Replace Head Studs?
Thread Starter
Former Vendor
Joined: Feb 2005
Posts: 15,228
Likes: 2,530
From: Anaheim
Thats your problem right there, I think. Gold yellow colored replacement studs must only be turned 90 degrees twice, not three times. 20Nm + 90 degrees + 90 degrees and leave them to that. See Service Info Tech 1983 document and correct Tech Spec Book. Its probably good idea to replace them all with similar new ones and use correct tightening method. At least this would be easy explanation for higher tq.
Read technical bulletin Group 1, 8703, in the 1987 Tech Book. Page 27 clearly talks about the "gold-yellow color head studs with the longer threads" and refers to a picture of these studs. Clearly three 90 degree turns. This bulletin "superceeds all previous repair instructions in the repair publications concerning the tightening of the cylinder heads." Everything prior is no longer valid, as of this publicaiton. This also agrees with the workshop "tightening proceedure" page, which was superceeded in 1986, at least for my copy.
Thread Starter
Former Vendor
Joined: Feb 2005
Posts: 15,228
Likes: 2,530
From: Anaheim
Thread Starter
Former Vendor
Joined: Feb 2005
Posts: 15,228
Likes: 2,530
From: Anaheim
Rennlist Member
Joined: Dec 2001
Posts: 25,186
Likes: 105
), I used a 4 foot breaker bar to get to the third 90. 
I guess well see. Since I had alot of time, and I knew that the ARPs sometimes were a problem with having to retorque, I did the total amount of steps over several days or a few weeks, to let everything settle. No, I understand that is different then a temperature cycle, but as you suggest, a retorque after the engine is installed is ridiculous. I would like to avoid that.
Addict
Rennlist Member
Rennlist Member
Joined: Nov 2003
Posts: 2,608
Likes: 12
From: Sydney AUS
Bearing in mind that the modulus of elasticity of steel (pretty much of any variety) does not vary from around the 205-210E3 N/mm2 region. But the stress at the top of the elastic portion of the stress strain curve can vary by a factor of three from the weakest (say G4.6) to the strongest commonly used (G12.9)
Tightening by degrees (to produce a specific extension of the stud) pretty much eliminates the frictional effects that can result in studs tightened by torqueing delivering varying clamping forces.
Greg Browns observations (varying torques when tightened by degrees) and those of BC (empirical torque values with ARP studs unless BC is a 5 stone weakling LOL) lead me to conclude that the OEM studs are designed to yield (transition from elastic phase to plastic phase) and sit in the plastic phase, while the ARP studs probably remain in the elastic phase.
This means that the OEM studss will exert a reasonably constant force or compression on the head gasket even though temperature rises (alu head growing more than the steel stud) causing the stud to be stretched. In contrast the ARP stud will exert a greater compression as temps rise. Thus the need to re-torque the ARP studs after a heat cycle becuase extra force in the heat cycle will have compressed the gasket and reduced the overall height of the compressed parts thus reducing the clamping force. I am presuming that gasket compression is a one time deal and has a finite limit. It should thus be possible to simulate the heat cycle by overtightening the ARP studs - leaving all to settle then loosening and retorquing. Assuming that you don't pull the threads out of the block - which I believe is more likely a limiting factor in all of this.
Tightening by degrees (to produce a specific extension of the stud) pretty much eliminates the frictional effects that can result in studs tightened by torqueing delivering varying clamping forces.
Greg Browns observations (varying torques when tightened by degrees) and those of BC (empirical torque values with ARP studs unless BC is a 5 stone weakling LOL) lead me to conclude that the OEM studs are designed to yield (transition from elastic phase to plastic phase) and sit in the plastic phase, while the ARP studs probably remain in the elastic phase.
This means that the OEM studss will exert a reasonably constant force or compression on the head gasket even though temperature rises (alu head growing more than the steel stud) causing the stud to be stretched. In contrast the ARP stud will exert a greater compression as temps rise. Thus the need to re-torque the ARP studs after a heat cycle becuase extra force in the heat cycle will have compressed the gasket and reduced the overall height of the compressed parts thus reducing the clamping force. I am presuming that gasket compression is a one time deal and has a finite limit. It should thus be possible to simulate the heat cycle by overtightening the ARP studs - leaving all to settle then loosening and retorquing. Assuming that you don't pull the threads out of the block - which I believe is more likely a limiting factor in all of this.
Rennlist Member
Joined: Jun 2008
Posts: 4,715
Likes: 89
From: Tasmania, Australia
Thanks for that logical analysis Jon. Tightening by degrees makes a bit more sense now. It would be interesting to know where in that plastic phase range the studs are (how close to failure they are), what conditions lead to permanent stretch and the consequent need for replacement, and how you can assess if replacement is warranted/necessary. Someone at Porsche came up with that precise specification .... it'd be nice to know the rationale behind it.
I've always done a tighten up ... release .. and then final tighten down to spec on all motors I've done that to get the variable of gasket compression out of the way ... what are everyones thoughts on that for OEM? ... or have Porsche already factored that in?
I've always done a tighten up ... release .. and then final tighten down to spec on all motors I've done that to get the variable of gasket compression out of the way ... what are everyones thoughts on that for OEM? ... or have Porsche already factored that in?
Last edited by Dave928S; Jun 17, 2010 at 06:51 AM.
Supercharged
Rennlist Member
Rennlist Member
Joined: May 2002
Posts: 18,926
Likes: 88
From: Back in Michigan - Full time!
Bearing in mind that the modulus of elasticity of steel (pretty much of any variety) does not vary from around the 205-210E3 N/mm2 region. But the stress at the top of the elastic portion of the stress strain curve can vary by a factor of three from the weakest (say G4.6) to the strongest commonly used (G12.9)
Tightening by degrees (to produce a specific extension of the stud) pretty much eliminates the frictional effects that can result in studs tightened by torqueing delivering varying clamping forces.
Greg Browns observations (varying torques when tightened by degrees) and those of BC (empirical torque values with ARP studs unless BC is a 5 stone weakling LOL) lead me to conclude that the OEM studs are designed to yield (transition from elastic phase to plastic phase) and sit in the plastic phase, while the ARP studs probably remain in the elastic phase.
This means that the OEM studss will exert a reasonably constant force or compression on the head gasket even though temperature rises (alu head growing more than the steel stud) causing the stud to be stretched. In contrast the ARP stud will exert a greater compression as temps rise. Thus the need to re-torque the ARP studs after a heat cycle becuase extra force in the heat cycle will have compressed the gasket and reduced the overall height of the compressed parts thus reducing the clamping force. I am presuming that gasket compression is a one time deal and has a finite limit. It should thus be possible to simulate the heat cycle by overtightening the ARP studs - leaving all to settle then loosening and retorquing. Assuming that you don't pull the threads out of the block - which I believe is more likely a limiting factor in all of this.
Tightening by degrees (to produce a specific extension of the stud) pretty much eliminates the frictional effects that can result in studs tightened by torqueing delivering varying clamping forces.
Greg Browns observations (varying torques when tightened by degrees) and those of BC (empirical torque values with ARP studs unless BC is a 5 stone weakling LOL) lead me to conclude that the OEM studs are designed to yield (transition from elastic phase to plastic phase) and sit in the plastic phase, while the ARP studs probably remain in the elastic phase.
This means that the OEM studss will exert a reasonably constant force or compression on the head gasket even though temperature rises (alu head growing more than the steel stud) causing the stud to be stretched. In contrast the ARP stud will exert a greater compression as temps rise. Thus the need to re-torque the ARP studs after a heat cycle becuase extra force in the heat cycle will have compressed the gasket and reduced the overall height of the compressed parts thus reducing the clamping force. I am presuming that gasket compression is a one time deal and has a finite limit. It should thus be possible to simulate the heat cycle by overtightening the ARP studs - leaving all to settle then loosening and retorquing. Assuming that you don't pull the threads out of the block - which I believe is more likely a limiting factor in all of this.
Best post of 2010! Serious.
This really makes a lot of sense to me - and explains why re-use of old studs/bolts will result in lower torque values and clamping force. But it also makes me cringe a bit becasue I re-used my head bolts. Well, I'm not going back in, so hopefully they will last.
Nordschleife Master
Joined: Oct 2006
Posts: 8,575
Likes: 5
From: Orange, Cal
Best post of 2010! Serious.
This really makes a lot of sense to me - and explains why re-use of old studs/bolts will result in lower torque values and clamping force. But it also makes me cringe a bit becasue I re-used my head bolts. Well, I'm not going back in, so hopefully they will last.
Rennlist Stories
The Best Porsche Posts for Porsche Enthusiasts
9 Vehicles Porsche Helped Engineer that Aren't Porsches
Verdad Gallardo
9 Features and Characteristics That Only Porsche People Understand
Verdad Gallardo
I've Written 500 Rennlist Articles: Here's How Porsche Has Changed Along the Way
Joe Kucinski
10 Most Unnecessary Porsches Ever Built (And Why We Love Them)
Verdad Gallardo
Porsche 911 GT3 S/C vs 718 Spyder RS: 10 Categories, One Winner
Joe Kucinski
This Builder Is Turning Heads With Its Slantnose 911 Creation
Verdad Gallardo
Porsche 911 GT3 Artisan Edition Pays Homage to Japanese Culture
Verdad Gallardo
Porsche Reveals Coupe Variant of the Electric Cayenne With a Fresh Look
Verdad Gallardo
10 Porsche Colors That Have More Personality Than Most People
Verdad Gallardo
Thread Starter
Former Vendor
Joined: Feb 2005
Posts: 15,228
Likes: 2,530
From: Anaheim
Best post of 2010! Serious.
This really makes a lot of sense to me - and explains why re-use of old studs/bolts will result in lower torque values and clamping force. But it also makes me cringe a bit becasue I re-used my head bolts. Well, I'm not going back in, so hopefully they will last.
Thinking back, we had terrible head gasket problems with the 924s. Porsche switched head bolts from a style that was solid in the middle, to a bolt that yielded (threaded all the way), much like a 928 bolt. This helped these cars, quite a bit. I'm not sure if these were re-usable, or not. I'll have to look back.
Used 928 head bolts vary so much that I've found them impossible to re-use, unless they are very, very new.....like you just bolted a head on and forgot the pistons and have to remove it.
Supercharged
Rennlist Member
Rennlist Member
Joined: May 2002
Posts: 18,926
Likes: 88
From: Back in Michigan - Full time!
I've been fine for almost 2 years now... time to buy a lottery ticket?
Well, I'm convinced, I'll not re-use the bolts (on a boosted engine... N/A stock engine would pro'lly be just fine) unless they are virtually new. This has been a very enlightening thread.
On the 944 turbos, the only way we could keep the heads gaskets from blowing was to replace the studs everytime they were loosened.
Thinking back, we had terrible head gasket problems with the 924s. Porsche switched head bolts from a style that was solid in the middle, to a bolt that yielded (threaded all the way), much like a 928 bolt. This helped these cars, quite a bit. I'm not sure if these were re-usable, or not. I'll have to look back.
Used 928 head bolts vary so much that I've found them impossible to re-use, unless they are very, very new.....like you just bolted a head on and forgot the pistons and have to remove it.
Thinking back, we had terrible head gasket problems with the 924s. Porsche switched head bolts from a style that was solid in the middle, to a bolt that yielded (threaded all the way), much like a 928 bolt. This helped these cars, quite a bit. I'm not sure if these were re-usable, or not. I'll have to look back.
Used 928 head bolts vary so much that I've found them impossible to re-use, unless they are very, very new.....like you just bolted a head on and forgot the pistons and have to remove it.
Rest in Peace
Rennlist Member
Rennlist Member
Joined: May 2006
Posts: 9,903
Likes: 2
From: Bird lover in Sharpsburg
For the price of a set of bolts VS redoing a head gasket it is silly not to use new bolts on a 928.
But people do it all the time.
But people do it all the time.
On the 944 turbos, the only way we could keep the heads gaskets from blowing was to replace the studs everytime they were loosened.
Thinking back, we had terrible head gasket problems with the 924s. Porsche switched head bolts from a style that was solid in the middle, to a bolt that yielded (threaded all the way), much like a 928 bolt. This helped these cars, quite a bit. I'm not sure if these were re-usable, or not. I'll have to look back.
Used 928 head bolts vary so much that I've found them impossible to re-use, unless they are very, very new.....like you just bolted a head on and forgot the pistons and have to remove it.
Thinking back, we had terrible head gasket problems with the 924s. Porsche switched head bolts from a style that was solid in the middle, to a bolt that yielded (threaded all the way), much like a 928 bolt. This helped these cars, quite a bit. I'm not sure if these were re-usable, or not. I'll have to look back.
Used 928 head bolts vary so much that I've found them impossible to re-use, unless they are very, very new.....like you just bolted a head on and forgot the pistons and have to remove it.
Thread Starter
Former Vendor
Joined: Feb 2005
Posts: 15,228
Likes: 2,530
From: Anaheim
I use bolts on all my hot rod engines. Mike Simard has beautiful (read: works of art) studs that probably are better. Mike thinks his stuff through, working out any problems/variables. I've not had any problems with new bolts, so I've had no reason to use his studs....but the first time I have an issue, that's who I'm going to call.
Burning Brakes
Joined: Nov 2008
Posts: 908
Likes: 2
From: West Vancouver B.C.
Thank you Greg , I used Mike Simards girdle and block studs on the short block , the engine builder loved it . As far as head studs , sounds like new porsche , or Mikes studs . Short cuts and cheep outs always seem to bit you
Thread Starter
Former Vendor
Joined: Feb 2005
Posts: 15,228
Likes: 2,530
From: Anaheim
Almost any engine that "hurts" one head gasket is going to "hurt" both, so it seems like we are always removing both heads. I will pull one head with the engine in the car, but if both have to come off, the engine is coming out.
I actually can't remember the last time I pulled just one head on a damaged engine...
Call MIke and use his studs....you've already seen the lower pieces.
Art.