rebuilt, or so I have heard from a source.

Now if I correct by a tooth, 12.5 - 9 would be 3.5 by the dial indicator method. That would be out of the 9 degree spec by the dial indicator method.

I would need to compare it to the cam mark line up check as well. I don't think the two matched.

I would also like to verify TDC of piston. I don't think the degree pointer is off as it take quite a bit to remove that. It doesn't look like its been touched at all. Is there a field service method for this? I am thinking to remove the injector, put a dial indicator in and record reading (intake) for 2 mm before TDC and 2mm after TDC and split the difference for the actual TDC.

Also, I'll replace the seals on my valve stems while I'm at it. Adding to my "while I'm at it" projects, I am nearly 50% done with the engine rebuild... At least i am learning a very important lesson in digging into my engine and parts, be prepared to replace anything made of rubber original to a 1984 car.

Attached Thumbnails

 

__________________
Theseus

Instragram: doss_project
1984 300SD - 930 Blue, OM 617
Murfreesboro, TN

Theseus owned a ship in Greek mythology, and when a part of the ship needed replacing, it was replaced. Eventually, everything was replaced. Is it the same ship? (I own the car of Theseus. )

Maybe another cup of coffee is in order.
Big sprocket, little number. Little sprocket, big number.

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.

Zeroing the TDC mark is time well invested as valve timing and injection pump timing are based upon it as their starting point for adjustments. If it is off by even two degrees all later adjusts will also be off.

Yes, measure it in both directions and split the degree difference. After the first adjustment reconfirm until it is in a repeatable accurate position.

Good luck!!!

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

This may help...crank at zero with piston precisely at TDC compression stroke, a zero offset key on the cam, the cam mark lined up to the one on the tower, you should be able to put the cam gear with the chain on it into place.

Since the timing has been out, I'd suggest also timing the injection pump.

__________________
"Rudeness is a weak man's imitation of strength" - Eric Hoffer

You are looking at it backwards. The crank gear is the DRIVE gear. The cam gear is the DRIVEN gear.

One full revolution of the crank gear equals one half revolution of the cam gear.

360 degrees on the crank and 180 degrees on the cam.

If each tooth corresponds to 18 degrees then a 180 degree advancement of the DRIVEN gear means the cam gear has only advanced 10 teeth.

The next revolution of the crank results in the cam gear completing its orbit.

Another way to look at it is the crank must do a 720 degree circuit per compression stroke.

__________________
“Whatever story you're telling, it will be more interesting if, at the end you add, "and then everything burst into flames.”
― Brian P. Cleary, You Oughta Know By Now

It makes absolutely no difference which gear is "driving."
The ratio is determined by the number of teeth on the sprockets, not by which sprocket is driving.
The cam-to-crank ratio is 1:2. Nothing backwards about that.



Notice how the big number goes with the little sprocket and the little number goes with the big sprocket?

With that in mind, how does 18º at the cam correlate to 9º at the crank?

__________________
When cryptography is outlawed, bayl bhgynjf jvyy unir cevinpl.