Why are head studs better than head bolts?

[Nasty Wendy]

Just thinking about random stuff here and my mind wonders to this question. Head studs always seam to be the choice method of securing heads to the block. What makes them better than quality head bolts? The more I think about it the more I realize that I do not know. As I wrestle with this question all I can come up with is advantages of studs in an aluminum block. I would think you can safely get more torque on a head stud in an aluminum block than with bolts due to not having to 'spin' the bolts under the torque load and strip the threads in the block. Is that the sole advantage?

 

[regan j]

Roughly the same as fine threads versus course threads. More surface area being used. Studs go to the bottom of the hole and bolts go till they bottom out at the top regardless if there is further to go. Hopefully someone with more technical knowledge will explain better.

 

[Nasty Wendy]

I have not compared the length of a stud to a bolt. I would imagine that ARP would design their bolts to go as deep as possible. I know they can't get all the way in like studs but do they leave enough threads unused to make a difference? Interesting.

 

[earlbrown]

You were on the right track with the varying pitch on the studs. Think of it this way. A threaded fastener is an inclined plane wrapped around a cylinder (technical term: "screwed"). A fine thread will be longer with less slope, where a course thread takes a shorter path with a greater slope.

Lets say you wanted to generate 1000 pounds of clamping load. That means you twist the wedge to the point the inclined plane has 1000 pounds exerted on it.....

To make that easier to mentally grasp lets unwind the threads and put a 1000# weight up the ramps. The shelf is 5 feets high so the course thread will be at a steep angle from being so short. It will be a bitch to push the weight up the ramp because pushing the weight forward (parallel to the ground) will put 1/2 the pressure into forcing the weight against the ramp and 1/2 actually sliding the load (huh huh, ''load''. huh huh)

Now for the fine threads.... The ramp is the same 5 feets tall but it's very long and only at a 20 degree angle. Since it's much less steep, it's easier to push the weight as most of the energy is spend actually moving the load. So you can achieve the same force with leff effort. Granted you have to walk further (more degrees of rotation per pound of clamping force) but your metaphoric nuts don't 'splode doing it. (huh huh ''doing it''. huh huh)

What you find is it take less torque on the wrench to generate the same clamping load in a frictionless environment. Since we don't use super anal-eze or my spit, there's going to be friction. With that limitation, clamp load = torque reading - drag. The finer threads have less drag so you can have more clamping force with the same amount of rotational torque at the tool. (huh huh, ''tool''). That's also why adding a hardened washer slathered in moly under a stock replacement bolt helps too.

 

[Nasty Wendy]

OK I understand that. At least I think I do. What I'm getting from that goes back to what I was thinking about not pulling the threads out of an aluminum block. The more gradual increase in clamping force over the finer grade threads will make for a less 'savage' pull on threads as the torque rinch turns. Being that the torque spec doesn't change does it matter? Is 85'lbs on a stud more than 85'lbs on a bolt? OK, the answer is technically is yes due to the decreased drag of the finer threads but dammit add 3lbs to compensate for that. The question is what is the 'practical advantage' of using studs over less expensive bolts? Its not like we are pushing the outer limits of how much torque we can apply to a bolt. If all we have to overcome is that higher drag coefficient then can't that be done by a slight increase in torque setting?

 

[novaderrik]

there is not only the fine thread thing- the stud isn't spinning as you tighten it down, so it can hold more clamping force without stretching.

or something like that.

 

[Nasty Wendy]

Well that's three reasons. Threads all the way in for a deeper pull - Smoother transfer of torque into clamping force - No twist and stretch

Man you guys are smert. What will my mind wonder to next?????

 

[earlbrown]

OK I understand that. At least I think I do. What I'm getting from that goes back to what I was thinking about not pulling the threads out of an aluminum block. The more gradual increase in clamping force over the finer grade threads will make for a less 'savage' pull on threads as the torque rinch turns. Being that the torque spec doesn't change does it matter? Is 85'lbs on a stud more than 85'lbs on a bolt? OK, the answer is technically is yes due to the decreased drag of the finer threads but dammit add 3lbs to compensate for that. The question is what is the 'practical advantage' of using studs over less expensive bolts? Its not like we are pushing the outer limits of how much torque we can apply to a bolt. If all we have to overcome is that higher drag coefficient then can't that be done by a slight increase in torque setting?

There really isn't a practical advantage to using studs over bolts. Yes studs are stronger but they only have to be as strong as needed to do the job. They're more of an insurance thing and a 'as long as I'm there thing'. The other reason they're nicer to an aluminum block is during the tightening process they threads are just getting pulled linearly. The threads on the block and the threads on the fastener 'marry' to each other and you're not introducing sliding friction from the turning.

And yes, 85 #/feet of torque is greater clamp load on a fine thread stud than a course thread bolt. Not just from the pitch but also due to the washer and the stud not being bottomed out. During the torquing process there's three sliding options depending on the path of least resistance. Either the stud can turn, the washer can spin against the head or the washer can slide against the nut. Increasing the amount of square lightyears of load area allows for more molecules of lube catch the load.

 

[Heisenberg]

You were on the right track with the varying pitch on the studs. Think of it this way. A threaded fastener is an inclined plane wrapped around a cylinder (technical term: "screwed"). A fine thread will be longer with less slope, where a course thread takes a shorter path with a greater slope.

Lets say you wanted to generate 1000 pounds of clamping load. That means you twist the wedge to the point the inclined plane has 1000 pounds exerted on it.....

To make that easier to mentally grasp lets unwind the threads and put a 1000# weight up the ramps. The shelf is 5 feets high so the course thread will be at a steep angle from being so short. It will be a bitch to push the weight up the ramp because pushing the weight forward (parallel to the ground) will put 1/2 the pressure into forcing the weight against the ramp and 1/2 actually sliding the load (huh huh, ''load''. huh huh)

Now for the fine threads.... The ramp is the same 5 feets tall but it's very long and only at a 20 degree angle. Since it's much less steep, it's easier to push the weight as most of the energy is spend actually moving the load. So you can achieve the same force with leff effort. Granted you have to walk further (more degrees of rotation per pound of clamping force) but your metaphoric nuts don't 'splode doing it. (huh huh ''doing it''. huh huh)

What you find is it take less torque on the wrench to generate the same clamping load in a frictionless environment. Since we don't use super anal-eze or my spit, there's going to be friction. With that limitation, clamp load = torque reading - drag. The finer threads have less drag so you can have more clamping force with the same amount of rotational torque at the tool. (huh huh, ''tool''). That's also why adding a hardened washer slathered in moly under a stock replacement bolt helps too.

What an interesting "thread"

 

[1952ray]

What an interesting "thread"

One more thing to think about, the industry standard in the screw machine world is % of threads is held to 60 %. ARP studs are held to 65 % of threads.

 

[notacarlo]

What an interesting "thread"

I see what you did there.

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[hiboom]

I'm thinking bolts or studs the bottom line is there are 8 bolts, studs holding are heads unless we are able to add more bolts or studs for holding force I'm not so sure a stud would save a head gasket over say using a bolt.or on a high boost motor I don't know it you lift a head because your tune was off a little whether a stud over abolt would make a difference either,so far I've been a chicken to go over 30 lbs of boost with stock head gaskets I have used arp bolts and studs and currently running studs I'm wondering if there is a real difference too?You always hear about studs and all the big boys use them must be some science to it. LOL

 

[Bigjsn250]

This goes to what Earl was saying and explains pretty well a little of the advantages of studs. Also many moons ago studs came without the allen head provision on top so unscrewing them with the head in place was a bear. Now that most if not all come with this provision it's much more practical to use studs in installs where things get tight when studs are used i.e. Buick's and Mustang's.



Torque Efficiency

During engine assembly or maintenance, a bolt must be installed by torqueing it into place. Due to the head bolt’s design, it has to be rotated into its slot in order to engage the threads and secure it into place. This process creates both twisting force and a vertical clamping force, which means that when the cylinders within the engine’s combustion chamber begin accumulating load, the bolt will both stretch and twist. Because the bolt has to react to two different forces simultaneously, its capacity to secure the head is slightly reduced and it forms a less reliable seal in high-powered engines.

By contrast, a head stud can be tightened into place without any direct clamping force applied through the tightening. A stud can be threaded into a slot up to “finger tightness,” or the degree to which it would be tightened by hand. Afterward, the cylinder head is installed and a nut is torqued into place against the stud. The nut torque provides the clamping force, rather than the torque of the fastener itself, and the rotational force is avoided entirely. Because the stud is torqued from a relaxed state, the pressure from the nut will make it stretch only along the vertical axis without a concurrent twisting load. The result is a more evenly distributed and accurate torque load compared to that of the head bolt. This ultimately translates into higher reliability and a lower chance of head gasket failure.

 

[sean lawrence]

studs have more clamping force but get ready to have a good time puling the heads after that

 

[sean lawrence]

I prefer arp head bolts

 

[sean lawrence]

and of course some good head gaskets

 

[pacecarta]

studs have more clamping force but get ready to have a good time puling the heads after that

studs are no real problem even with ac , easier than bolts in my experience
once the nut has been backed off it literally can be spun of with your fingers
with the allen head studs its really easy , just pull the rear studs and the two long center studs and head will slide off

and the 12 point 1/2" nuts makes it easier to retorque even with headers on

 

[notacarlo]

They do slide right off. Its nice.

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[turbofabricator]

They also save the threads in the block. The studs are spun in with no load on the threads. Everytime you tighten a fastener under load you are wearing the threads down. There's FOUR for you "Wendy". (And a nasty one at that")