TBF'd GTS Crank viability of crank recovery
10-10-2020, 03:18 PM
#1
Rennlist Member
Thread Starter
TBF'd GTS Crank viability of crank recovery
As some of you well know I have a 92 GTS with a 90S4 motor that was recovered from my late [wrecked] S4. The motor is running as well as it ever has and I must say I am inclined to leave it that way.
That being said when I stripped the original GTS motor I kept a number of components in the hope that one day I might be able to build a full GTS or maybe even a stroker motor but those dreams are fading into the sunset as the years pass and my bones creak ever more as I age.
Nonetheless I have amongst the kit recovered a pair of 5R heads with a little of the corrosion I have described, a set of unmodifed 86.5 cams, a set of GTS pistons and a set of 1R rods plus a GTS crank that got chewed up somewhat on the rear thrust face. I had a local engine shop spin the crank up on a crank grinding machine and she ran true. The thrust journal main bearing to my surprise appeared unmolested and measured correctly as we could tell. The damaged thrust face when cleaned up and polished suggested that the attrition had taken about 0.5mm off the thrust face so clearly not serviceable in its current condition.
This left me wondering whether the crank is worth recovering or whether it is just better to throw it or make it into an ornament. It is quite possible to build the thrust face up to recover the stock dimension. I doubt it would be feasible to get a custom thrust bearing made with a compensatory face. I am assuming the crank passageways will need thorough cleaning and presumably the crank heat treatment will have been compromised and need redoing.
Just wondered if anyone has been down a similar path and the viability of what might need doing together with likely cost. I am currently thinking it is not going to be a viable project but given the dearth of GTS cranks maybe it is not a lost cause entirely.
That being said when I stripped the original GTS motor I kept a number of components in the hope that one day I might be able to build a full GTS or maybe even a stroker motor but those dreams are fading into the sunset as the years pass and my bones creak ever more as I age.
Nonetheless I have amongst the kit recovered a pair of 5R heads with a little of the corrosion I have described, a set of unmodifed 86.5 cams, a set of GTS pistons and a set of 1R rods plus a GTS crank that got chewed up somewhat on the rear thrust face. I had a local engine shop spin the crank up on a crank grinding machine and she ran true. The thrust journal main bearing to my surprise appeared unmolested and measured correctly as we could tell. The damaged thrust face when cleaned up and polished suggested that the attrition had taken about 0.5mm off the thrust face so clearly not serviceable in its current condition.
This left me wondering whether the crank is worth recovering or whether it is just better to throw it or make it into an ornament. It is quite possible to build the thrust face up to recover the stock dimension. I doubt it would be feasible to get a custom thrust bearing made with a compensatory face. I am assuming the crank passageways will need thorough cleaning and presumably the crank heat treatment will have been compromised and need redoing.
Just wondered if anyone has been down a similar path and the viability of what might need doing together with likely cost. I am currently thinking it is not going to be a viable project but given the dearth of GTS cranks maybe it is not a lost cause entirely.
Last edited by FredR; 10-10-2020 at 03:22 PM.
10-11-2020, 12:28 AM
#2
Former Vendor
As some of you well know I have a 92 GTS with a 90S4 motor that was recovered from my late [wrecked] S4. The motor is running as well as it ever has and I must say I am inclined to leave it that way.
That being said when I stripped the original GTS motor I kept a number of components in the hope that one day I might be able to build a full GTS or maybe even a stroker motor but those dreams are fading into the sunset as the years pass and my bones creak ever more as I age.
Nonetheless I have amongst the kit recovered a pair of 5R heads with a little of the corrosion I have described, a set of unmodifed 86.5 cams, a set of GTS pistons and a set of 1R rods plus a GTS crank that got chewed up somewhat on the rear thrust face. I had a local engine shop spin the crank up on a crank grinding machine and she ran true. The thrust journal main bearing to my surprise appeared unmolested and measured correctly as we could tell. The damaged thrust face when cleaned up and polished suggested that the attrition had taken about 0.5mm off the thrust face so clearly not serviceable in its current condition.
This left me wondering whether the crank is worth recovering or whether it is just better to throw it or make it into an ornament. It is quite possible to build the thrust face up to recover the stock dimension. I doubt it would be feasible to get a custom thrust bearing made with a compensatory face. I am assuming the crank passageways will need thorough cleaning and presumably the crank heat treatment will have been compromised and need redoing.
Just wondered if anyone has been down a similar path and the viability of what might need doing together with likely cost. I am currently thinking it is not going to be a viable project but given the dearth of GTS cranks maybe it is not a lost cause entirely.
That being said when I stripped the original GTS motor I kept a number of components in the hope that one day I might be able to build a full GTS or maybe even a stroker motor but those dreams are fading into the sunset as the years pass and my bones creak ever more as I age.
Nonetheless I have amongst the kit recovered a pair of 5R heads with a little of the corrosion I have described, a set of unmodifed 86.5 cams, a set of GTS pistons and a set of 1R rods plus a GTS crank that got chewed up somewhat on the rear thrust face. I had a local engine shop spin the crank up on a crank grinding machine and she ran true. The thrust journal main bearing to my surprise appeared unmolested and measured correctly as we could tell. The damaged thrust face when cleaned up and polished suggested that the attrition had taken about 0.5mm off the thrust face so clearly not serviceable in its current condition.
This left me wondering whether the crank is worth recovering or whether it is just better to throw it or make it into an ornament. It is quite possible to build the thrust face up to recover the stock dimension. I doubt it would be feasible to get a custom thrust bearing made with a compensatory face. I am assuming the crank passageways will need thorough cleaning and presumably the crank heat treatment will have been compromised and need redoing.
Just wondered if anyone has been down a similar path and the viability of what might need doing together with likely cost. I am currently thinking it is not going to be a viable project but given the dearth of GTS cranks maybe it is not a lost cause entirely.
My custom crankshaft builder's main business and reputation is repairing all kinds of crankshafts. He's got an uber dollar high tech crank welding machine, with a guy that is a specialist at this. I've had him weld and regrind several...back to standard. This work is nothing short of amazing!
10-11-2020, 02:38 AM
#3
Archive Gatekeeper
Rennlist Member
Rennlist Member
Here's the GTS crank from my '93 I tried to ruin back in 2009.
Before:
After Greg's guy rescued it:
Before:
After Greg's guy rescued it:
10-11-2020, 04:50 PM
#5
Former Vendor
At any given time, there's probably 250 crankshafts in various stages of repair....
Before I found this guy, I tossed anything that wasn't a perfect "standard/standard" crankshaft in the trash.
Look at the radius where the rod journal meets the counterweight....perfection. (That radius is where the stress risers start, leading to broken crankshafts.)
There's people that claim to be able to do repairs/rebuilding, there's people that do "adequate" work, and then there are perfectionists.
And it's terribly difficult to assemble "perfection", unless what you start with is perfect.
Trending Topics
10-13-2020, 08:59 AM
#8
Rennlist Member
Thread Starter
A little teaser if I may-
The stock crank journal is 30.2mm and the crank end float when new is typically in the range 0.15mm to 0.2mm so logic suggests the face to face dimension of the thrust bearing in going to be around 30.0mm.
The retirement end float for the thrust bearing is specified as 0.4mm so one imagines that the intent was that the crank can float up to plus or minus 0.2mm from the centerline due to long term wear. This presumes Porsche assumed even wear on both thrust faces [not of course the TBF scenario].
Now, as in the case of my TBF'd GTS crank, the crank journal is now 30.6mm and thus 0.4mm missing from the rear most crank face. There will be a small amount of float on the small end bearing and also on the big end bearing. I know that oversize thrust bearings are available although I do not know if the flange to flange dimension is oversized. If a thrust bearing with a face to face dimension of 30.2mm were available the end float would be 0.4mm but obviously biased to the rear side.
My instincts tell me that such a scenario would not be acceptable- any thoughts?
The stock crank journal is 30.2mm and the crank end float when new is typically in the range 0.15mm to 0.2mm so logic suggests the face to face dimension of the thrust bearing in going to be around 30.0mm.
The retirement end float for the thrust bearing is specified as 0.4mm so one imagines that the intent was that the crank can float up to plus or minus 0.2mm from the centerline due to long term wear. This presumes Porsche assumed even wear on both thrust faces [not of course the TBF scenario].
Now, as in the case of my TBF'd GTS crank, the crank journal is now 30.6mm and thus 0.4mm missing from the rear most crank face. There will be a small amount of float on the small end bearing and also on the big end bearing. I know that oversize thrust bearings are available although I do not know if the flange to flange dimension is oversized. If a thrust bearing with a face to face dimension of 30.2mm were available the end float would be 0.4mm but obviously biased to the rear side.
My instincts tell me that such a scenario would not be acceptable- any thoughts?
10-13-2020, 05:00 PM
#9
Former Vendor
A little teaser if I may-
The stock crank journal is 30.2mm and the crank end float when new is typically in the range 0.15mm to 0.2mm so logic suggests the face to face dimension of the thrust bearing in going to be around 30.0mm.
The retirement end float for the thrust bearing is specified as 0.4mm so one imagines that the intent was that the crank can float up to plus or minus 0.2mm from the centerline due to long term wear. This presumes Porsche assumed even wear on both thrust faces [not of course the TBF scenario].
Now, as in the case of my TBF'd GTS crank, the crank journal is now 30.6mm and thus 0.4mm missing from the rear most crank face. There will be a small amount of float on the small end bearing and also on the big end bearing. I know that oversize thrust bearings are available although I do not know if the flange to flange dimension is oversized. If a thrust bearing with a face to face dimension of 30.2mm were available the end float would be 0.4mm but obviously biased to the rear side.
My instincts tell me that such a scenario would not be acceptable- any thoughts?
The stock crank journal is 30.2mm and the crank end float when new is typically in the range 0.15mm to 0.2mm so logic suggests the face to face dimension of the thrust bearing in going to be around 30.0mm.
The retirement end float for the thrust bearing is specified as 0.4mm so one imagines that the intent was that the crank can float up to plus or minus 0.2mm from the centerline due to long term wear. This presumes Porsche assumed even wear on both thrust faces [not of course the TBF scenario].
Now, as in the case of my TBF'd GTS crank, the crank journal is now 30.6mm and thus 0.4mm missing from the rear most crank face. There will be a small amount of float on the small end bearing and also on the big end bearing. I know that oversize thrust bearings are available although I do not know if the flange to flange dimension is oversized. If a thrust bearing with a face to face dimension of 30.2mm were available the end float would be 0.4mm but obviously biased to the rear side.
My instincts tell me that such a scenario would not be acceptable- any thoughts?
However, here's some other thoughts/information:
No wear on the crankshaft occurs until the thrust bearing hits the steel backing. Once the two steel surfaces touch each other, things go really bad, really quick, and the damaged surface on the crank is never flat, but always "dished".
Therefore. you would have to have the damaged thrust surface reground square until you'd know the actual dimension of the thrust bearing you would require.
I can not recall the thrust width of the 1st oversize bearing, but they are cheap, and you could certainly order one to see if there is enough meat to accomplish what you are thinking.
Last edited by GregBBRD; 10-13-2020 at 05:02 PM.
10-13-2020, 05:41 PM
#10
Rennlist Member
Thread Starter
My first thought is that you are in dreamland.
However, here's some other thoughts/information:
No wear on the crankshaft occurs until the thrust bearing hits the steel backing. Once the two steel surfaces touch each other, things go really bad, really quick, and the damaged surface on the crank is never flat, but always "dished".
Therefore. you would have to have the damaged thrust surface reground square until you'd know the actual dimension of the thrust bearing you would require.
I can not recall the thrust width of the 1st oversize bearing, but they are cheap, and you could certainly order one to see if there is enough meat to accomplish what you are thinking.
However, here's some other thoughts/information:
No wear on the crankshaft occurs until the thrust bearing hits the steel backing. Once the two steel surfaces touch each other, things go really bad, really quick, and the damaged surface on the crank is never flat, but always "dished".
Therefore. you would have to have the damaged thrust surface reground square until you'd know the actual dimension of the thrust bearing you would require.
I can not recall the thrust width of the 1st oversize bearing, but they are cheap, and you could certainly order one to see if there is enough meat to accomplish what you are thinking.
No dreamland here and no "La La Land" either-
The dimension I quoted of 30.6mm is the status quo after having had the thrust face reground and polished so that on first glance it looks like it "should be" once more. I was impressed by how easy it was for the machine shop to get it to that point- took them about 15 minutes as I recall. My calcs/common sense tell me the offset is not likely to be correctable by any oversize thrust bearing with equal geometry thus why I floated the post to test whether folks like your good self had any thoughts to offer in that regard. At the back of my mind was the info that came out following that engine failure in a car Mark A was driving on track wherein there was a finite element analysis showing just how sensitive the rods are to lateral thrust. Thus the crank as currently is would need a custom one off thrust bearing to work or more likely, welding up and refinishing to the stock 30.2mm dimension as highlighted. The other possibility theoretically might be to grind the other thrust face to compensate for the 0.4mm "lost" material but then the total dimension would be 31.0mm and an oversize thrust bearing with face to face dimension of 30,8mm would be required and my assumption is that no such item exists. According to the Rose passion website some form of oversize thrust bearings to the tune of 0.25mm and 0.5mm are available but it was not clear to me what dimension they were referring to - I assumed it was with respect to a main journal regrind.
FYI- the thrust bearing that came out of the motor had not only the white metal and the copper ground out but it also had a most of the steel flange missing thus why and how the end float was something like 1.2mm before the crank was thrust restrained by the crankcase webbing. Sadly the crankcase was well and truly cracked after the bearing had spun- a real mess. Thus I got the car for a song and stuck my trusty 90 S4 motor in it.
10-13-2020, 08:13 PM
#11
Former Vendor
Greg,
No dreamland here and no "La La Land" either-
The dimension I quoted of 30.6mm is the status quo after having had the thrust face reground and polished so that on first glance it looks like it "should be" once more. I was impressed by how easy it was for the machine shop to get it to that point- took them about 15 minutes as I recall. My calcs/common sense tell me the offset is not likely to be correctable by any oversize thrust bearing with equal geometry thus why I floated the post to test whether folks like your good self had any thoughts to offer in that regard. At the back of my mind was the info that came out following that engine failure in a car Mark A was driving on track wherein there was a finite element analysis showing just how sensitive the rods are to lateral thrust. Thus the crank as currently is would need a custom one off thrust bearing to work or more likely, welding up and refinishing to the stock 30.2mm dimension as highlighted. The other possibility theoretically might be to grind the other thrust face to compensate for the 0.4mm "lost" material but then the total dimension would be 31.0mm and an oversize thrust bearing with face to face dimension of 30,8mm would be required and my assumption is that no such item exists. According to the Rose passion website some form of oversize thrust bearings to the tune of 0.25mm and 0.5mm are available but it was not clear to me what dimension they were referring to - I assumed it was with respect to a main journal regrind.
FYI- the thrust bearing that came out of the motor had not only the white metal and the copper ground out but it also had a most of the steel flange missing thus why and how the end float was something like 1.2mm before the crank was thrust restrained by the crankcase webbing. Sadly the crankcase was well and truly cracked after the bearing had spun- a real mess. Thus I got the car for a song and stuck my trusty 90 S4 motor in it.
No dreamland here and no "La La Land" either-
The dimension I quoted of 30.6mm is the status quo after having had the thrust face reground and polished so that on first glance it looks like it "should be" once more. I was impressed by how easy it was for the machine shop to get it to that point- took them about 15 minutes as I recall. My calcs/common sense tell me the offset is not likely to be correctable by any oversize thrust bearing with equal geometry thus why I floated the post to test whether folks like your good self had any thoughts to offer in that regard. At the back of my mind was the info that came out following that engine failure in a car Mark A was driving on track wherein there was a finite element analysis showing just how sensitive the rods are to lateral thrust. Thus the crank as currently is would need a custom one off thrust bearing to work or more likely, welding up and refinishing to the stock 30.2mm dimension as highlighted. The other possibility theoretically might be to grind the other thrust face to compensate for the 0.4mm "lost" material but then the total dimension would be 31.0mm and an oversize thrust bearing with face to face dimension of 30,8mm would be required and my assumption is that no such item exists. According to the Rose passion website some form of oversize thrust bearings to the tune of 0.25mm and 0.5mm are available but it was not clear to me what dimension they were referring to - I assumed it was with respect to a main journal regrind.
FYI- the thrust bearing that came out of the motor had not only the white metal and the copper ground out but it also had a most of the steel flange missing thus why and how the end float was something like 1.2mm before the crank was thrust restrained by the crankcase webbing. Sadly the crankcase was well and truly cracked after the bearing had spun- a real mess. Thus I got the car for a song and stuck my trusty 90 S4 motor in it.
That's great news that the thrust face cleaned up in that tiny amount....usually people are not that lucky.
Yes, the parts people are referring to the ID of the main bearing being .25 and .50 oversize...but the thrust surface is also oversize. If I was you, I'd order up the 1st oversize bearing and see if you can get close.
Worse case scenario, you might have to have someone grind the center main to .25mm undersize.
10-15-2020, 08:30 AM
#12
Rennlist Member
Fred, don't be silly - call your local Porsche Centre and drop off your damaged crank. GBP1750 and 2 weeks later you would be in a possession of a brand new (refurbished, first oversize 0.25mm) GTS crank with a full set of matched Genuine Porsche bearings - both crank and rods - mine were made in 1996! For the money/2yrs warranty and hassle free exchange, this is very hard to beat...
I was the first customer Worldwide of their programme, thus I got a slight discount, but still think GBP1750 isn't bad, when you factor in the cost of new full set of Genuine Porsche crank and rod bearings...
Hope this helps,
Alex
I was the first customer Worldwide of their programme, thus I got a slight discount, but still think GBP1750 isn't bad, when you factor in the cost of new full set of Genuine Porsche crank and rod bearings...
Hope this helps,
Alex
10-15-2020, 08:52 AM
#13
Rennlist Member
Thread Starter
Fred, don't be silly - call your local Porsche Centre and drop off your damaged crank. GBP1750 and 2 weeks later you would be in a possession of a brand new (refurbished, first oversize 0.25mm) GTS crank with a full set of matched Genuine Porsche bearings - both crank and rods - mine were made in 1996! For the money/2yrs warranty and hassle free exchange, this is very hard to beat...
I was the first customer Worldwide of their programme, thus I got a slight discount, but still think GBP1750 isn't bad, when you factor in the cost of new full set of Genuine Porsche crank and rod bearings...
Hope this helps,
Alex
I was the first customer Worldwide of their programme, thus I got a slight discount, but still think GBP1750 isn't bad, when you factor in the cost of new full set of Genuine Porsche crank and rod bearings...
Hope this helps,
Alex
An interesting thought given as you say a full set of bearing shells is probably going to be the best part of a grand or so I rather suspect! I would have thought the part exchange route would be out of the question given the damage it has taken but....?
At the moment I am a long way from decision time- more like I am at the decision to I keep the thing or dump it in the trash- I think I have passed that hurdle!
Seems an age since we last met at Castle Coombe UK 928 track day- must be what- close to 10 years ago? It is 15 years since I wrecked my S4 and 14 years I have had the GTS the crank came from.
Trust you are keeping well and virus free!
10-15-2020, 09:34 AM
#14
Rennlist Member
Alex,
An interesting thought given as you say a full set of bearing shells is probably going to be the best part of a grand or so I rather suspect! I would have thought the part exchange route would be out of the question given the damage it has taken but....?
At the moment I am a long way from decision time- more like I am at the decision to I keep the thing or dump it in the trash- I think I have passed that hurdle!
Seems an age since we last met at Castle Coombe UK 928 track day- must be what- close to 10 years ago? It is 15 years since I wrecked my S4 and 14 years I have had the GTS the crank came from.
Trust you are keeping well and virus free!
An interesting thought given as you say a full set of bearing shells is probably going to be the best part of a grand or so I rather suspect! I would have thought the part exchange route would be out of the question given the damage it has taken but....?
At the moment I am a long way from decision time- more like I am at the decision to I keep the thing or dump it in the trash- I think I have passed that hurdle!
Seems an age since we last met at Castle Coombe UK 928 track day- must be what- close to 10 years ago? It is 15 years since I wrecked my S4 and 14 years I have had the GTS the crank came from.
Trust you are keeping well and virus free!
I am well and virus free so far
As for the crank and the damage - Porsche doesn't care one bit... The one I handed in was damaged while racing at Snetterton. The dry sump run dry due to a crack in the oil cooler return line. Both 2 and 6 rod bearings had evaporated and the engine still run. The 2R rods had assumed a rainbow shape, but luckily for me held on. As you can imagine the rod journal was in a much worse shape than the one that Greg posted earlier. Porsche took it and told me that it would be refurbished to as new condition. In the words of the parts manager at Porsche Reading - a dealer owned directly by Porsche AG "Why do you think we are selling/exchanging you an oversized crank?"
Alex
10-15-2020, 09:58 AM
#15
Rennlist Member
Thread Starter
Been a long time indeed!
I am well and virus free so far
As for the crank and the damage - Porsche doesn't care one bit... The one I handed in was damaged while racing at Snetterton. The dry sump run dry due to a crack in the oil cooler return line. Both 2 and 6 rod bearings had evaporated and the engine still run. The 2R rods had assumed a rainbow shape, but luckily for me held on. As you can imagine the rod journal was in a much worse shape than the one that Greg posted earlier. Porsche took it and told me that it would be refurbished to as new condition. In the words of the parts manager at Porsche Reading - a dealer owned directly by Porsche AG "Why do you think we are selling/exchanging you an oversized crank?"
Alex
I am well and virus free so far
As for the crank and the damage - Porsche doesn't care one bit... The one I handed in was damaged while racing at Snetterton. The dry sump run dry due to a crack in the oil cooler return line. Both 2 and 6 rod bearings had evaporated and the engine still run. The 2R rods had assumed a rainbow shape, but luckily for me held on. As you can imagine the rod journal was in a much worse shape than the one that Greg posted earlier. Porsche took it and told me that it would be refurbished to as new condition. In the words of the parts manager at Porsche Reading - a dealer owned directly by Porsche AG "Why do you think we are selling/exchanging you an oversized crank?"
Alex
Good info! I have a set of 1R rods from the motor and a full set of pistons that look OK but would need more inspection. I also have a pair of 5R heads with some corrosion damage that could easily be recovered. A good block and I could be in business [theoretically]. My S4 powered GTS is running rather sweetly these days and even the ac works well at 40C ambient temps. The 955 Turbo S provides the "straight line fun" and it is no slouch in the corners either but for reasons I know not the 928 still puts a silly grin on my face every time I drive it. With a spare transaxle unit in my garage, I figured that if I had a spare motor the car will literally last forever and way longer than my limited existence on this planet notwithstanding the Covid 19 impact.