Head Studs vs. Bolts

[jdogg05]

Hi,

I understand that head studs are generally prefered to head bolts because they "eliminate" the torsional force experienced by the bolt and thus provide a more accuracte clamping force...

How does this work? Isn't the idea behind this that the nut "slips" on the threads thus "eliminating" the torsional force? From a FBD perspective the force holding the nut from coming loose is the (torsional) friction force between the threads, generated from the coefficient of friction and the (axial) normal force between those threads. Thus, my first point to make is that the torsional force is definitely not "eliminated" as a lot of people state. My QUESTION regarding this is:

How is the bolt any different than the stud if this is the case? I understand we add a threaded connection when a stud is used, but we already have a threaded connection with the bolt... why is two better than one (if that is the argument). In essence, this reduction of torsional force is due to the interface of a threaded connection. Thus, this same reduction must still be taking place at the interface of the bolt and the threaded hole in the engine block... How does adding a second threaded connection (nut and stud) reduce the torsional force on the stud so substantially as compared to the bolt? I feel like I am missing something simple mathematically but I just can't seem to figure it out.

I also read that studs are better for wear in the sense that repeated disassembly/assembly results in wearing the threads on the stud and not in the block... I don't understand the reasoning behind this.

 

[MikeHalloran]

WRT to your last point, it's easier to replace a stud than to repair the internal threads of a block.

Elsewise, it's a religious issue.


Mike Halloran
Pembroke Pines, FL, USA

 

[TheBlacksmith]

With a bolt you apply/measure torque on one end of a shaft that has to rotate through the head to tighten the threads on the other end of the shaft to provide the proper clamping force on the head gasket. With a stud you are applying the torque directly to the nut, eliminating several variables.

 

[Tmoose]

"I also read that studs are better for wear in the sense that repeated disassembly/assembly results in wearing the threads on the stud and not in the block... I don't understand the reasoning behind this."

No sliding while supporting big load = no wear

 

[jdogg05]

SO I think I understand my second question. When you disassemble/assemble a head with studs you don't have to remove the studs from the block; therefore, that interface doesn't get worn. With bolts you have to take the bolts out of the block each time...

Sorry, Tmoose I don't know what you mean by no sliding while supporting big load.

 

[inline6]

the studs ive used a very high precision fine metric thread at the top as apposed to a coarse one at the bottom and therefore the clamping is more precisely controlled? the better surface finish must produce a more consistent preload even with the torque wrench which is known to be at best 30% accurate conventionally

with a stud properly installed (i.e. not bottomed out) i think a smaller amount of the stud sees the torsion you get from a bolt which twists about its "whole length"

 

[Pud]

Way back in the early '80's, at the engine manufacturer I used to work for, bolts were introduced instead of studs and nuts as they were cheaper to install on a production line - one operation instead of two (One man less on line). This coincided with the introduction of equipment that torqued the bolts into 5% yield.

H

www.tynevalleyplastics.co.uk

It's ok to soar like an eagle, but weasels don't get sucked into jet engines.

 

[RacinNdrummin]

Generally studs have a fine thread on the fastener side, essentially increasing clamping load for the same amount of torque, normally in the neighborhood of 30-40%. Of course this only matters of your yield/tinsile strength is enough to warrant the extra load.

 

[tbuelna]

jdogg05-

An engine that gets torn down on a regular basis (ie. a race engine) will usually be better off with studs/nuts, since the screw threads that will experience wear or damage are on components that are easy to replace. Production engines almost always use bolts because they only require producing a single threaded connection, which reduces cost. And as others noted, most production engines now use torque-to-yield cylinder head fasteners which can be made from lower cost materials. It is probably easier for automated machines to install/tighten torque-to-yield bolts as opposed to studs/nuts.

For cylinder head fasteners, both bolts and studs/nuts can give excellent results if they are designed, manufactured and installed properly. And both types are subject to torsional strain in the fastener body at installation due to prevailing torque in the threads, especially with long grip lengths. So the best choice would depend upon the particular application.

Regards,
Terry

 

[Tmoose]

"Sorry, Tmoose I don't know what you mean by no sliding while supporting big load."

The bolt threads are still sliding along and gainst the threads in the block just as the full clamping load of 7000 lbs is achieved.
When a stud is used the threads in the block don't slide.

( some would argue fervently that when the block is threaded they are head screws, not head bolts )

 

[jdogg05]

Tmoose and tbuelna,

I see. They don't slide during subsequent disassembly/assembly of the cylinder head so there is no wear of those important threaded connections. However, I would make the point that they do slide (and thus plastically deform) during the first installation.

Also Tmoose, I would agree that they are technically machine screws and not bolts.

RacinNdrummin and inline6,

I think the point about the finer thread on the nutted end of the head stud is the key to explaining how the torsional force is reduced and thus more accurate clamping force is acheived with head studs. IT'S THE CRUX!

Thanks everyone for your input.

 

[NormPeterson]

I don't think there should be appreciable torque, friction, or axial force involved as a stud is threaded into the block.


Norm

 

[jdogg05]

Not when you are first hand tightening them in, no. But when you torque the nut down it will also turn the stud tight into the block.

 

[MikeCaruso]

Screws or studs test for load/length in MTS.
Parallel grind ends screws or studs.
Install screws or studs no load and check free length using ultrasonics.
Tighten using ultrasonic checking for length required to produce correct clamp load.
Keeping track of required torque which will blow you away.
I have seen 7/16 GM SBC head screws long ones take 48 lb/ft and 85 lb/ft to reach the same clamp load.
That is why you see TTY (use once) and TTA (reuse OK) fasteners being used more and more as they offer less clamp load scatter.

P-51 Mustang WWII Fighter plane V-12 crankshaft main studs were TTA.
Old School Not So Bad



Mike Caruso

http://www.aera.org

 

[yoshimitsuspeed]

Mike

Is that variance seen while keeping as many variables as similar as possible or is that variance seen with big changes to variables?
Main ones I can think of are brand of screw or stud, brand and hardness of washer used or even no washer used, lubrication used or even none at all etc?

 

[MikeCaruso]

Hi,
Yes you are correct (Main ones I can think of are brand of screw or stud, brand and hardness of washer used or even no washer used, lubrication used or even none at all etc?)
The testing I did they was Factory OEM GM NEW head bolts. Lube, Material and washers being stamped or ground.
EP Grease, ARP lube, Anti-seize all can and do have different fricton reductions when using a toruqe wrench.
Nothing is easy as it seems, just screw the heads onto the engine.
TTY, TTA make it easier and better for mass production.



Mike Caruso

http://www.aera.org

 

[tbuelna]

TTY engine fasteners are used for mass production applications entirely for cost reasons. These fasteners get installed under carefully controlled conditions at the factory, and they don't normally need to be replaced for 150,000 miles or more. You can buy a replacement set of TTY head fasteners for most engines for around $30-$40.

 

[MikeCaruso]

Agreed,
Yes the OEM have wonderful TTY tightening machines on the assembly line. I know Ford has since the head fastener change in 1992.5 on the Ford 5.0L H.O. engines and I am sure there are many more.
Bottom line......The OEM knew by reducing scatter of fastener clamp load they would pay out less warranty moneys.


Mike Caruso

http://www.aera.org

 

[BigClive]

I did think the main advantage of bolts (as Tmoose says, surely they are screws?) was that a greater length of thread could be used with a bolt allowing higher torque on the bolts and thus greater clamping force. I have read over the years several engine modification books (notably the one by Vizard on the A-Series engine) that recommend changing the engine from "stud" to "bolt" to achieve this greater clamping force.

A disadvantage I have found with bolts is that I am always worried that he various passages in the head gasket will line up perfectly - it is easy to disturb the head gasket when placing the head over the engine. These days I use a few "temporary" studs for alignment when putting the head on a "bolt" engine.

 

[MikeCaruso]

BigClive,
Check out this site very good easy for all to understand. Nose around as he has layer after layer great information.

http://www.boltscience.com/

No I have nothing to do with him or the site.

Regards,

Mike

Mike Caruso

http://www.aera.org