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Chain Failure Engine. What Next?

 
Old 11-25-2018, 11:05 PM
  #16  
Crumpler
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Originally Posted by Mrmerlin View Post
I would not use either of the cams again as the cam without any broken teeth could have gotten stress cracks from the upset ,
maybe the cam could be magnifluxed ,
still better to get another set
Okay, that sounds reasonable to me, thanks Stan.

Originally Posted by Landseer View Post
Likely original
Yeah, I just didn't think they would look that good. I guess I'm in less hurry to change the head gaskets in the 86 now, seeing these.


So I got the valves off today.
Pictured are the exhaust side which had the cam failure, intake looked the same but not pictured.
If they are bent I can't tell. Is there a best practice to evaluate if good or not?



The lifter's were interesting. Eight good and spongy to touch, and eight frozen. I can't tell for sure b/c I didn't mark all of them coming off but it would be wild if the eight bad ones were from the exhaust side in which the cam got chewed up?




It doesnĺt matter b/c the head is getting replaced anyway but this scoring was present and seems to be the only damage, in addition to the exhaust cam and tensioner bracket.




As an aside, I want to thank Seth. His thread on head reconditioning was a big help.


Last edited by Crumpler; 11-25-2018 at 11:26 PM.
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Old 11-25-2018, 11:22 PM
  #17  
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put the valve in a drill chuck and spin it near a straight edge if the valve is bent you will see it
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Old 11-25-2018, 11:29 PM
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Or put them back in the guides and fill the chambers with kerosene and see if it leaks out. If so, the valves or seats are damaged and go from there.
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Old 11-28-2018, 06:42 PM
  #19  
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Great ideas thank you, I will probably do both.

So bent valves get replaced without question but from my research of old threads:

our valve springs usually don't fail and don't need to be replaced...right?

lifters can be rejuvenated, but I got the impression the consensus was to replace all if you are in there?

valve guides always replaced while you are in there...?
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Old 11-28-2018, 06:48 PM
  #20  
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If you know a machine shop have them put the valves in their grinder- tell him to just barely grind them,. If they are bent it will show,
You are not looking to have them ground ready for use- just checking for bent
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Old 11-28-2018, 10:12 PM
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As an aside, I want to thank Seth. His thread on head reconditioning was a big help.
You are quite welcome, Dave!
I am glad to be of help.
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Old 11-29-2018, 01:17 PM
  #22  
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Chain/tensioner failure is apparently somewhat common on the 944 engines, although this is the first such failure I've heard of on a 928 engine.

Is this failure as rare on the 928 engine as I suspect...and if so, why?
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Old 11-29-2018, 07:05 PM
  #23  
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Originally Posted by skpyle View Post
You are quite welcome, Dave!
I am glad to be of help.

And help you did.
The only thing about that thread I would change is that one certain party's behavior. That was unfounded and irritated me just knowing you had to put up with it.

Originally Posted by GregBBRD View Post
Chain/tensioner failure is apparently somewhat common on the 944 engines, although this is the first such failure I've heard of on a 928 engine.

Is this failure as rare on the 928 engine as I suspect...and if so, why?
So this is why I thought the autopsy would be interesting to the forum. I couldn't find another documented example.
Penny for your thoughts GB.
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Old 11-29-2018, 10:14 PM
  #24  
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You can do a quick valve grind yourself for $30 for the suction cup tool and lapping compound. Then mark the seats and valves with a magic marker and repeat. Some people hate this process, but I really enjoy it and find useful. This will allow you to see what is going on with the valves clearly. Also the liquid leak test, but it can be done with WD-40 or anything. I would not necessarily replace the lifters, unless I was sure they needed it, but depends on cost. The reason being that if there is a problem later you can change them without pulling the head. You can easily check the guides just by rattle/feel, but I think 1) they are inexpensive to replace relative the cost the decking the heads (which you MUST do), and 2) they are soft and will normally need replacement in even a medium mileage 928 motor. The first motor I rebuilt had 60K miles on it and the guides were loose.
Regarding the chains, you are totally right. I have not done them on my 944, and everyone who knows the cars yells at me. But it never even comes up in 928 land.
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Old 11-29-2018, 10:25 PM
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Originally Posted by Crumpler View Post
:



So this is why I thought the autopsy would be interesting to the forum. I couldn't find another documented example.
Penny for your thoughts GB.
Anything mechanical can fail at any time, that's a given. And the early '85 tensioners were really bad (although I did not look at the serial number to see if your engine had the early '85 version tensioners.....I will.)

If a tensioner collapses completely, it makes sense that the "pads" are going to get really hammered. Once that chain hits metal, I'd think it would be over quickly.

I think we would all benefit from a bit more very close inspection of the pieces....did the plastic chain piece break first and then the chain grabbed?

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Old 11-29-2018, 11:38 PM
  #26  
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Originally Posted by Jim Devine View Post
If you know a machine shop have them put the valves in their grinder- tell him to just barely grind them,. If they are bent it will show,
You are not looking to have them ground ready for use- just checking for bent
Thank you JIm!

Originally Posted by karl ruiter View Post
You can do a quick valve grind yourself for $30 for the suction cup tool and lapping compound. Then mark the seats and valves with a magic marker and repeat. Some people hate this process, but I really enjoy it and find useful. This will allow you to see what is going on with the valves clearly. Also the liquid leak test, but it can be done with WD-40 or anything. I would not necessarily replace the lifters, unless I was sure they needed it, but depends on cost. The reason being that if there is a problem later you can change them without pulling the head. You can easily check the guides just by rattle/feel, but I think 1) they are inexpensive to replace relative the cost the decking the heads (which you MUST do), and 2) they are soft and will normally need replacement in even a medium mileage 928 motor. The first motor I rebuilt had 60K miles on it and the guides were loose.
Regarding the chains, you are totally right. I have not done them on my 944, and everyone who knows the cars yells at me. But it never even comes up in 928 land.
Thank you Karl. I was watching you tube videos on this and thought it would be cathartic

Originally Posted by GregBBRD View Post
Anything mechanical can fail at any time, that's a given. And the early '85 tensioners were really bad (although I did not look at the serial number to see if your engine had the early '85 version tensioners.....I will.)

If a tensioner collapses completely, it makes sense that the "pads" are going to get really hammered. Once that chain hits metal, I'd think it would be over quickly.

I think we would all benefit from a bit more very close inspection of the pieces....did the plastic chain piece break first and then the chain grabbed?
Ok so I pulled the other tensioner tonight...
I was under the impression that they should match config side to side just upside down on one head right?
These don't seem to match, are they different MY tensioners?


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Old 11-30-2018, 02:53 AM
  #27  
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Originally Posted by Crumpler View Post
Thank you JIm!



Thank you Karl. I was watching you tube videos on this and thought it would be cathartic



Ok so I pulled the other tensioner tonight...
I was under the impression that they should match config side to side just upside down on one head right?
These don't seem to match, are they different MY tensioners?

Same design/vintage, just different sides of the engine.
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Old 11-30-2018, 09:54 AM
  #28  
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Originally Posted by GregBBRD View Post
Same design/vintage, just different sides of the engine.
Got it.
So if everything is original then I'm back at the "one in a million" failure scenario.
Are there any other angles, or pics, I can provide Greg?
In terms of the pieces, that's all debris that was in there during my necropsy. I can go back to Landseer's thread and see if he took any pics of the bottom pad or chain.
Anything else to look at?
Thanks for looking.

Dave.

Edit: Actually, there are good pictures on the first page of the thread, pad looks fine, chain broke one place?

https://rennlist.com/forums/928-foru...erailment.html


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Old 11-30-2018, 10:41 AM
  #29  
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I have an 86 S3 head that had the same failure some 10 years back from Big Dave's Black car, pads were not the issue then but can cause the same failure as Greg stated, it's that flat washer inside the tube w/ the spring, it gets jammed, also binds causing oil pressure issues (clacking lifters).
There was a Porsche TSB that replaced the flat washer with a cupped one IIRC.
If the Cam covers are off, just replace them w/ S4's and put on new shoes, everyone likes new shoes

Dave K
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Old 11-30-2018, 12:02 PM
  #30  
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Originally Posted by davek9 View Post
I have an 86 S3 head that had the same failure some 10 years back from Big Dave's Black car, pads were not the issue then but can cause the same failure as Greg stated, it's that flat washer inside the tube w/ the spring, it gets jammed, also binds causing oil pressure issues (clacking lifters).
There was a Porsche TSB that replaced the flat washer with a cupped one IIRC.
If the Cam covers are off, just replace them w/ S4's and put on new shoes, everyone likes new shoes

Dave K
So Dave, it sure looks like there was damage to that flat disk.
Close up shows it is chewed into a triangle shape and the channel it sits over on the tensioner is distorted as well.
Crazy. Thanks for the tip.




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