GTS Cam broke
02-08-2012, 12:51 PM
#76
Scoped the passenger side this morning, all is good, no contact between valves and pistons. Told ya 
In order, cyl 1,2,3,4
In order, cyl 1,2,3,4
02-08-2012, 12:54 PM
#78
02-08-2012, 12:56 PM
#79
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Agreed. Good news is always a good thing when everything seems to be going bad. I am glad I got to feel how the car ran before.... Now I can't wait to see how it runs after all this is done!
02-10-2012, 01:07 AM
#80
Drifting
Yesterday we checked the cam gear retaining bolts ( one per alloy gear) on a new customers 928S4 that we have never seen before ( 1989 year type Hong Kong import ) and along with the mandatory torque tube rear coupling check /inhex bolt replacement , thrust bearing measurement etc , well both the cam gear bolts were very low on torque value , one would almost say they were just a very short time away from the alloy cam gear gear slip = breaking a cam + all the other damage that follows( great money saving find if I say so myself).
It looked as if they had never been tightened , but that's understandable because I have never seen or read in any Porsche official literature a widespread warning that these bolts along with the Torque tube rear coupling bolts ( manual & auto ) are to be checked & tightened on a" Very" regular basis
It looked as if they had never been tightened , but that's understandable because I have never seen or read in any Porsche official literature a widespread warning that these bolts along with the Torque tube rear coupling bolts ( manual & auto ) are to be checked & tightened on a" Very" regular basis
02-10-2012, 02:55 AM
#81
Rennlist Member
As I read this thread through once more I find it kind of disturbing that we read things like "regular check of the cam bolt is required" or "regularly check the cam tower bolts". Nothing wrong with the comments but crikey, if Porsche cannot engineer these things to stay together what the hell are they doing making anything mechanical? Clearly something not right here.
The root cause of Richard's problem would seem to be the cam bolt was not tight enough but that begs the obvious question- "why"? Perhaps Richard can enlighten us as to when the bolt was last removed and why. I can accept a factory error might occur but once bedded in it should never come undone again unless distrubed-surely? If it can come undone on it's own with time then we really are driving around in one huge POS!
I adjusted my cam timing a few weeks ago after purchasing Ken's bumpstick [excellent kit] but I never thought to check the calibration of my [trusty old] torque wrench- all be it my "arm torque wrench" felt fair measure had been dished out as I tightened the 5/8 bank cam bolt.
TBF, 2/6 failure, breathing issues, oil retention in the 1/4 bank, and now loose cam wheels to worry about? Upgrade these things? -the word in my mind at the moment sad to say is off load!
Did you see the polished condition of the cam lobes on the GB modded engine- has anyone seen anything like that on their cams? If one [clever] man can do that how come Porsche could not?
Sad to say perhaps Porsche really did know what they were doing when they decided to abandon the 928! Some one please tell me I am not getting paranoid!
Do we need to start loctiting the cam bolts? Is there a way to do this such that we can undo them when needed?
Interesting how rice buners do not seem to have these problems.
Regards
Fred
The root cause of Richard's problem would seem to be the cam bolt was not tight enough but that begs the obvious question- "why"? Perhaps Richard can enlighten us as to when the bolt was last removed and why. I can accept a factory error might occur but once bedded in it should never come undone again unless distrubed-surely? If it can come undone on it's own with time then we really are driving around in one huge POS!
I adjusted my cam timing a few weeks ago after purchasing Ken's bumpstick [excellent kit] but I never thought to check the calibration of my [trusty old] torque wrench- all be it my "arm torque wrench" felt fair measure had been dished out as I tightened the 5/8 bank cam bolt.
TBF, 2/6 failure, breathing issues, oil retention in the 1/4 bank, and now loose cam wheels to worry about? Upgrade these things? -the word in my mind at the moment sad to say is off load!
Did you see the polished condition of the cam lobes on the GB modded engine- has anyone seen anything like that on their cams? If one [clever] man can do that how come Porsche could not?
Sad to say perhaps Porsche really did know what they were doing when they decided to abandon the 928! Some one please tell me I am not getting paranoid!
Do we need to start loctiting the cam bolts? Is there a way to do this such that we can undo them when needed?
Interesting how rice buners do not seem to have these problems.
Regards
Fred
02-10-2012, 03:59 AM
#83
Drifting
Glad you asked the question , never ever use any form of loctite at all on any of these bolts , meaning either the cam gear retaining bolts or the Torque Tube coupling bolts , the reasons are as follows }
A) In relation the the alloy cam gear retaining bolts , the bolts do not undo at all , its the crush ( compression if you like) of the alloy , and over time the shock waves that are continuous through this item finally gets too much and the gear is not retained as strongly against its boss as one would like( this takes years and years to do, so its not overnight)
This is why we do not see this at all on the same set up in the 944 single or twin cam engines , because these very similar gears ( one per engine in the 944) are made of steel and the crush is just not there and consequently these gears never slip at all once torqued correctly
Is it an issue on a 928 ?
Answer = not to me , because I have been attending to these ( re torqueing 928 alloy cam gear bolts) since 1986 , and in these last 26 years I / we have never had any breakages at all , because we just routinely just re tighten these easy to get at bolts during a service .
Now, why not apply Loctite to these bolts ?
Answer = because if you do and then say 30,000Kms later when you recheck the torque value , the Loctite will grip the threads ( as it meant to) and you run the very real risk of not actually reapplying the holding force needed to retain the alloy cam gear in place securely , meaning we want the thick washer behind the bolt head to apply max pressure to the gear centre( via the washer) , if the Loctite held the thread , then the bolt head is not actually applying the force you think you applied with the torque wrench
So just lube the thread lightly & torque , then recheck half way between cam belt replacement ( easy to do )
You could probably just do it every cam belt change , but I do it as well half way through , as I mentioned above , just to be sure .
Now to Torque Tubes }
The T/Tube single coupling inhex bolts , be it manual or auto , its the rear coupling inhex bolt that looses its torque value long before the front one does ( Auto & manual trans)
In the case of these inhex bolts ( one per coupling ) these like the cam gear bolts do not loose their torque value by undoing , these inhex bolts stretch , and in this way they loose their torque value .
Same thing applies , do not apply Loctite , because this will only lock the thread in place , so when one goes back to recheck ( say every major service ) like we have done for many many years then if Loctite is retaining the threads then the torque value will be way too little in the clamping action of this coupling
Highly recommend new inhex bolts every third or fourth re torqueing or when ever the trans is removed etc
Like the cam gears , we see a lot of the T /T rear couplings almost loose ( red rust dust in that area ) on the ones we have not seen before , naturally all that is needed is to tighten from time to time .
This is not an issue at all if maintained , its just common sense after inspecting the basic design , and just tighten certain things at service intervals ( this is not rocket science ) its just maintenance .
A) In relation the the alloy cam gear retaining bolts , the bolts do not undo at all , its the crush ( compression if you like) of the alloy , and over time the shock waves that are continuous through this item finally gets too much and the gear is not retained as strongly against its boss as one would like( this takes years and years to do, so its not overnight)
This is why we do not see this at all on the same set up in the 944 single or twin cam engines , because these very similar gears ( one per engine in the 944) are made of steel and the crush is just not there and consequently these gears never slip at all once torqued correctly
Is it an issue on a 928 ?
Answer = not to me , because I have been attending to these ( re torqueing 928 alloy cam gear bolts) since 1986 , and in these last 26 years I / we have never had any breakages at all , because we just routinely just re tighten these easy to get at bolts during a service .
Now, why not apply Loctite to these bolts ?
Answer = because if you do and then say 30,000Kms later when you recheck the torque value , the Loctite will grip the threads ( as it meant to) and you run the very real risk of not actually reapplying the holding force needed to retain the alloy cam gear in place securely , meaning we want the thick washer behind the bolt head to apply max pressure to the gear centre( via the washer) , if the Loctite held the thread , then the bolt head is not actually applying the force you think you applied with the torque wrench
So just lube the thread lightly & torque , then recheck half way between cam belt replacement ( easy to do )
You could probably just do it every cam belt change , but I do it as well half way through , as I mentioned above , just to be sure .
Now to Torque Tubes }
The T/Tube single coupling inhex bolts , be it manual or auto , its the rear coupling inhex bolt that looses its torque value long before the front one does ( Auto & manual trans)
In the case of these inhex bolts ( one per coupling ) these like the cam gear bolts do not loose their torque value by undoing , these inhex bolts stretch , and in this way they loose their torque value .
Same thing applies , do not apply Loctite , because this will only lock the thread in place , so when one goes back to recheck ( say every major service ) like we have done for many many years then if Loctite is retaining the threads then the torque value will be way too little in the clamping action of this coupling
Highly recommend new inhex bolts every third or fourth re torqueing or when ever the trans is removed etc
Like the cam gears , we see a lot of the T /T rear couplings almost loose ( red rust dust in that area ) on the ones we have not seen before , naturally all that is needed is to tighten from time to time .
This is not an issue at all if maintained , its just common sense after inspecting the basic design , and just tighten certain things at service intervals ( this is not rocket science ) its just maintenance .
02-10-2012, 05:21 AM
#84
Rennlist Member
^^^^^^^ Good advice that I heeded and noted years ago.
Here is what happens when the rear coupling inhex bolt stretches and is not applying the correct clamping force anymore, as Bruce points out. The shaft slides forward and the bolt shank rides on the drive shaft splines (and you have to wonder about the chance of TBF, even on a manual, if the shaft is left too long pushing on the end of the crank pilot bearing).
This pic is from the rear of my 82S TT ... if only the previous owner had taken the simple step of periodically tightening/renewing the bolt ... cheap easy insurance. The design is obviously a bit lacking when you look at that coupling, and asking a lot of one bolt ... but you can compensate for its deficiency by tightening/replacing (and perhaps adding a secondary clamp).
Luckily I caught my GTS in time, before damage. It had a loose rear bolt, with shaft migration, and it had been serviced regularly from new by the selling Porsche dealership. It had obviously never been tightened or replaced.
Tightening cam gear and torque tube bolts should go on to everyones routine maintenance list, considering the possible consequences.
Here is what happens when the rear coupling inhex bolt stretches and is not applying the correct clamping force anymore, as Bruce points out. The shaft slides forward and the bolt shank rides on the drive shaft splines (and you have to wonder about the chance of TBF, even on a manual, if the shaft is left too long pushing on the end of the crank pilot bearing).
This pic is from the rear of my 82S TT ... if only the previous owner had taken the simple step of periodically tightening/renewing the bolt ... cheap easy insurance. The design is obviously a bit lacking when you look at that coupling, and asking a lot of one bolt ... but you can compensate for its deficiency by tightening/replacing (and perhaps adding a secondary clamp).
Luckily I caught my GTS in time, before damage. It had a loose rear bolt, with shaft migration, and it had been serviced regularly from new by the selling Porsche dealership. It had obviously never been tightened or replaced.
Tightening cam gear and torque tube bolts should go on to everyones routine maintenance list, considering the possible consequences.
Last edited by Dave928S; 02-10-2012 at 05:41 AM.
02-10-2012, 08:13 AM
#85
Drifting
Wow good Photo Dave , I have kept a badly damaged rear coupling inhex bolt ( at work to show customers ) that would fit very nicely into the damage you have shown , meaning this sort of damage is wide spread( world wide ) and all brought about by lack of basic maintenance( torqueing of bolts regularly)
In simple terms, what happens is that the 928 series }
Manual Trans , all models and years have a hollow female input shaft ( and split in three places for grip purposes) naturally this emanates from within the trans ( 5th gear input shaft) , which slides over the solid Torque Tube quill shaft ( drive shaft within the T/T)
This hollow shaft ( input to trans) has one only coupling and one inhex bolt , now this bolt stretches with hundreds / thousands of acceleration events ( standing starts are the worst) , now if this stretch is kept in check ( meaning periodic tightening) then all is well .
But I do not see it mentioned anywhere in the literature ??????
Answer = well now you do
Now What if the bolt stretches ( which in turn reduces the clamping force ) ?
Answer = the male & female splines are now free to move against each other = they wear away( red dust) , this in turn makes things even worse ( see photo above ) you will see the badly worn male splines .
Now what happens in this condition ?
Answer = the quill shaft ( T/T drive shaft ) rides forward , by the way this is what happens in Auto & manual Transmissions and this forces the rear coupling inhex bolt to ride up & over the male splines ( wearing away as it goes) into the male splines ( basicily machining them away ( see photo)
Auto Trans 4 speed Auto ( in Aust it was from 1984 onwards) }
The set up is the same , female split & hollow shaft emanating from the rear ( trans) bellhousing ,and the same single rear coupling with its single inhex bolt , now the same result if left unattended for too many years, meaning worn splines & often broken input shaft ( metal fatigue) from too much unneeded movement ( male to female spline free play) over a too longer time span.
Now while this is happening the quill shaft emigrates forward ( yes forward ) don't really know why & more importantly I don't care why , it just does , and that's all I care about and how to stop it to stop it , and yes it causes front flex plate deflection ,
I discovered this basic issue well over 15 years ago ( so its not new )
Question = Does the front T / T coupling bolt suffer from as much streatch , meaning does it loose as much holding/ clamping force at the same rate ?
Answer = No
Why = Have no idea and don't care why , it just does
Question = If the rear coupling is maintained ( read kept tight ) will the front flex plate symptom return with a vengance ?
Answer = No
Question } What if I have a 928S4 auto ( example) & how & what do I do ?
Answer = First of all , remove engine bell housing lower section , look at the flex plate , is it pushed forward ( Yes Or No) , if yes or not sure }
Remove the front coupling bolt ( noticing how tight it was to release its clamping force against coupling)
Then connect a dial gauge ( we made a special tool to do this) and with the front coupling disconnected , move the flywheel forward and back and read out the movement and note .
From Memory I think on an S4 is up to .2mm is within new spec & up to .4mm is within absolute spec , (read Max )
Remove rear coupling inhex bolt & compare how much easier it may be to undo .
With the front flex plate relaxed , insert a new 931....... inhex bolt but do not tighten just yet( leave loose )
Inspect rear coupling half moon cut out on the quill shaft that should just be visible through the rear coupling bolt hole , if not exactly aligned , then move the quill shaft back so it aligns in the hole exactly, this is done with the help of the front coupling and its loose bolt , just lever back .
Once the rear coupling hole aligns perfectly insert new 931.... bolt .
Now and only now the TT quill shaft is aligned + the flex plate is at rest
Torque both new inhex bolts
Retorque at regular intervals ( mainly the rear ) which in the auto is out of sight & out of mind
Note } do not apply Loctite to inhex bolts , apply only light lubrication to threads
928 & S 3 speed Auto
This was the only one to have a sliding sleeve ( which actually went onto the 944 951 range manual trans models)
This 3 speed Auto version in the 928 (78 - 83) had a solid shaft emanating out of the auto trans ( not hollow) , so in effect we have two solid shafts coming together with a sliding sleeve with two couplings & two inhex bolts ( this system is very strong) , as I mentioned this exact sliding sleeve and couplings & inhex bolts went straight onto the 944 series ( 82 - 91 ) and 951
Interestingly the 968 manual ( last of the 944 series ) 1992 - 1995 year types , evolved to have two half moon cutouts on the quill shaft & two half moon cutouts on the solid output shaft from the trans , meaning the sliding sleeve has four ( Yes 4) 931....... inhex bolts , this is just stunning , I strongly suspect that if Porsche evolved the 928 from the GTS it seems that is four bolt coupling would of probably have appeared on the next generation 928 , because it appeared on the 968
In simple terms, what happens is that the 928 series }
Manual Trans , all models and years have a hollow female input shaft ( and split in three places for grip purposes) naturally this emanates from within the trans ( 5th gear input shaft) , which slides over the solid Torque Tube quill shaft ( drive shaft within the T/T)
This hollow shaft ( input to trans) has one only coupling and one inhex bolt , now this bolt stretches with hundreds / thousands of acceleration events ( standing starts are the worst) , now if this stretch is kept in check ( meaning periodic tightening) then all is well .
But I do not see it mentioned anywhere in the literature ??????
Answer = well now you do
Now What if the bolt stretches ( which in turn reduces the clamping force ) ?
Answer = the male & female splines are now free to move against each other = they wear away( red dust) , this in turn makes things even worse ( see photo above ) you will see the badly worn male splines .
Now what happens in this condition ?
Answer = the quill shaft ( T/T drive shaft ) rides forward , by the way this is what happens in Auto & manual Transmissions and this forces the rear coupling inhex bolt to ride up & over the male splines ( wearing away as it goes) into the male splines ( basicily machining them away ( see photo)
Auto Trans 4 speed Auto ( in Aust it was from 1984 onwards) }
The set up is the same , female split & hollow shaft emanating from the rear ( trans) bellhousing ,and the same single rear coupling with its single inhex bolt , now the same result if left unattended for too many years, meaning worn splines & often broken input shaft ( metal fatigue) from too much unneeded movement ( male to female spline free play) over a too longer time span.
Now while this is happening the quill shaft emigrates forward ( yes forward ) don't really know why & more importantly I don't care why , it just does , and that's all I care about and how to stop it to stop it , and yes it causes front flex plate deflection ,
I discovered this basic issue well over 15 years ago ( so its not new )
Question = Does the front T / T coupling bolt suffer from as much streatch , meaning does it loose as much holding/ clamping force at the same rate ?
Answer = No
Why = Have no idea and don't care why , it just does
Question = If the rear coupling is maintained ( read kept tight ) will the front flex plate symptom return with a vengance ?
Answer = No
Question } What if I have a 928S4 auto ( example) & how & what do I do ?
Answer = First of all , remove engine bell housing lower section , look at the flex plate , is it pushed forward ( Yes Or No) , if yes or not sure }
Remove the front coupling bolt ( noticing how tight it was to release its clamping force against coupling)
Then connect a dial gauge ( we made a special tool to do this) and with the front coupling disconnected , move the flywheel forward and back and read out the movement and note .
From Memory I think on an S4 is up to .2mm is within new spec & up to .4mm is within absolute spec , (read Max )
Remove rear coupling inhex bolt & compare how much easier it may be to undo .
With the front flex plate relaxed , insert a new 931....... inhex bolt but do not tighten just yet( leave loose )
Inspect rear coupling half moon cut out on the quill shaft that should just be visible through the rear coupling bolt hole , if not exactly aligned , then move the quill shaft back so it aligns in the hole exactly, this is done with the help of the front coupling and its loose bolt , just lever back .
Once the rear coupling hole aligns perfectly insert new 931.... bolt .
Now and only now the TT quill shaft is aligned + the flex plate is at rest
Torque both new inhex bolts
Retorque at regular intervals ( mainly the rear ) which in the auto is out of sight & out of mind
Note } do not apply Loctite to inhex bolts , apply only light lubrication to threads
928 & S 3 speed Auto
This was the only one to have a sliding sleeve ( which actually went onto the 944 951 range manual trans models)
This 3 speed Auto version in the 928 (78 - 83) had a solid shaft emanating out of the auto trans ( not hollow) , so in effect we have two solid shafts coming together with a sliding sleeve with two couplings & two inhex bolts ( this system is very strong) , as I mentioned this exact sliding sleeve and couplings & inhex bolts went straight onto the 944 series ( 82 - 91 ) and 951
Interestingly the 968 manual ( last of the 944 series ) 1992 - 1995 year types , evolved to have two half moon cutouts on the quill shaft & two half moon cutouts on the solid output shaft from the trans , meaning the sliding sleeve has four ( Yes 4) 931....... inhex bolts , this is just stunning , I strongly suspect that if Porsche evolved the 928 from the GTS it seems that is four bolt coupling would of probably have appeared on the next generation 928 , because it appeared on the 968
02-10-2012, 08:42 AM
#86
Rennlist Member
Your TT information
I appreciate the logic and information and made my notes to check on mine if I would not have followed this thread I don't think I would have looked for the TT info here, perhaps reposting under a thread title about TT maintenance may benefit future searches on this subject.
Best H
Best H
02-10-2012, 04:07 PM
#87
Under the Lift
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Although I have not encountered a loose cam gear bolt, I am surprised by how little clamping force the cam gear bolt at the 47 ft-lb torque spec exerts on the gear. It's just barely over the force required to stop the gear from slipping on the cam nose. This has always bothered me. I am reluctant to overtighten this bolt, but I always make sure it is at the torque spec. And whenever I do a torque check, I check the gear timing before and after with Ken's tool to make sure it hasn't moved.
02-24-2012, 06:34 PM
#89
Just started the tear down today. It's been a really busy month.
02-24-2012, 06:36 PM
#90
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