Rear Pinch Bolts, check 'em.
06-18-2015, 08:32 PM
#31
Rennlist Member
The spline damage/TT pic I posted is from my '82, when I pulled it out years ago, and which had stuffed bolts with red dust on them. The bolt with the teeth marks is from my GTS, which has only 120,00kms on it, and has a service book stamped for every single service by a 'leading' Sydney Porsche dealer, where it was purchased ... and was obviously never checked. I replaced the shaft, even though the splines weren't all that bad, and renewed the bolts. Luckily the 5 speed input shaft looks OK.
Bruce Buchanan (Jet 951) alerted me to this problem years ago, and he is who I thank for saving me from catastrophic damage.
The bolts are overstressed and need to be routinely torqued/replaced. Think about it .... one clamp bolt on a shaft taking thousands of clockwise/counterclockwise torque twists. It's not at all surprising that the bolt goes to lunch.
06-18-2015, 08:51 PM
#32
"Aircraft" quality maintenance on these cars really pays off.
06-18-2015, 09:41 PM
#33
Former Vendor
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I don't believe the pinch bolts stretch out and loosen much at all.
I strongly believe that many mechanics/owners do not use a torque wrench to correctly set the pinch bolts at their proper torque value. This then allows some slop between the mated spline surfaces which only worsens with the miles causing damage of the splines, which causes more looseness until the failure of the joint.
One way to see if these bolts stretch is to measure them against a brand new one. The measurements taken should also include the distance between the threads of the pinch bolts, besides overall lengths.
Cheers,
I strongly believe that many mechanics/owners do not use a torque wrench to correctly set the pinch bolts at their proper torque value. This then allows some slop between the mated spline surfaces which only worsens with the miles causing damage of the splines, which causes more looseness until the failure of the joint.
One way to see if these bolts stretch is to measure them against a brand new one. The measurements taken should also include the distance between the threads of the pinch bolts, besides overall lengths.
Cheers,
06-18-2015, 10:41 PM
#34
Rennlist Member
Helping Out
That is an awesome Idea.
If I were in the area I would gladly participate in one or more "Pinch Bolt Parties". Especially those cars that have had the fronts checked and not the rears.
Maybe they could be filmed and made into a "Pinch Bolt Prowess" sticky
But I'm not, so I can't.
However, if someone is willing to set up a collection site, I'll be one of the first to donate toward the cost of this "expensive lesson" and help out the poor bloke looking at a 2 to 5 K$ repair. Unless, of coarse, they are so loaded they don't care.
If I were in the area I would gladly participate in one or more "Pinch Bolt Parties". Especially those cars that have had the fronts checked and not the rears.
Maybe they could be filmed and made into a "Pinch Bolt Prowess" sticky
But I'm not, so I can't.
However, if someone is willing to set up a collection site, I'll be one of the first to donate toward the cost of this "expensive lesson" and help out the poor bloke looking at a 2 to 5 K$ repair. Unless, of coarse, they are so loaded they don't care.
Last edited by depami; 06-19-2015 at 12:18 AM. Reason: Add title
06-18-2015, 11:03 PM
#35
Rennlist Member
Thread Starter
Looking back at my books last year, I worked on 64 928's, 12 BMW's, 9 Audi's, 8 Mercedes, 2 Jags, 4 Chevy's and two Kia's. There might have been a Ford in there but they don't count. Am I supposed to catch every lose bolt that some other idiot wrench didn't tighten up? I am helping the guy out, I'm not charging $135 per hour to do this job like everyone else in the area and I know what I'm doing. Although with the amount of guys like you who would bitch and moan I should be so I have less things to worry about.
I have put that rear pinch bolt on my "check this now so I don't have to charge you more later" list. Hope that makes you feel better.
06-18-2015, 11:12 PM
#36
Drifting
I know its the single inhex bolt stretching because back in the late 1980's and all through the 1990's ( smack bang in middle of the time frame when I was at a large Porsche dealer in Sydney working on Porsche's ) we were seeing the beginning of these " Out Of Sight / Out of Mind" hidden rear coupling single inhex bolt being a bit too loose when it was time for us to remove the trans( man or Auto ) on a given 928S/928S4 etc for the first time(warranty oil leak reasons etc ) on higher than usual km cars ( higher mileage examples) as opposed to the more average 928S or S4 than was doing one third of the Kms / miles
So here we were at the Porsche dealer & starting to notice the rear coupling inhex bolt being a little loose or a lot more loose in its meant to be tension , but yet ( out of curiosity ) if on the very same car we went to the front coupling to check it was Always much tighter( normal tight tension) , now this was on these higher Km travelled 928S/928S4 cars
So what we did was to check the rear coupling inhex bolts on a basis of something to check on a Major Service & what we discovered was that the lower km travelled 928S/ 928S4's , meaning the majority of the ones we saw , the rear coupling inhex bolt in each of these cars was NOT loose at all , because the said 928S/928S4 had not travelled the higher Kms / acceleration events like the other 928S/928S4 models that had twice the Kms travelled or more
So that showed me without a shadow of a doubt that the rear inhex bolt that holds the rear coupling together was just simply stretching as all bolts do when they are subjected to constant strain events in this case shock from hard acceleration events which naturally want to spread the CUT in three places quite weak hollow splined input shaft into the trans & this poor little bolt that no one wants to check , over the years just looses its tension because it has no choice other than stretch , because thats what bolts do ( this is not rocket science )
That's why we have been seeing front flex plate deflection , because the rear coupling , its single bolt stretches much much much quicker than the front one ( always have & always will ) I can not change that simple fact
Remember ,( 4 speed auto ) you have to have the T Tube shaft moving forward to get it the deflect the main front flex plate , and for the front coupling to hold the deflection , this naturally has to be tight or tighter that the rear coupling ( very simple kindergarten type of stuff )
So that's why we do not have any issues with these , because we just maintain the rear couplings on the 928 manual trans ( ALL ) and the 4 speed 928 Auto , meaning the ones with the stupid hollow cut in three places splined input shaft
Interesting subnote } I / we at the Porsche dealer ( circa 1990 ) mentioned what we were noticing to the visiting in field Porsche Service Engineer ( this guy was from the Porsche importer at the time ) & would do quarterly visits to the main dealers and ask questions & take notes , and all that was said at the time was , good find , keep checking & tighten when need be , and considering we were having much more important warranty issues at the time with the 911 C4 model at the time , this issue was deemed minor & can be dealt in field by us during major services etc
Remember , this was not causing any breakages at the time( circa later 1980's early 1990's) , because the lost tension was only minor "ish" & the cars were only a few years old , no metal fatigue had made matters worse , this stuff would turn up later on cars that were NOT checked on a regular basis
It would of been nice for Porsche to have put up a service " Technical Information" bulletin about checking/ tightening/ replacing the rear coupling bolt , because we were talking about all those decades ago , but **** happens & its only a bolt that more people know about now days
Regards
Bruce Buchanan
Buchanan Automotive , working on Porsche cars since 1977
So here we were at the Porsche dealer & starting to notice the rear coupling inhex bolt being a little loose or a lot more loose in its meant to be tension , but yet ( out of curiosity ) if on the very same car we went to the front coupling to check it was Always much tighter( normal tight tension) , now this was on these higher Km travelled 928S/928S4 cars
So what we did was to check the rear coupling inhex bolts on a basis of something to check on a Major Service & what we discovered was that the lower km travelled 928S/ 928S4's , meaning the majority of the ones we saw , the rear coupling inhex bolt in each of these cars was NOT loose at all , because the said 928S/928S4 had not travelled the higher Kms / acceleration events like the other 928S/928S4 models that had twice the Kms travelled or more
So that showed me without a shadow of a doubt that the rear inhex bolt that holds the rear coupling together was just simply stretching as all bolts do when they are subjected to constant strain events in this case shock from hard acceleration events which naturally want to spread the CUT in three places quite weak hollow splined input shaft into the trans & this poor little bolt that no one wants to check , over the years just looses its tension because it has no choice other than stretch , because thats what bolts do ( this is not rocket science )
That's why we have been seeing front flex plate deflection , because the rear coupling , its single bolt stretches much much much quicker than the front one ( always have & always will ) I can not change that simple fact
Remember ,( 4 speed auto ) you have to have the T Tube shaft moving forward to get it the deflect the main front flex plate , and for the front coupling to hold the deflection , this naturally has to be tight or tighter that the rear coupling ( very simple kindergarten type of stuff )
So that's why we do not have any issues with these , because we just maintain the rear couplings on the 928 manual trans ( ALL ) and the 4 speed 928 Auto , meaning the ones with the stupid hollow cut in three places splined input shaft
Interesting subnote } I / we at the Porsche dealer ( circa 1990 ) mentioned what we were noticing to the visiting in field Porsche Service Engineer ( this guy was from the Porsche importer at the time ) & would do quarterly visits to the main dealers and ask questions & take notes , and all that was said at the time was , good find , keep checking & tighten when need be , and considering we were having much more important warranty issues at the time with the 911 C4 model at the time , this issue was deemed minor & can be dealt in field by us during major services etc
Remember , this was not causing any breakages at the time( circa later 1980's early 1990's) , because the lost tension was only minor "ish" & the cars were only a few years old , no metal fatigue had made matters worse , this stuff would turn up later on cars that were NOT checked on a regular basis
It would of been nice for Porsche to have put up a service " Technical Information" bulletin about checking/ tightening/ replacing the rear coupling bolt , because we were talking about all those decades ago , but **** happens & its only a bolt that more people know about now days
Regards
Bruce Buchanan
Buchanan Automotive , working on Porsche cars since 1977
06-18-2015, 11:14 PM
#37
Rennlist Member
Thread Starter
Bruce, what would you say about those cars that are showing flex plate migration and not a peep in rear bolt stretch? I've checked dozens and the rear bolt was still at or above factory torque specs. My standard practice has been to check that rear bolt if the front flex plate has had more movement than I like to see, 3-5mm. Almost all of them have been tight as a virgin on prom night.
06-18-2015, 11:24 PM
#38
Rennlist Member
06-18-2015, 11:32 PM
#39
Rennlist Member
Thread Starter
Looking at the records, the last time I had this car in for any work that involved getting it off the ground was 1-3-13 for engine mounts and I looked at the front then. 3-4-11 was the previous check. Miles between those two were about 2000 and there have been 2k since then. I worked on his 951 and 996 in 2014.
06-19-2015, 12:27 AM
#40
Rennlist Member
What exactly should I feel guilty about here?
Looking back at my books last year, I worked on 64 928's, 12 BMW's, 9 Audi's, 8 Mercedes, 2 Jags, 4 Chevy's and two Kia's. There might have been a Ford in there but they don't count. Am I supposed to catch every lose bolt that some other idiot wrench didn't tighten up? I am helping the guy out, I'm not charging $135 per hour to do this job like everyone else in the area and I know what I'm doing. Although with the amount of guys like you who would bitch and moan I should be so I have less things to worry about.
I have put that rear pinch bolt on my "check this now so I don't have to charge you more later" list. Hope that makes you feel better.
Looking back at my books last year, I worked on 64 928's, 12 BMW's, 9 Audi's, 8 Mercedes, 2 Jags, 4 Chevy's and two Kia's. There might have been a Ford in there but they don't count. Am I supposed to catch every lose bolt that some other idiot wrench didn't tighten up? I am helping the guy out, I'm not charging $135 per hour to do this job like everyone else in the area and I know what I'm doing. Although with the amount of guys like you who would bitch and moan I should be so I have less things to worry about.
I have put that rear pinch bolt on my "check this now so I don't have to charge you more later" list. Hope that makes you feel better.
Sean, I have no doubt you are a good mechanic and know these cars very well. That is quite obvious to me just from hanging around here. I usually enjoy your posts, respect your opinions and appreciate your technical contributions to this forum.
But, your first post really rubbed me the wrong way as you were quick to blame another for error and appear completely oblivious to your own role in the big picture here.
It is very possible that someone had been in there and done sub-standard work, but one could argue that the “expensive mistake” was yours, for not checking the rear bolt in the three years you’ve been servicing the car. Had the rear bolt been checked three years ago, one year ago, or even six months ago, today’s damage may not exist.
True that the rear bolt is a PITA to check, but after all, just as important, or more, than the front, and should be checked right along with the front, especially if the front appears “too perfect”. The customer may not have any clue what a pinch bolt is, let alone know how important the rear one is. Instead they rely on your expertise. The drive shaft should be treated as a whole and have both ends checked at the same time, just like the drive shaft and u-joints on more conventional vehicles. The extra time to drop the exhaust, or whatever needs to be done, should be sold to the customer as part of checking the front. If the customer persistently declines, then at least you’ve done your best to warn them of potential pending failure.
But, hindsight is 20/20 and I have no desire to get in a pissing contest with you. Surely I would lose.
So, let’s move forward and instead of calling this an “expensive mistake”, call it an “expensive lesson” for all to learn the importance of checking the rear bolt.
Also, please see my other post about “helping out”.
06-19-2015, 12:44 AM
#41
Rennlist Member
Thread Starter
Maybe you shouldn't feel guilty, I just know I would. "Damn! Why didn't I check that rear bolt! I know better!"
Sean, I have no doubt you are a good mechanic and know these cars very well. That is quite obvious to me just from hanging around here. I usually enjoy your posts, respect your opinions and appreciate your technical contributions to this forum.
But, your first post really rubbed me the wrong way as you were quick to blame another for error and appear completely oblivious to your own role in the big picture here.
It is very possible that someone had been in there and done sub-standard work, but one could argue that the “expensive mistake” was yours, for not checking the rear bolt in the three years you’ve been servicing the car. Had the rear bolt been checked three years ago, one year ago, or even six months ago, today’s damage may not exist.
True that the rear bolt is a PITA to check, but after all, just as important, or more, than the front, and should be checked right along with the front, especially if the front appears “too perfect”. The customer may not have any clue what a pinch bolt is, let alone know how important the rear one is. Instead they rely on your expertise. The drive shaft should be treated as a whole and have both ends checked at the same time, just like the drive shaft and u-joints on more conventional vehicles. The extra time to drop the exhaust, or whatever needs to be done, should be sold to the customer as part of checking the front. If the customer persistently declines, then at least you’ve done your best to warn them of potential pending failure.
But, hindsight is 20/20 and I have no desire to get in a pissing contest with you. Surely I would lose.
So, let’s move forward and instead of calling this an “expensive mistake”, call it an “expensive lesson” for all to learn the importance of checking the rear bolt.
Also, please see my other post about “helping out”.
Sean, I have no doubt you are a good mechanic and know these cars very well. That is quite obvious to me just from hanging around here. I usually enjoy your posts, respect your opinions and appreciate your technical contributions to this forum.
But, your first post really rubbed me the wrong way as you were quick to blame another for error and appear completely oblivious to your own role in the big picture here.
It is very possible that someone had been in there and done sub-standard work, but one could argue that the “expensive mistake” was yours, for not checking the rear bolt in the three years you’ve been servicing the car. Had the rear bolt been checked three years ago, one year ago, or even six months ago, today’s damage may not exist.
True that the rear bolt is a PITA to check, but after all, just as important, or more, than the front, and should be checked right along with the front, especially if the front appears “too perfect”. The customer may not have any clue what a pinch bolt is, let alone know how important the rear one is. Instead they rely on your expertise. The drive shaft should be treated as a whole and have both ends checked at the same time, just like the drive shaft and u-joints on more conventional vehicles. The extra time to drop the exhaust, or whatever needs to be done, should be sold to the customer as part of checking the front. If the customer persistently declines, then at least you’ve done your best to warn them of potential pending failure.
But, hindsight is 20/20 and I have no desire to get in a pissing contest with you. Surely I would lose.
So, let’s move forward and instead of calling this an “expensive mistake”, call it an “expensive lesson” for all to learn the importance of checking the rear bolt.
Also, please see my other post about “helping out”.
This was 99% the error of the guy who went in and put the bolts in place of the rivets on the flex plate. He didn't' torque the torque tube to the trans bell housing, he didn't torque the pinch bolt to the shaft and somehow in your mind I'm supposed to cover part of the cost of this failure? Right. I'll take it on the chin for the other 1%. Do you blame the guy who gets his 200k transmission serviced for the first time and it fails or the guy who did the service? I had some good respect for you until your first post on this thread. Not sure what you do for a living but I'm sure we can find a similar correlation that would make you think "wtf is he thinking".
There isn't anything I can say to ease your thoughts on this, but thanks for putting your thoughts in.
06-19-2015, 01:00 AM
#42
Rennlist Member
I've just measured three new bolts, which measure at 54.60 mm, and a damaged one which I believe has stretched, at 55.05 mm. I'm searching for a couple of measuring pins so that I can take thread dimensions, and I'm also looking for more used and new bolts, which I know I have somewhere. The difference I'm measuring could, of course, be manufacturing differences between new and old bolts, but IMO stretching seems more likely.
Nothing short of measuring a bolt when it goes in new, and then measuring that same bolt when it comes out, after loosening, will categorically tell if it has stretched, or just rotated loose.
Once I find those other bolts I'll get back with dimensions.
Nothing short of measuring a bolt when it goes in new, and then measuring that same bolt when it comes out, after loosening, will categorically tell if it has stretched, or just rotated loose.
Once I find those other bolts I'll get back with dimensions.
06-19-2015, 01:41 AM
#43
Drifting
Hi Sean , yes I know what you mean , the ones that I have been nearly fooled by ,on at least three occasions over the years, the inhex bolt had been loctite'ed into place( by someone else years ago ) & gave me a completely false reading ( but I didn't know that at that time ) until I removed the front of T Tube / rear of engine bell housing lower cover & saw the flex plate pushed in , so I removed the rear coupling inhex bolt & it was tight all the way out & had heaps of Loctite on the threads , so the thread was held in place but the bolt was stretched & the head of the bolt was not holding the coupling tight & on quite a few of these bolts the side , meaning the flank of the bolt has the tell tale teeth marks of the splines touching / cutting into the flank of the bolt , this can only happen as the shaft migrates forward
On another occasion a long term friend of mine (another Porsche independent specialist) refereed me one of his 928S4 customers ( he only wants to do older 911's these days ) and he eventually refereed the three 928 customers he had over to us as he was no longer interested ( another story ) , but the first one ( we had never seen the car before ) 1987 928S4 Auto, and one of the first things we do( here) is to reset the T Tube shaft , because in old age they are nearly all migrated forward , well the flex plate was pushed in all the way , but the rear coupling bolt was tight & I mean properly tight , I was puzzled until I rang the other specialist ( who I worked with at the Porsche dealers years ago ) & I mentioned what I had on this car , then it clicked , he had his mechanic to check & tighten the rear coupling bolt on the two or so services they did over a few years period previously , but they never reset the T Tube shaft , because they didn't know it was pushed forward when they first serviced the car , he just remembered what we did all those years before at the dealer & that was to just tighten the bolt ( nothing else ), so when they first saw the car ( say 15 years ago ) the shaft was already pushed forward & the rear coupling bolt was loose , but they just tightened there & that was it
The Big saving thing for the engine thrust bearing on that car( 87 S4) was the fact that ( they ) other Porsche specialist & I only use High Oil Film Strength engine oils in last century Porsche's , meaning 15W-50 or 20w-50 with good levels of ZDDP , this was the car that showed me the importance of the oil film strength in regards to engine thrust bearings , because this car had a very very very pushed in flex plate for years & years & when we reset the T Tube shaft we measure the thrust bearing wear & on this car it was well within spec
So there is quite a few ways we can be caught out by a bolt that appears tight but is not & a bolt that is tight , but why is it tight ( all Human inspired mini disasters )
Regards
Bruce Buchanan
Buchanan Automotive
On another occasion a long term friend of mine (another Porsche independent specialist) refereed me one of his 928S4 customers ( he only wants to do older 911's these days ) and he eventually refereed the three 928 customers he had over to us as he was no longer interested ( another story ) , but the first one ( we had never seen the car before ) 1987 928S4 Auto, and one of the first things we do( here) is to reset the T Tube shaft , because in old age they are nearly all migrated forward , well the flex plate was pushed in all the way , but the rear coupling bolt was tight & I mean properly tight , I was puzzled until I rang the other specialist ( who I worked with at the Porsche dealers years ago ) & I mentioned what I had on this car , then it clicked , he had his mechanic to check & tighten the rear coupling bolt on the two or so services they did over a few years period previously , but they never reset the T Tube shaft , because they didn't know it was pushed forward when they first serviced the car , he just remembered what we did all those years before at the dealer & that was to just tighten the bolt ( nothing else ), so when they first saw the car ( say 15 years ago ) the shaft was already pushed forward & the rear coupling bolt was loose , but they just tightened there & that was it
The Big saving thing for the engine thrust bearing on that car( 87 S4) was the fact that ( they ) other Porsche specialist & I only use High Oil Film Strength engine oils in last century Porsche's , meaning 15W-50 or 20w-50 with good levels of ZDDP , this was the car that showed me the importance of the oil film strength in regards to engine thrust bearings , because this car had a very very very pushed in flex plate for years & years & when we reset the T Tube shaft we measure the thrust bearing wear & on this car it was well within spec
So there is quite a few ways we can be caught out by a bolt that appears tight but is not & a bolt that is tight , but why is it tight ( all Human inspired mini disasters )
Regards
Bruce Buchanan
Buchanan Automotive
06-19-2015, 01:56 AM
#44
The Parts Whisperer
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There are a small group of you ( Greg B, Rob E, Mark K, and a few more) that have wrenched with me side by side and will probably attest to my speed but possibly my lack of attention to detail. Pinch bolts are one that I break out my torque wrench every time. I've seen first hand the repercussions from improper installation
06-19-2015, 02:21 AM
#45
Rennlist Member
Then we can call it even, I took offense to your post saying I should take ownership of this. On a car that has seen no more than 4000 miles in 4 years. Do I feel bad about it? Yea I do, but if I felt bad about every issue on a car that came in I'd never sleep. I wake up at night wondering if I torqued a wheel correctly and have called the customer to ask them to check it.
This was 99% the error of the guy who went in and put the bolts in place of the rivets on the flex plate. He didn't' torque the torque tube to the trans bell housing, he didn't torque the pinch bolt to the shaft and somehow in your mind I'm supposed to cover part of the cost of this failure? Right. I'll take it on the chin for the other 1%. Do you blame the guy who gets his 200k transmission serviced for the first time and it fails or the guy who did the service? I had some good respect for you until your first post on this thread. Not sure what you do for a living but I'm sure we can find a similar correlation that would make you think "wtf is he thinking".
There isn't anything I can say to ease your thoughts on this, but thanks for putting your thoughts in.
This was 99% the error of the guy who went in and put the bolts in place of the rivets on the flex plate. He didn't' torque the torque tube to the trans bell housing, he didn't torque the pinch bolt to the shaft and somehow in your mind I'm supposed to cover part of the cost of this failure? Right. I'll take it on the chin for the other 1%. Do you blame the guy who gets his 200k transmission serviced for the first time and it fails or the guy who did the service? I had some good respect for you until your first post on this thread. Not sure what you do for a living but I'm sure we can find a similar correlation that would make you think "wtf is he thinking".
There isn't anything I can say to ease your thoughts on this, but thanks for putting your thoughts in.
I design, and provide support for, passenger and freight elevator controls. When a mechanic needs help with a problem he calls me. Some days I take over fifty calls from elevator mechanics. Some problems are easy and some not so. Many times answers and solutions are not readily apparent and that’s when I really shine, figuring things out and creating solutions. Sometimes speculation is all there is to go on and I am more often right than not. I deal with all sorts of people and many that are not qualified for the job they have. Even then, I treat them with respect and never belittle them. The good ones I rarely hear from as they solve many problems themselves. I am extremely good at what I do, which is, ultimately, analytical problem solving. I don’t stop at getting the car running (the part of an elevator that goes up and down is called the car), I explain to them what took place and how to prevent it from happening again. Every call is a training session to some extent, some more than others. Technically I only deal with the controller itself, but realistically, I troubleshoot the entire system including mechanical, hydraulic and electrical including 100+ HP 3 phase motors, starters and variable frequency drives. It’s amazing the number of electricians that have no clue how electricity works.
So, to service one end of a drive shaft and not at least inspect the other is, in my mind, mostly unacceptable.
But once again, hindsight is always 20/20 so hopefully we can all agree to learn what we can learn and move on.
I checked my rear pinch bolt when I changed motor mounts, checked crank shaft end play and added PKlamp. The rear bolt was loose and showed signs of wear. I just tightened it but, given what I’ve learned (or re-learned) form this thread, I think I’ll replace it.
Thanks to all that have contributed technical content to this thread and I will now end my rant.
Rant over!
Last edited by depami; 06-19-2015 at 02:23 AM. Reason: punctuation