Replace Head Studs?
06-22-2010, 06:03 PM
#91
Drifting
06-22-2010, 09:27 PM
#92
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The 18 long studs were only in Germany. I ordered them "car down" and got them in 5 days. I also ordered another set "regular" for inventory (no hurry...there wasn't a car sitting there waiting for them). That could take a month. Porsche had the short studs in Atlanta...so that only took a week!
06-23-2010, 07:25 PM
#93
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Thread Starter
Well, I'm officially frustrated. I've had the heads installed and removed so many times on this one car, that I'm no longer sure how many times I've done it. Where did I put my set of metric head wingnuts?
I'm on my 4th set of head nuts, my 3rd set of studs, and my 4th set of head gaskets.
I know this for sure: There is no stinking way that old or new Porsche studs will take 20NM followed by 3 separate steps of 90 degree turns, without stretching the crap and yielding some of the studs! Hell, I backed off the specifications to 15nm followed by 3 separate steps of 80 degree turns and it still ruins studs!
Here's what I found out:
The first step of 15nm is fine. The second step of 80 degress makes the nuts somewhere between 35 to 45 ft. lbs. The second step of 80 degrees makes the nuts somewhere between 65 to 75 ft. lbs. The third step takes some of the nuts to 90, some to 105, some to 65, and both of the "short studs" on the upper left corners to 50 ft.lbs. When I loosen the nuts, some take 3/4 of a turn to loosen to zero torque and some take 1/4 of a turn to loosen to zero torque. That can't be right!
Crap.
Removal of the head (again) and removal of the studs (again) finds that the some of the studs are fine and some of the studs are now longer. The two short corner studs are 1.5mm (.060" longer). Those yielded and are junk, for sure.
Looking back at the Porsche specifications for torque, the early engines get torqued to 15, 37, and then 66 ft.lbs. This is just about the same as 20nm, followed by 2 steps of 90 degrees.
I think the third step of 90 degrees is nothing but a huge misprint that they kept repeating. None of the other water cooled cars do this (from 924 through 968)...and they should all basically have the same pieces.
I'm going to start over, one more time...and torque the fricking thing to the early specification. Got to be better than stretching the studs into junk.
I'm on my 4th set of head nuts, my 3rd set of studs, and my 4th set of head gaskets.
I know this for sure: There is no stinking way that old or new Porsche studs will take 20NM followed by 3 separate steps of 90 degree turns, without stretching the crap and yielding some of the studs! Hell, I backed off the specifications to 15nm followed by 3 separate steps of 80 degree turns and it still ruins studs!
Here's what I found out:
The first step of 15nm is fine. The second step of 80 degress makes the nuts somewhere between 35 to 45 ft. lbs. The second step of 80 degrees makes the nuts somewhere between 65 to 75 ft. lbs. The third step takes some of the nuts to 90, some to 105, some to 65, and both of the "short studs" on the upper left corners to 50 ft.lbs. When I loosen the nuts, some take 3/4 of a turn to loosen to zero torque and some take 1/4 of a turn to loosen to zero torque. That can't be right!
Crap.
Removal of the head (again) and removal of the studs (again) finds that the some of the studs are fine and some of the studs are now longer. The two short corner studs are 1.5mm (.060" longer). Those yielded and are junk, for sure.
Looking back at the Porsche specifications for torque, the early engines get torqued to 15, 37, and then 66 ft.lbs. This is just about the same as 20nm, followed by 2 steps of 90 degrees.
I think the third step of 90 degrees is nothing but a huge misprint that they kept repeating. None of the other water cooled cars do this (from 924 through 968)...and they should all basically have the same pieces.
I'm going to start over, one more time...and torque the fricking thing to the early specification. Got to be better than stretching the studs into junk.
06-23-2010, 07:28 PM
#94
Thanks for the info Greg. This may not matter, but what color are the studs?
06-23-2010, 07:30 PM
#95
What is considered a good torque? Can someone just go with, say, 75lbs and be done? 75lbs after a 20lb torque, sit, release, retorque at 20, let sit, torque to 50, and let sit, then torque to 75?
06-23-2010, 08:13 PM
#96
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I would say the short studs are definitely yielding - (Dunno how much shorter they are) but if it was say half the length of the long studs then the tightening process should be calling for a total angle turned that is half that of the long studs to achieve the same clamping force.
Are you certain that the washers under the nuts aren't turning on the head ? (IIRC the washer should be roughened underneath to prevent this happening) My understanding is that if this happens you reduce the frictional losses at the nut/washer/head interface and in so doing the stud can untwist after you remove the torque wrench thus altering the applied torque.
Do we know what grade the studs are and how long they are - based on x number of turns and thread pitch it's a fairly simple calculation to figure if they are supposed to be stretching into the yield zone or not.
Are you certain that the washers under the nuts aren't turning on the head ? (IIRC the washer should be roughened underneath to prevent this happening) My understanding is that if this happens you reduce the frictional losses at the nut/washer/head interface and in so doing the stud can untwist after you remove the torque wrench thus altering the applied torque.
Do we know what grade the studs are and how long they are - based on x number of turns and thread pitch it's a fairly simple calculation to figure if they are supposed to be stretching into the yield zone or not.
06-23-2010, 08:27 PM
#97
Former Sponsor
Thread Starter
I would say the short studs are definitely yielding - (Dunno how much shorter they are) but if it was say half the length of the long studs then the tightening process should be calling for a total angle turned that is half that of the long studs to achieve the same clamping force.
Are you certain that the washers under the nuts aren't turning on the head ? (IIRC the washer should be roughened underneath to prevent this happening) My understanding is that if this happens you reduce the frictional losses at the nut/washer/head interface and in so doing the stud can untwist after you remove the torque wrench thus altering the applied torque.
Do we know what grade the studs are and how long they are - based on x number of turns and thread pitch it's a fairly simple calculation to figure if they are supposed to be stretching into the yield zone or not.
Are you certain that the washers under the nuts aren't turning on the head ? (IIRC the washer should be roughened underneath to prevent this happening) My understanding is that if this happens you reduce the frictional losses at the nut/washer/head interface and in so doing the stud can untwist after you remove the torque wrench thus altering the applied torque.
Do we know what grade the studs are and how long they are - based on x number of turns and thread pitch it's a fairly simple calculation to figure if they are supposed to be stretching into the yield zone or not.
No grade marking on studs.
Agree that the short studs should have a different stretch factor, due to the shorter length...Porsche missed that.
06-23-2010, 08:27 PM
#98
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06-23-2010, 08:30 PM
#99
Former Sponsor
Thread Starter
I usually convert all the engines to head bolts. Most of my work is with later engines. Been quite some time since I dug into a 2 valve engine. I'll go back and see what specifications I used on the others I've done.
06-23-2010, 08:34 PM
#100
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06-23-2010, 08:53 PM
#101
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06-23-2010, 08:58 PM
#102
06-23-2010, 09:14 PM
#103
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06-23-2010, 09:29 PM
#104
I waited quite a while between turns. I also (for better or worse) used NO threadlocker on the studs because I was afraid of having some issue with... 100% of the stuff I have changed, modified, or replaced and not being able to pull a head quickly in the car.
And the last variable - its a cometic gasket. I let it compress over a three week period - simply because I had the time.
I may have screwed up, but I will not know until the first time I hit 24psi, which is what the supercharger from Todd is geared to make.
Of course, maybe they have all stretched. It sure didn't SEEM like it was stretching when I had to have my wife counter hold the engine against the wall on the stand as I turned the bar.
06-23-2010, 10:04 PM
#105
Drifting