Replace Head Studs?
02-18-2016, 02:38 PM
#196
Rennlist Member
IIRC...studs are better...
Because the stud threads go into the block relaxed..which is important..
As you tighten the nut..the stud distributes force better along more threads, but the nut...is more efficient because only about 6 threads of a bolt or a nut when torque is allied to it, do anything.
Past 6 threads, there is almost no grip at all.
A long bolt as you tighten it, grips no more than the nut.
Read that in an engineering book a long time ago...
Because the stud threads go into the block relaxed..which is important..
As you tighten the nut..the stud distributes force better along more threads, but the nut...is more efficient because only about 6 threads of a bolt or a nut when torque is allied to it, do anything.
Past 6 threads, there is almost no grip at all.
A long bolt as you tighten it, grips no more than the nut.
Read that in an engineering book a long time ago...
02-19-2016, 03:49 PM
#198
Former Sponsor
Thread Starter
02-19-2016, 03:55 PM
#199
Rennlist Member
02-19-2016, 05:10 PM
#200
Former Sponsor
Thread Starter
You should ask Mike.....no stock specification will apply.
Porsche's original specifications put the fasteners (studs and bolts) into stretch, without putting them into yield. This design is extremely good, since the head gasket is "pinched" down both hot and cold. (Note that this applies to both 928 and 951 models.)
When aftermarket companies started making higher quality head stud kits for the 951 models, I tried them, of course. The aftermarket companies used the same torque specifications as the stock application, however because the material was so much better, the stock torque specifications did not stretch the hardware. It was virtually impossible to retain the head gaskets on the 951 engines, using this hardware, simply because the studs were not stretched.
To get that hardware to stretch, the torque required to get them into the required stretch was so high that the threads simply would not hold in the aluminum blocks. Many people went to 14mm hardware to increase the torque.....again, they could not get the hardware into stretch, but they compensated by increasing the torque. The 944 GTR engines used huge studs (.625")....and the heads would actually crack sometimes, when tightening them down.
Mike's a very smart engineer/machinist. I'm sure he knows exactly where his hardware needs to be torqued to apply the proper preload.
02-19-2016, 05:25 PM
#201
Rennlist Member
Stretch is needed to keep constant force when hot or cold?
the 3- 90s vs the 2- 90s is the nuts vs the bolts, right? and is that because the threads deform more on the nuts vs the bolts?
the 3- 90s vs the 2- 90s is the nuts vs the bolts, right? and is that because the threads deform more on the nuts vs the bolts?
You should ask Mike.....no stock specification will apply.
Porsche's original specifications put the fasteners (studs and bolts) into stretch, without putting them into yield. This design is extremely good, since the head gasket is "pinched" down both hot and cold. (Note that this applies to both 928 and 951 models.)
When aftermarket companies started making higher quality head stud kits for the 951 models, I tried them, of course. The aftermarket companies used the same torque specifications as the stock application, however because the material was so much better, the stock torque specifications did not stretch the hardware. It was virtually impossible to retain the head gaskets on the 951 engines, using this hardware, simply because the studs were not stretched.
To get that hardware to stretch, the torque required to get them into the required stretch was so high that the threads simply would not hold in the aluminum blocks. Many people went to 14mm hardware to increase the torque.....again, they could not get the hardware into stretch, but they compensated by increasing the torque. The 944 GTR engines used huge studs (.625")....and the heads would actually crack sometimes, when tightening them down.
Mike's a very smart engineer/machinist. I'm sure he knows exactly where his hardware needs to be torqued to apply the proper preload.
Porsche's original specifications put the fasteners (studs and bolts) into stretch, without putting them into yield. This design is extremely good, since the head gasket is "pinched" down both hot and cold. (Note that this applies to both 928 and 951 models.)
When aftermarket companies started making higher quality head stud kits for the 951 models, I tried them, of course. The aftermarket companies used the same torque specifications as the stock application, however because the material was so much better, the stock torque specifications did not stretch the hardware. It was virtually impossible to retain the head gaskets on the 951 engines, using this hardware, simply because the studs were not stretched.
To get that hardware to stretch, the torque required to get them into the required stretch was so high that the threads simply would not hold in the aluminum blocks. Many people went to 14mm hardware to increase the torque.....again, they could not get the hardware into stretch, but they compensated by increasing the torque. The 944 GTR engines used huge studs (.625")....and the heads would actually crack sometimes, when tightening them down.
Mike's a very smart engineer/machinist. I'm sure he knows exactly where his hardware needs to be torqued to apply the proper preload.
02-19-2016, 06:28 PM
#202
Three Wheelin'
So after reading this thread and others I'm not quite clear. Is current best practice for an S3 reusing the studs 20NM + 3 x 90 degrees, then wait a day and remove/reapply 90 degrees?
02-19-2016, 10:01 PM
#204
Rennlist Member
Rennlist Site Sponsor
Somewhat off topic, but related...
I remember some very special fasteners (but can't remember the application - getting old sucks!) from years ago. The bolts were basically hollow tubes. Inside was a precise steel rod with the bottom anchored at the very end of the threaded part of the bolt, while the exposed end of the rod was a few thousandths proud of the bolt head. You simply torqued the bolt until the rod was flush with the bolt head - resulting in perfect stretch in the fastener. You could use an open-end wrench if you wanted to. Only problem was an exorbitant cost - I don't remember the numbers, but I remember swallowing several times...
I remember some very special fasteners (but can't remember the application - getting old sucks!) from years ago. The bolts were basically hollow tubes. Inside was a precise steel rod with the bottom anchored at the very end of the threaded part of the bolt, while the exposed end of the rod was a few thousandths proud of the bolt head. You simply torqued the bolt until the rod was flush with the bolt head - resulting in perfect stretch in the fastener. You could use an open-end wrench if you wanted to. Only problem was an exorbitant cost - I don't remember the numbers, but I remember swallowing several times...
02-19-2016, 10:08 PM
#205
Former Sponsor
Thread Starter
I wish it was simple....but here is what will work 90% of the time.
Original studs, in good condition....not rusted. New "wide" head nuts:
Follow the factory manual for angle tightening. If any of the washers rotate at any time, start over.
If you have one bad stud, you will need to repalce all of the studs on that one side....becasue the new studs from Porsche are not like the old studs.
New Factory studs. "Wide" replacement head nuts:
Follow the factory manual for a very early engine....torque instead of angle tightening. If any of the washers rotate at any time, start over. If you attempt to follow the proceedure for angle tightening, order another set of studs....you just ruined the first set.
Late model 928 with bolts:
Replace the bolts, follow the factory instructions for angle tightening. If any of the washers rotate at any time, start over.
Re-using my original nuts, because they look fine, and I'm cheap:
Good luck in the hell where crappy mechanics go. Do not refer to what you just did as "freshened the top end", "valve job", or "rebuilt".
Using the "old" bolts, because they look fine and I'm cheap:
You might get away with this....but you need to have a torque wrench that gives you a torque reading after the 90 degree angle. If your torque readings vary more than about 20 ft. lbs, see above....or buy new bolts.
"Aftermarket hardware": Consult with the manufacturer of the hardware. Ask them how much stretch they designed into the hardware at whatever torque they tell you to use. If they can't answer that question, you are in deep trouble.
Original studs, in good condition....not rusted. New "wide" head nuts:
Follow the factory manual for angle tightening. If any of the washers rotate at any time, start over.
If you have one bad stud, you will need to repalce all of the studs on that one side....becasue the new studs from Porsche are not like the old studs.
New Factory studs. "Wide" replacement head nuts:
Follow the factory manual for a very early engine....torque instead of angle tightening. If any of the washers rotate at any time, start over. If you attempt to follow the proceedure for angle tightening, order another set of studs....you just ruined the first set.
Late model 928 with bolts:
Replace the bolts, follow the factory instructions for angle tightening. If any of the washers rotate at any time, start over.
Re-using my original nuts, because they look fine, and I'm cheap:
Good luck in the hell where crappy mechanics go. Do not refer to what you just did as "freshened the top end", "valve job", or "rebuilt".
Using the "old" bolts, because they look fine and I'm cheap:
You might get away with this....but you need to have a torque wrench that gives you a torque reading after the 90 degree angle. If your torque readings vary more than about 20 ft. lbs, see above....or buy new bolts.
"Aftermarket hardware": Consult with the manufacturer of the hardware. Ask them how much stretch they designed into the hardware at whatever torque they tell you to use. If they can't answer that question, you are in deep trouble.
02-20-2016, 02:16 PM
#206
Rennlist Member
bolt or nuts, and you can get (verify) to near the same torque value by angle torquing and certainly torqued to a specific torque via non angle, what is the drawback by not using new studs or bolts?
I wish it was simple....but here is what will work 90% of the time.
Original studs, in good condition....not rusted. New "wide" head nuts:
Follow the factory manual for angle tightening. If any of the washers rotate at any time, start over.
If you have one bad stud, you will need to repalce all of the studs on that one side....becasue the new studs from Porsche are not like the old studs.
New Factory studs. "Wide" replacement head nuts:
Follow the factory manual for a very early engine....torque instead of angle tightening. If any of the washers rotate at any time, start over. If you attempt to follow the proceedure for angle tightening, order another set of studs....you just ruined the first set.
Late model 928 with bolts:
Replace the bolts, follow the factory instructions for angle tightening. If any of the washers rotate at any time, start over.
Re-using my original nuts, because they look fine, and I'm cheap:
Good luck in the hell where crappy mechanics go. Do not refer to what you just did as "freshened the top end", "valve job", or "rebuilt".
Using the "old" bolts, because they look fine and I'm cheap:
You might get away with this....but you need to have a torque wrench that gives you a torque reading after the 90 degree angle. If your torque readings vary more than about 20 ft. lbs, see above....or buy new bolts.
"Aftermarket hardware": Consult with the manufacturer of the hardware. Ask them how much stretch they designed into the hardware at whatever torque they tell you to use. If they can't answer that question, you are in deep trouble.
Original studs, in good condition....not rusted. New "wide" head nuts:
Follow the factory manual for angle tightening. If any of the washers rotate at any time, start over.
If you have one bad stud, you will need to repalce all of the studs on that one side....becasue the new studs from Porsche are not like the old studs.
New Factory studs. "Wide" replacement head nuts:
Follow the factory manual for a very early engine....torque instead of angle tightening. If any of the washers rotate at any time, start over. If you attempt to follow the proceedure for angle tightening, order another set of studs....you just ruined the first set.
Late model 928 with bolts:
Replace the bolts, follow the factory instructions for angle tightening. If any of the washers rotate at any time, start over.
Re-using my original nuts, because they look fine, and I'm cheap:
Good luck in the hell where crappy mechanics go. Do not refer to what you just did as "freshened the top end", "valve job", or "rebuilt".
Using the "old" bolts, because they look fine and I'm cheap:
You might get away with this....but you need to have a torque wrench that gives you a torque reading after the 90 degree angle. If your torque readings vary more than about 20 ft. lbs, see above....or buy new bolts.
"Aftermarket hardware": Consult with the manufacturer of the hardware. Ask them how much stretch they designed into the hardware at whatever torque they tell you to use. If they can't answer that question, you are in deep trouble.
02-20-2016, 05:24 PM
#207
Nordschleife Master
Thanks, Greg. I don't always get new nuts but have in the past and now bin pick ones with a pretty face. I might not meet St. Peter but I'll be where the party's at.
02-20-2016, 08:22 PM
#208
Former Sponsor
Thread Starter
Now that's confusing. Remember that it's not the torque around the bolt (or stud) that holds the heads down; it's the force down the bolt. That's the entire problem! Measuring stretch is much superior. That's why "angle torquing" works because you're stretching the bolt (and compressing the gasket) a specific amount. Just torque deals with friction of the nut on the threads and on the washer and those vary with condition and lubrication.
That's why the original Raceware head studs for the 944's worked so poorly. The material was very good (too good) and the studs did not stretch at the stock specification. There was certainly torque, but no stretch. Virtually impossible to keep head gaskets intact in engines using this hardware.
The "latest generation" of Porsche studs for the early 928 engines is made to a different specification that the original hardware. Using the stock angle torque instructions, the hardware is stretched beyond its elasticity limit and is "yielded". Once a fastener is yielded, it will continue to stretch at a torque well below it's design specification. (With a torque wrench that records both angles and the resulting torque, this is very easy to see.....if the second 90 degree turn gave you an "ending" torque of 80 ft lbs and the third 90 degree turn gives you an ending torque less than that.....the stud just yielded.) Yielded hardware is junk.
In large diameter hardware, where torque or angle tightening is impossible, they have hydraulics that hook onto the threads of the stud. They then pull the stud to the specification that is given by the engineers that designed the studs, spin the nuts down, and release the hydraulics. Because it is a simple matter to measure how much hydraulic force is being applied, it is a simple matter to know if the stud was yielded. If the required hydraulic pressure suddenly drops....oops....ruined that stud. It's a very cool process to watch, BTW.
02-20-2016, 08:35 PM
#209
Rennlist Member
Exactly!
That's why the original Raceware head studs for the 944's worked so poorly. The material was very good (too good) and the studs did not stretch at the stock specification. There was certainly torque, but no stretch. Virtually impossible to keep head gaskets intact in engines using this hardware.
The "latest generation" of Porsche studs for the early 928 engines is made to a different specification that the original hardware. Using the stock angle torque instructions, the hardware is stretched beyond its elasticity limit and is "yielded". Once a fastener is yielded, it will continue to stretch at a torque well below it's design specification. (With a torque wrench that records both angles and the resulting torque, this is very easy to see.....if the second 90 degree turn gave you an "ending" torque of 80 ft lbs and the third 90 degree turn gives you an ending torque less than that.....the stud just yielded.) Yielded hardware is junk.
In large diameter hardware, where torque or angle tightening is impossible, they have hydraulics that hook onto the threads of the stud. They then pull the stud to the specification that is given by the engineers that designed the studs, spin the nuts down, and release the hydraulics. Because it is a simple matter to measure how much hydraulic force is being applied, it is a simple matter to know if the stud was yielded. If the required hydraulic pressure suddenly drops....oops....ruined that stud. It's a very cool process to watch, BTW.
That's why the original Raceware head studs for the 944's worked so poorly. The material was very good (too good) and the studs did not stretch at the stock specification. There was certainly torque, but no stretch. Virtually impossible to keep head gaskets intact in engines using this hardware.
The "latest generation" of Porsche studs for the early 928 engines is made to a different specification that the original hardware. Using the stock angle torque instructions, the hardware is stretched beyond its elasticity limit and is "yielded". Once a fastener is yielded, it will continue to stretch at a torque well below it's design specification. (With a torque wrench that records both angles and the resulting torque, this is very easy to see.....if the second 90 degree turn gave you an "ending" torque of 80 ft lbs and the third 90 degree turn gives you an ending torque less than that.....the stud just yielded.) Yielded hardware is junk.
In large diameter hardware, where torque or angle tightening is impossible, they have hydraulics that hook onto the threads of the stud. They then pull the stud to the specification that is given by the engineers that designed the studs, spin the nuts down, and release the hydraulics. Because it is a simple matter to measure how much hydraulic force is being applied, it is a simple matter to know if the stud was yielded. If the required hydraulic pressure suddenly drops....oops....ruined that stud. It's a very cool process to watch, BTW.
The studs are about 14" wide, and perhaps 80-90 studs for the cap.
They would stretch about...3/4"..and the nut would come off/on by hand.
02-20-2016, 08:35 PM
#210
Former Sponsor
Thread Starter
I've re-used the special "tall" head nuts on 951 engines, in the past, without issue.....especially if they just got replaced and the engine comes back apart in a short period of time.
I retire any nuts that have been in use for multiple years.