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Why exhaust studs and not bolts?

I am curious as to why the 911 motor used exhaust studs with nuts to secure the exhaust system. Why not bolts? Is it only because on some studs barrel nut fasteners that must be used b/c therre is limited space with the heat exchangers?
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Old 03-01-2003, 07:12 AM
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If that were the only reason...you could use allen head bolts (small heads).
If you have bolts...and they go in and out several times...you will need to repair the holes on a regular basis.
I have wondered in the past why not use stainless steel studs??
The are strong enough for headers....will not corrode...and if you use brass nuts, they will not seize.
Bob
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Old 03-01-2003, 07:17 AM
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it's probably a matter of them freezing in the head, like studs do. at least with studs, you can heat the nut and remove it, and it's fine if the stud just stays in the head like it's supposed to do. with a frozen bolt, there's no way to heat it, and it would twist off, requiring a lot of drilling, tapping, inserting, and often ruining the head in the process. not that studs don't break, but it's not that often.
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Old 03-01-2003, 08:15 AM
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Paul, I posed a similar question a while back. If I recall, the responses had to do with the *stretching* ability of the studs and tinsel strength. Also for reasond JW mentions...

Here's the link..

Exhaust: use hex bolts instead of studs?
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Old 03-01-2003, 08:38 AM
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THe BOLTs are torque to yield so that they stretch after the first torque. Then they are never reusable any more. The studs do not stretched like bolts and do not destroy the holes like bolts do.
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Old 03-01-2003, 08:51 AM
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Quote:
Originally posted by yelcab1
THe BOLTs are torque to yield so that they stretch after the first torque.. . .
That's a stretch.
Most bolts are NOT used that way.
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Old 03-01-2003, 09:05 AM
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I might as well throw in my 2 cents. Blast away

Bolts are made to stretch. That is what keeps them snug.

Now the stud vs bolt. I saw a guy from ARP on Shadetree Mechanic that was talking about the difference. I can't remember exactly what he said but it had something about a bolt has a stretch and a twist at the same time when it is tightened. A stud is not being twisted so it works better than a bolt.

Make sense?
Old 03-01-2003, 12:15 PM
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Fasteners, unless they're specifically designed to take a permanent "stretch", are seldom torqued to the yield point. Exhaust manifold fasteners are not the "stretch to yield" type. In addition, most automotive fasteners are not of this type with the exception of the connecting rod and flywheel bolts on 911s and cylinder head fasteners on other cars.

I agree that stainless steel would be an ideal material for the exhaust studs as corrosion is an issue with anthing on the underside of the engine. If bolts are used in this application, not matter the material, the threads in the cylinder head (aluminum) will eventually fatigue.

Bolts, studs and nuts all work on the same principle of the inclined ramp.

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Old 03-01-2003, 01:04 PM
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too many beers...

anyway: explanation ... bolts: in-and-out action trough treads = no good

studs = screwed in with finger-action, torqued nuts by torque-wrench = less wear in the head treads = cheaper in the long term


studs = KINDA better
nuts = not good in engineers eyes
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Old 03-01-2003, 01:14 PM
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Cool Studs vs Bolts

Actually, IMHO, the reason bolts are used instead of studs is to
keep from over loading the threads in those expensive aluminum
Porsche cylinder heads. Using a bolt as a fastener in a soft metal
is ok if there is not much load or torque put on it, but while tightening a bolt into soft threads, the first few turns of the threads generally have to take too much load and will strip out. However, with a stud, the threads are fully engaged prior to any
load being applied. When the nut is torqued, the load is evenly distributed over all the threads. Just a thought.

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Old 03-01-2003, 03:40 PM
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Cool

Actually a lot of the previous answers make sense to me. I figured that Porsche just used them to raise our exasperation level.

I like the use of brass nuts and make sure the barrel nuts are clean and use antiseize on the threads.

Good luck,
David Duffield
Old 03-01-2003, 04:02 PM
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Quote:
Originally posted by philfran
I might as well throw in my 2 cents. Blast away

Bolts are made to stretch. That is what keeps them snug.

Now the stud vs bolt. I saw a guy from ARP on Shadetree Mechanic that was talking about the difference. I can't remember exactly what he said but it had something about a bolt has a stretch and a twist at the same time when it is tightened. A stud is not being twisted so it works better than a bolt.

Make sense?
Nope! Only if there is no thread friction does the stud see zero torsional stress. All real materials exhibit friction. When the nut is tightened down it generates torsional stress in the stud. The torsional stress relaxes (edit gradually) to zero in both studs and bolt or screws as soon as the tightening is complete. Cheers, Jim

Last edited by Jim Sims; 03-01-2003 at 08:48 PM..
Old 03-01-2003, 04:35 PM
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From the ARP web site. These guys I trust

We are talking head bolts and not header bolts in the following.

Stud vs bolt:

ARP®’s factory Tech Representatives are often asked which is better, cylinder head studs or bolts. The answer, invariably, depends on the installation. On many street-driven vehicles, where master cylinders and other items protrude into the engine compartment, it’s probably necessary to use head bolts so that the cylinder heads can be removed with the engine in the car.

For most applications, however, studs are recommended. And for good reason. Using studs will make it much easier to assemble an engine (especially a racing powerplant which must be serviced frequently and quickly!) with the cylinder head and gasket assured of proper alignment.

Studs also provide more accurate and consistent torque loading. Here’s why. When you use bolts to secure the head, the fastener is actually being “twisted” while it’s being torqued to the proper reading. Accordingly, the bolt is reacting to two different forces simultaneously. A stud should be installed in a “relaxed” mode—never crank it in tightly using a jammed nut.

If everything is right, the stud should be installed finger tight. Then, when applying torque to the nut, the stud will stretch only on the vertical axis. Remember, an undercut shorter stud will have a rate similar to a longer, standard shank stud. This provides a more even clamping force on the head. Because the head gasket will compress upon initial torquing, make sure studs and bolts are re-torqued after the engine has been run.

Bolt or stud stretch: Notice they say fastener.

It is important to note that in order for a fastener to function properly it must be “stretched” a specific amount. The material’s ability to “rebound” like a spring is what provides the clamping force. You should know that different materials react differently to these conditions, and ARP® engineers have designed each fastener to operate within specific ranges.
Old 03-01-2003, 05:18 PM
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I guess I'd have to say ARP is wrong on this point. There's just no way to totally avoid torsion using a moving screw thread to tension a fastener. A nut's friction generates a traction on the stud's threads at the thread diameter which produces a torque about the centerline. This torque produces a torsional stress in the stud that climbs with the increasing axial tension (which is producing the tensile or clamping stress). Once the nut stops moving the torsional stress (edit gradually) disappears. This condition of combined torsional and axial stress is often the highest a fastener will ever experience which leads to the principle that if a fastener doesn't fail during tightening it isn't likely to fail under it's service tensile stress. Studs can be tightened, and tightened very accurately, with essentially zero torsional stress but it requires a hydraulic stud tensioner which pulls on the stud while the nut is being run down with no load on the nut. I have used such a system combined with a temperature compensated ultrasonic extensiometer to tension hardened 2-1/4" diameter 4140 studs on a 72 ft long rail gun (electromagnetic launcher). Like trying to tension a 72 ft long engine head, talk about fastener cross-talk and tensioning patterns! Cheers, Jim

Last edited by Jim Sims; 03-01-2003 at 08:46 PM..
Old 03-01-2003, 05:50 PM
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Cool

I love Jim's threads
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Old 03-01-2003, 06:02 PM
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I agree that you cannot eliminate torsional load but might not a stud exhibit less that a bolt?

ARP states to install the stud finger tight. No torsional load to the fastener at any point during installation. The nut would then have some torsional load while it is tightened.

A bolt, on the other hand, might have more torsional load due to the fact that it "stops" when bottomed and then when the correct torque is applied it would tend to twist more?

Just guessing here.

Cheers to you also
Old 03-01-2003, 06:06 PM
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Some of the tightening torsion is "locked in" due to friction under the bolt head or under the nut; if there is a lock washer under the head or nut it is even more locked. After a few load cycles the torsion usually relaxes and goes completely to zero. On some critical fastener applications the torsional stress is deliberately relieved by immediately turning the nut or bolt back slightly (around 1/8 of a turn) after tightening to specified torque. The studs may go to zero torsional stress more readily due to easier movement of the nut as opposed to the fixed head of a bolt or screw. This may be what ARP means.

Now why are "stretch" bolts used in connecting rods and flywheel screws? I have a theory having to do with the stress-strain diagram and achieving consistent and uniform clamping loads. I'm not sure though. Does anyone know why these bolts are torqued to yield? Cheers, Jim
Old 03-01-2003, 08:41 PM
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Jim,

Since you are in Los Alamos you must be a very technical person so you might like the following read on the ARP site. It is beyond me in most cases

http://www.arp-bolts.com/pages/tech/fastener.html

Phil
Old 03-02-2003, 02:55 AM
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Quote:
Originally posted by Jim Sims
a 72 ft long rail gun (electromagnetic launcher).
heehee...........do i even want to ask
great thread
Old 03-02-2003, 06:35 AM
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I would advise against using stainless studs on the aluminum case (and stainless for any exhaust application). The aluminum and stainless steel become a ganvanic couple with the aluminum being the anode and the stainless being the cathode. Since there is high potential difference between these two metals, the aluminum will corrode at a much faster rate than if mild steel is used.

Using austinetic stainless steel like a 309 or 316 is not recommended for fasteners especially on high temp applications. It's high temperature strength is not good and it galls easily. A martensitic stainless steel like 403 or 422 is a good high temp bolting material, but it's not readily available. FYI, martensitic stainless steel is magnetic.
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