Hey guys,
I've been wondering why a stud is stronger than a bolt. I mean in general, not specifically Head Bolts.
It seems like a stud would be the same strength as a bolt, if they are both Grade 8.
The only advantage I can think of is extra threads are being utilized, to spread the strain when the fastener stretches.
A bolt has only one section of threads holding it into place, with the head on top, fastening down the part.
A stud has the same section of threads holding it into place, while the nut is providing another few threads to hold down the part.
As a matter of fact, my Dad and I discussed the topic, and came to the conclusion that a stud will probably be weaker and less reliable. Thick about it, a bolt has only one place for it to loosen while a stud has two potential places to loosen. When the vibrations start stretching out the fastener, a bolt will only loosen in one area while the stud will loosen in two areas.
I see studs used for the Heads and Intake Manifolds. I've always taken the strength information for granted.
Recently, members on a Jeep forum I belong to have advised using studs for the lock-out hubs because the studs are stronger than a bolt. But they could never tell me why. I am eager to learn new information any time, and will listen, as long as there is good logic behind it.

 

it is not loosening it is stretching. arp bolts and studs takes a lot more pressure to stretch than a stock bolt.

 

Actually to get right down to it there is a couple factors that make studs stronger then bolts. I know this stuff in my head... let's see if I can get it out properly lol.

First one being the head of the bolt, when you machine metal up to a corner like that (going from threaded shaft to head) you automatically create a week point. Machinist always put a radius at this point to reduce the stress. Notice any bolt or shaft has a radius in the corner, you will seldom see a sharp 90 degrees. This does not eliminate the stress point it just reduces it!

With thread's 90% of their holding grip is done on the first three threads, that is why nuts are usually 5 to 6 threads max on a coarse thread.

So the the area of holding on those 3 threads is much greater then the corner were the bolt head would be on a normal bolt and much stronger.

 

In the case of a Head Bolt, the bolt is receiving max. pressure while running, more at full throttle, and a LOT more with a turbo.
I figure as the head tries to lift off the block, the bolt wil stretch a little TINY bit. After a long time, the bolt will be stretched out enough to possibly create a situation for the threads to loosen. Why would an ARP bolt or stud take more pressure? Are they stronger than Grade 8?

That's cool information! I never thought of that, but it sure makes sense now!
A bolt has 3 threads doing the hard work, with a "weak link" at the head. Essentially, a stud has 6 theads doing the hard work, with extras for "strength in numbers".
Another question now...why are there fine thread bolts?
I always just figured "strength in numbers". Now that you mention the first 3 threads hold most of the weight, I wonder why fine threads even exist?
Thank you, excellent lesson to learn!

 

I have never even looked into ARP studs but if I had to guess I would say they are stronger then grade 8. It is possible to get stronger with proper heat/chemical treatments.

The main reason for fine threads is to be able to get the needed 3 threads into a smaller area thus being able to use shorter bolts & thinner nuts when needed. Bearing nuts are generally fine threads for this reason. I have never seen anything saying one is stronger then the other.

 

ARP and others are selling on the basis of high or higher tensile strength, or the ability to be pulled apart before stretching. That's why they are more desirable for stretch resistance. Which is also why majority of the time you will up the torque value to increase clamping load without the increase chance of stretching the bolt or stud.

 

Warooz your absolutly right about not having a 90 degree corner. I work in the aircraft industry and we never machine a corner without a radius, never. That is where the part will break, at the corner without a radius. If you make a part without a radius that's where a crack will begin.

 

Speeking of cracks, if you have a crack in steel to stop it you drill a hole at either end and it will stop the spread. Then you could weld it up.

Here's a little from ARP:

• ARP HEAD STUD KITS

Obtaining the optimum cylinder head-to-block sealing is especially critical in small displacement engines employing high compression pistons or power adders like turbochargers, nitrous oxide and superchargers. That’s why ARP head studs are popular among leading Sport Compact/Import racers.
You should know that ARP uses a premium grade 8740 alloy that is rated far superior to “aircraft” quality. Then, each stud is precisely heat-treated to 200,000 psi. Following heat-treat, each stud is centerless ground to make it as close to perfectly concentric as possible. This procedure involves about ten very slight cuts and results in an exceptionally straight part. It’s important to note that lesser quality studs are not even centerless ground – the material is thread rolled in bar stock form (mostly before heat-treat, when the material is easier to machine). Because ARP studs are manufactured to such exacting tolerances, you will note that gaskets and cylinder heads literally glide into position and are perfectly aligned – something that won’t happen with inferior quality head studs.
ARP studs are thread rolled after heat-treat, which gives them about 1000% (that’s ten times) better fatigue strength than those studs that are threaded prior to heat-treat. You will also note that ARP offers specially undercut studs for several engines. This procedure (done only to the shorter studs) more equalizes the “stretch” of both studs, which makes for a more consistent clamping force – one that compensates for head gasket compression when the head is installed. This helps prevent blown head gaskets, and assures optimum sealing!
Premium parallel ground washers are also included with each kit.

• ARP HEAD BOLTS KITS

ARP Head Bolt Kits are offered in two configurations for optimal performance.
HIGH PERFORMANCE SERIES
High Performance head bolts are made of 8740 chrome moly and available with a reduced wrenching hex or 12-point with a wide area flanged head. They are nominally rated at 180,000 psi and kits come complete with hardened parallel-ground washers.
PROFESSIONAL SERIES
All Pro Series bolts are designed for competition applications and are rated nominally at 200,000 psi. Available with undercut short bolts that can help eliminate head gasket failures.

 

Edit: Bah I didn't get right the first time.

I spent 15 years doing aerospace machining. Now doing transit case work for the defense industry.

 

That explains why they are so strong. Aside from that massive heat treating thread rolling also known as thread forming is much stronger then cut threads. The idea is instead of removing material, which would weaken the shaft you simply displace the material.

If I put on a turbo I will for sure spend the extra cash on these head bolts.

 

Studs are not stronger then bolts, they give more clamping force, by the fact that all the force is clamping the head down not TWISTING the bolt shaft.

 

OK there not stronger if they are made the same.

 

Bolts and studs are not and cannot be made the same. To make a bolt you HAVE to start with bar stock the size of the bolt head or bigger and then machine down the threaded part of the shaft. That machining is what inherently makes them weaker.

 

LMAO yea were talking arp not all thread.

 

OK starmilt are the stronger if both are grade 8? some studs threads are turned, but the REAL strength comes from the the way they don't twist when tightend....right???