Modify PK Tensioner for Black edition
07-07-2014, 09:31 PM
#76
Rennlist
Basic Site Sponsor
Basic Site Sponsor
All three M8 fasteners were snug, but not torqued-to-20NM-and-left-in-place-for-years "tight". There was none of the typical "pop" as they came loose, as is typical for steel fasteners threaded into aluminum. That may mean nothing, or may be significant--I do not know.
You may have forgotten that our early PKT bracket had a 8.0mm thru-hole, not the 9mm of later ones. Here's a picture of the flat-head screw that came out of our GT. Note that the underside of the head shows contact only at the very outer edge, not a scratch elsewhere.
Attachment 851318
The thru-hole for this early bracket was 8.0 mm, which is a tight fit for a M8 screw. So it is basically acting as a dowel pin, and the forces on the fastener were primarily shear. Fasteners are pretty strong in shear.
I believe that the later brackets all have 9.0mm thru-holes for the countersink. If that is correct then that is a different situation: The combination of the incorrect countersink (making contact only at the top edge) and the oversize hole means that the fastener is stressed in bending, and not shear. Fasteners don't like bending, and tend to fatigue and break. Correcting the countersink reduces the lever-arm and the bending stress but I believe that the 8.0mm thru-hole was a better choice.
You may have forgotten that our early PKT bracket had a 8.0mm thru-hole, not the 9mm of later ones. Here's a picture of the flat-head screw that came out of our GT. Note that the underside of the head shows contact only at the very outer edge, not a scratch elsewhere.
Attachment 851318
The thru-hole for this early bracket was 8.0 mm, which is a tight fit for a M8 screw. So it is basically acting as a dowel pin, and the forces on the fastener were primarily shear. Fasteners are pretty strong in shear.
I believe that the later brackets all have 9.0mm thru-holes for the countersink. If that is correct then that is a different situation: The combination of the incorrect countersink (making contact only at the top edge) and the oversize hole means that the fastener is stressed in bending, and not shear. Fasteners don't like bending, and tend to fatigue and break. Correcting the countersink reduces the lever-arm and the bending stress but I believe that the 8.0mm thru-hole was a better choice.
I'm not sure what the function of this bolt is....but that bolt is not appear to be in very good condition. How many miles was that bolt in use? This bolt was tight when it was removed?
It might just be the picture, but the shiny tapered surface below the very top of that bolt looks like it has been "working" and has tiny areas where pieces of metal appear to be missing....much like a bearing that is loosing it's top layer. (Fretting is the proper term.) The rest of that surface, where the tiny "divots" are not present seems overly shiny and has the appearance of "working". Could it be that the entire bolt was moving around in the hole it went into and the very top edge of that area was actually above the taper of that hole?
Additionally, it appears that the threads below the shiny angled section were not engaged into a female threaded area....like they passed through a piece of metal, without threads. Those threads appear to be "packed" with a large amount of tiny metal filings....look at how rough that threaded portion, just below the tapered surface, appears in the picture.....that looks like metal debris.
The actual part of the bolt that probably was threaded into something is blurry....It would be good to see that area.
How about a well lighted picture of the hole that that bolt came out of?
That would tell volumes.
__________________
greg brown
714 879 9072
GregBBRD@aol.com
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
greg brown
714 879 9072
GregBBRD@aol.com
Semi-retired, as of Feb 1, 2023.
The days of free technical advice are over.
Free consultations will no longer be available.
Will still be in the shop, isolated and exclusively working on project cars, developmental work and products, engines and transmissions.
Have fun with your 928's people!
07-07-2014, 09:36 PM
#77
Instructor
Unsubscribed
07-07-2014, 10:21 PM
#78
Rennlist Member
I'll email some more pics, but I'm pretty sure that the bracket was working around the two flange-heads and not at the flat-head, as presumably designed. The "grunge" in the middle threads is silver anti-seize (probably not the best choice given the potential to self-loosen).
07-07-2014, 11:06 PM
#79
Inventor
Rennlist Member
Rennlist Member
Just got through reading this lot, I've gone from totally happy with the PKT to almost to scared to run the car.
I've had Ken's PKT fitted since 2009 and run 20,000 miles, I have not seen the email Roger sent out, as a coincidence I had one of the belt covers off last Thursday to check the belt and pulleys, all seemed fine.
Not sure what to do next, are the early PKT's OK, do I need to replace any hardware, i. e. bolts.
I've had Ken's PKT fitted since 2009 and run 20,000 miles, I have not seen the email Roger sent out, as a coincidence I had one of the belt covers off last Thursday to check the belt and pulleys, all seemed fine.
Not sure what to do next, are the early PKT's OK, do I need to replace any hardware, i. e. bolts.
Out of an abundance of caution, check the tightness of both flange head bolts.
At belt change time, contact Roger for updated hardware for your application.
07-07-2014, 11:18 PM
#80
Nordschleife Master
There's a term for this attack: FUD. Fear, Uncertainty, Doubt. Microsoft used to use it in the 1990's.
Nothing is foolproof. I think some people (including those who are not staging a FUD campaign) hold the Porsche engineers from the 1970's and 1980's in too high of a regard. Studying these engines, they are full of what I think are design errors. I don't see how the factory belt tensioning system is going to have lower failure rates than Porkensioner, when you take into account the possible installation errors (too tight, too loose), worn cam gears, worn water pumps, tired belts. Judged against the record of timing belt failures we see in the wreckers' yards, the PKT record even with it's imperfections is going to stand up on its own merits just fine. That's my opinion anyway.
Nothing is foolproof. I think some people (including those who are not staging a FUD campaign) hold the Porsche engineers from the 1970's and 1980's in too high of a regard. Studying these engines, they are full of what I think are design errors. I don't see how the factory belt tensioning system is going to have lower failure rates than Porkensioner, when you take into account the possible installation errors (too tight, too loose), worn cam gears, worn water pumps, tired belts. Judged against the record of timing belt failures we see in the wreckers' yards, the PKT record even with it's imperfections is going to stand up on its own merits just fine. That's my opinion anyway.
07-07-2014, 11:50 PM
#81
Rennlist Member
Are there any other cars that have a belt tension warning light (that's NOT triggered by mileage)? The very existence of that tells me the Porsche engineers weren't feeling overly confident about their own system. Of course if you all tell me they're commonplace then I'll feel stupid, but I come from the rotary world where we didn't have to worry about that crap :-)
07-07-2014, 11:58 PM
#82
Nordschleife Master
Belt tension warning is not by mileage, it is by lack of tension picked up by the sensor breaking a ground and notifying us via the warning light(s). I think the engineers were pretty suave in their design.
http://members.rennlist.com/v1uhoh/timing.htm
http://members.rennlist.com/v1uhoh/timing.htm
07-08-2014, 12:02 AM
#83
Rennlist Member
Oh I know that...that's my point, did any other car do something like this/feel the need for something like this? I'm not aware of any (I am aware of some cars who have a mileage-triggered light to tell you it's time to change your belt...which is different from this thing here of course).
07-08-2014, 12:08 AM
#84
Nordschleife Master
Oh I know that...that's my point, did any other car do something like this/feel the need for something like this? I'm not aware of any (I am aware of some cars who have a mileage-triggered light to tell you it's time to change your belt...which is different from this thing here of course).
I still think on an interference engine it would be risky not to design a warning system.
07-08-2014, 01:40 AM
#86
Rennlist
Basic Site Sponsor
Basic Site Sponsor
I've given this whole thread a lot of thought, today.
I'm certain that Ken, Roger, Sean, and others have studied the potential problems and the potential results of those potential problems, at length.
In the final analysis, nothing I think, see, or have a theory on really matters....it's simply not any of my business.
Ken says that everything is fine, there are no problems that need to be addressed, until a belt change is due. He's the designer, he's the builder, he's the expert on this tensioner. If he says everything is fine, that should be good enough!
I'm completely withdrawing from this thread and from this entire tensioner conversation.
I appologize for butting into something that is clearly not my business and has no affect on my life or what I do on a daily basis.
I'm certain that Ken, Roger, Sean, and others have studied the potential problems and the potential results of those potential problems, at length.
In the final analysis, nothing I think, see, or have a theory on really matters....it's simply not any of my business.
Ken says that everything is fine, there are no problems that need to be addressed, until a belt change is due. He's the designer, he's the builder, he's the expert on this tensioner. If he says everything is fine, that should be good enough!
I'm completely withdrawing from this thread and from this entire tensioner conversation.
I appologize for butting into something that is clearly not my business and has no affect on my life or what I do on a daily basis.
07-08-2014, 09:25 AM
#88
Basic Sponsor
Rennlist
Site Sponsor
Rennlist
Site Sponsor
From 92 the Porsche 968 used a "similar" automatic tensioning system - did this have a warning light for the belt tension? Anyone with a 968 that can comment - Doc Mirror?
__________________
Does it have the "Do It Yourself" manual transmission, or the superior "Fully Equipped by Porsche" Automatic Transmission?
George Layton March 2014
928 Owners are ".....a secret sect of quietly assured Porsche pragmatists who in near anonymity appreciate the prodigious, easy going prowess of the 928."
Does it have the "Do It Yourself" manual transmission, or the superior "Fully Equipped by Porsche" Automatic Transmission?
George Layton March 2014928 Owners are ".....a secret sect of quietly assured Porsche pragmatists who in near anonymity appreciate the prodigious, easy going prowess of the 928."
Last edited by ROG100; 07-08-2014 at 10:45 AM.
