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  #291 (permalink)  
Old 06-17-2012, 02:40 PM
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Originally Posted by raVenX View Post
Desert Bum did a very thorough break down of his engine after the timing chain failure and although he was not sure what caused the timing chain to break, his best guess was the tensioner. I would give his guess a lot of weight because he appears to be very knowledgeable.

See: Timing Chain Failure Update

That said, and with all due respect to Desert Bum, I am not 100% convinced that the tensioner is the cause, and I come to that conclusion based on almost all of the incidents being reported here have seen the vehicle cruising down the highway with MDS presumably engaged and active. If the tensioner was the cause, then we would see just as many failures at low rates of speed or sitting at a stop light, because once the tensioner reaches the point that it could fail, it wouldn't much matter if the car was under stress or not.

That being the case, I'd be more inclined to believe that something is going on within the MDS system, like an abrupt lockup. Maybe the solenoid being sent an errant command, or the oil passages getting sludged up, or the cam phaser having a bad moment.

As more reports come in, it will become easier to narrow down possible causes. It's still anyone's guess right now.

I have taken a good hard look at the tensioner, guide, and chain, and between me and my machinist we are almost certain the guide shoe went causing the chain to break one link while the engine continued to run normally. The remaining link finially gave up at some point thus wiping the whole thing out.

I have attempted to re-create the guide shoe as it should have been in one piece. Take notice of the thinnest section, that is where it broke. The tensioner was in one piece, had it not been for the timing chain falling off it would be intact.

Look closely at the two sections of guide I am holding together. The wear mark closest to my thumb is wore unevenly, the upper half has severe grooving, while the lower half does not show that amount of wear. This would indicate the guide broke, and the upper half remained in contact with the broken link, causing the severe wear pictured. The very lowest piece of the guide, the part that fastens to the block, shows the exact same wear pattern as the broken piece.

Now look at the chain. The camera would not focus as close as I would have liked but you can easily make out the broken links. Notice how the one broken link is shiny?? that is the link that went first, it polished itself smoothly rubbing on what was left of the guide. the other link held on for dear life, but it eventually failed. Download the picture and zoom in on the links.

BTW.... my machinist is one of the finest engine builders you have never heard of.....
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  #292 (permalink)  
Old 06-17-2012, 02:59 PM
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Originally Posted by desert bum View Post
i have taken a good hard look at the tensioner, guide, and chain, and between me and my machinist we are almost certain the guide shoe went causing the chain to break one link while the engine continued to run normally. The remaining link finially gave up at some point thus wiping the whole thing out.

I have attempted to re-create the guide shoe as it should have been in one piece. Take notice of the thinnest section, that is where it broke. The tensioner was in one piece, had it not been for the timing chain falling off it would be intact.

Look closely at the two sections of guide i am holding together. The wear mark closest to my thumb is wore unevenly, the upper half has severe grooving, while the lower half does not show that amount of wear. This would indicate the guide broke, and the upper half remained in contact with the broken link, causing the severe wear pictured. The very lowest piece of the guide, the part that fastens to the block, shows the exact same wear pattern as the broken piece.

Now look at the chain. The camera would not focus as close as i would have liked but you can easily make out the broken links. Notice how the one broken link is shiny?? That is the link that went first, it polished itself smoothly rubbing on what was left of the guide. The other link held on for dear life, but it eventually failed. Download the picture and zoom in on the links.

Btw.... My machinist is one of the finest engine builders you have never heard of.....

Hey Chrysler, hire this guy!
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  #293 (permalink)  
Old 06-17-2012, 03:14 PM
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Originally Posted by Desert Bum View Post
I have taken a good hard look at the tensioner, guide, and chain, and between me and my machinist we are almost certain the guide shoe went causing the chain to break one link while the engine continued to run normally. The remaining link finially gave up at some point thus wiping the whole thing out.

I have attempted to re-create the guide shoe as it should have been in one piece. Take notice of the thinnest section, that is where it broke. The tensioner was in one piece, had it not been for the timing chain falling off it would be intact.

Look closely at the two sections of guide I am holding together. The wear mark closest to my thumb is wore unevenly, the upper half has severe grooving, while the lower half does not show that amount of wear. This would indicate the guide broke, and the upper half remained in contact with the broken link, causing the severe wear pictured. The very lowest piece of the guide, the part that fastens to the block, shows the exact same wear pattern as the broken piece.

Now look at the chain. The camera would not focus as close as I would have liked but you can easily make out the broken links. Notice how the one broken link is shiny?? that is the link that went first, it polished itself smoothly rubbing on what was left of the guide. the other link held on for dear life, but it eventually failed. Download the picture and zoom in on the links.

BTW.... my machinist is one of the finest engine builders you have never heard of.....
It's not the tensioner, it's not the chain, it's the guide shoe. Right? Is this a part that can be upgraded? If so, how easy/difficult would it be to do it?

Great information!
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  #294 (permalink)  
Old 06-17-2012, 03:18 PM
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Originally Posted by pdx.challenger View Post
It's not the tensioner, it's not the chain, it's the guide shoe. Right? Is this a part that can be upgraded? If so, how easy/difficult would it be to do it?

Great information!
You are correct, you are looking at the guide shoe. The tensioner is also a big piece of ****e, but it didn't break, it fell apart when the chain went away. The piston and spring that apply the tension landed in the oil pan.
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Old 06-17-2012, 04:53 PM
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Originally Posted by Desert Bum View Post
You are correct, you are looking at the guide shoe. The tensioner is also a big piece of ****e, but it didn't break, it fell apart when the chain went away. The piston and spring that apply the tension landed in the oil pan.
So can the guide shoe be upgraded? If so, how easy/difficult is it?

P.S. Sorry if it's a dumb question, but I'm not mechanically inclined.

Last edited by pdx.challenger; 06-17-2012 at 07:09 PM.
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Old 06-17-2012, 06:34 PM
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I wonder if it was really necessary for the Hemi valvetrain to be an "interference" design?...and if so, was it a cost-cutting measure to not cut reliefs into the pistons, or is it that they are hyper-eutectic pistons which makes machining impractical?

You'd think Dodge would have put more safeguards in, to better contain a situation from going catastrophic? Remember a time when domestic design engines were a staple in reliability as far as the cam drive chain being the last thing to ever break, while import engines clearly had an Achilles heel with rubber timing belts that snapped with certainty and fatality given their typical interference valve clearance design? Now, many import engines come with everlasting chains, and we are stuck with a chain drive that is unusually prone to self-destruct and take the whole engine with it? That's not so cool!

You can argue that the failures are very low by percentage, but the fact remains that chain failure should be utterly unheard of, altogether, for the very reason that a chain is used for the design in the first place.
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Old 06-17-2012, 08:25 PM
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From what I have read via chrysler training materials, VVT was invented for the sole purpose of deleting the EGR valve. The fat, flat torque curve was an added bonus....

VVT is not the problem in my opinion, its the MDS. If you have ever built an engine with a roller cam think of it this way.....

When you adjust the valves on the first cylinder, there is a lot of spring pressure resistance when the cam comes up on the ramp, normalizes when the lifter peaks, and on the way down, the spring pressure pushes the cam lobe to the point were it can almost, and sometimes does roll the motor over by mere spring pressure.

When all the valves are adjusted, the motor turns over with an equal amount of resistance, placing an even amount of pressure on the cam.

Now MDS comes into play....
Take away eight lifters (four cylinders) and what do you have??? A ton of harmonics due to uneven pressures being placed on the camshaft, which would most certainly tranfer to the timing set. To add, the camshafts in the 5.7 are..... wait for it.......

Hollow.

In my opinion, that is why the timing chain failure.
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Old 06-17-2012, 08:27 PM
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The camshaft harmonic vibration may be causing a condition called cordial whip.

This condition is easily to witness........

Open the hood of any single belt driven A/C compressor vehicle, not a serpentine setup. Look at the belt with the A/C off...... nothing to really see.
Now turn the the A/C on and again look at the belt. On the slack side the fanbelt is violently pitching, yet there is a constant load on the belt.

What you have witnessed is the hamonics from the piston assy in the compressor being transfered into the fanbelt. All the start stop and uneven pressures cause the cordial whip condition, same as MDS??

My two cents anyway.......
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Old 06-17-2012, 08:33 PM
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So we know the tensioner is spring loaded, that means it can apply tension to the chain, but the vibration of the chain can push back on the tensioner causing it to vibrate as well.
Now if the tensioner was solid, it would severly limit the amount of chain vibration. Fluid will not compress, so if you used oil pressure, combined with spring pressure it would allow the tensioner to adjust for wear and start ups while oil pressure would "lock up" the tensioner and would not be influenced nearly as much from the vibration. The problem being the vibration is still there.............

maximum vibration occurs at a certain RPM, that condition is called resonance frequency. An example we have all witnessed at some point is tire balance. Going down the road at 50mph is nice and smooth...... pick up the pace to 60mph and here comes the vibration( resonance frequency) speed up to 75mph and the vibration goes away(out of resonance frequency) crank it up to 120mph, and the vibration is back, only waaay worse then it was at 60mph. There is an explination on why the vibration effectively doubles at higher speeds but I think everyone will get he point I am making.......

Serious race engines use a belt drive system like a Jesel or Comp. One of the main reasons for doing so is to help absorb valve train vibrations which we have discussed in an earlier thread.
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Old 06-18-2012, 08:29 AM
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I think the "absorb valvetrain vibrations" part is addressed at least in part by the modern crankshaft damper on the crank pulley.

Your thoughts on resonance in the adjuster and chain are compelling, nonetheless.

Last edited by randycat99; 06-18-2012 at 08:32 AM.
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