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.

 

[golfpin]

Good day Gentlemen,
highly intrigued by the info coming through on this thread. Mike please clarify a small point do I understand correctly that the main bolts/studs on the WW2 Merlin/mustang, were tightened to a "Torque to Angle" basis I am assuming TTY is torque to yield and TTA is torque to angle.

In the late 60,s I was fortunate enough to assemble both Repco F1 engines and then went on to Cosworth F1 engines, and following that quite a stint building SBC and Ford 350 to a very similar Imsa spec, what is intriguing is all values were in FT/Lbs but I am staggered to hear that TTA was used way back in the 1940 or before, or have I got it wrong. Comments appreciated,
with thanks Golfpin

 

[MikeCaruso]

Hi Golfpin,
Yes sir TTA was used on the mains. In the GM service manual it showed you have to check the crankshaft main housing bore for size. Next GM supplied a Ground Bar or Tube that you slipped into the block. Next installed and tightened the caps in place. Now the test is if the bar could be rotated by hand the block was ok for true.
I lost about 30 years of paper work in a flood of sewer water in my house or I would have scanned the pages for everyone.
Mike

Mike Caruso

http://www.aera.org

 

[dgallup]

We did the exact same test when I worked in the Mack Trucks Engine Development Lab 35 years ago.

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The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

 

[MikeCaruso]

No kidding it was a good test.

Mike Caruso

http://www.aera.org

 

[golfpin]

Good morning all,
Mike thanks for the reply, but am still a little puzzled. Your answer stops short at the mains, referring to the Merlin engine? Because the next sentence refers to the GM practice of using a test bar, does that mean the big ends on the Merlin were not tightened on a TTA basis?
The mandrel type of test is very familiar to me, it was used by quite a few, if not all the major manufacturers, even made my own.
Now here is my true dilemma, quite some years ago when assembling SBC,s the rule for big ends was torque to 65 f/t lbs measure the stretch , if not [.005] 5 thou increase torque to a max of 75 ft/lbs. If the bolt does not achieve the required stretch or exceeded it, within the 65-75 ft lb range DESTROY bolt and use another one, this was clearly before the era of ARP. Does this mean that the quality of the big end bolt was that much better than the current factory bolts? As pointed out by Tmoose most of the changes seem to be money/cost driven. Hope that this is not too far from the bolt vs stud issue but info begets questions not so?
Thanks Golfpin.

 

[MikeCaruso]

Golfpin,
I did not say Merlin I think you did.
P-51 Mustang used the GM Allision V12 that's why you are asking about GM my GM reference.
The tube or bar is in place of the crankshaft with-out bearings to check for true.
Bad screws/bolts are just that screws/bad bolts.
You can also us ultrasonics to check free lenght and tightened (stretch)length.
The cost for changing to TTY was pennies for Ford to reduce monies paid out to replace the leaking head gaskets.
Ford already had automated machines head screws/bolts just needed to be reprogramed for TTY steps.
Hope that helps.
Mike

Mike Caruso

http://www.aera.org

 

[golfpin]

Mike,
thank you for your patience and the courtesy of your replies, was not aware that Allison was built by GM, thks again
Golfpin.

 

[MikeCaruso]

Golfpin,
No Problem sooooo many things I don't know Yet Ha.
Anytime,
Mike


Mike Caruso

http://www.aera.org

 

[140Airpower]

Since the P51 was mentioned, the vast majority of P-51s that saw combat had the Merlin which was very superior to the then available Allison.

 

[Lou Scannon]

Not a fault of the basic design of the Allison, which had no shortcomings I'm aware of, but it did not receive the relentless development that the Merlin did, particularly in the area of supercharging. It seems that US aero engine development prioritized air-cooled radial engines over liquid cooled inline engines. Not that that was a mistake, but that left the field open to the Merlin and some of its British inline liquid cooled contemporaries to take all the accolades for this engine type.

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz

 

[140Airpower]

The Army wanted single mechanical superchargers combined with turbos which worked superbly in the P-38 and P-47, but could not be fitted to the P-39, P-40 or P-51, leaving them with only the supercharger and poor high altitude performance. Meanwhile GM neglected to develop a two-stage supercharged Allison for those installations. I think the Merlin was the best package possible for the P-51 up to 1945.

 

[tbuelna]

For anyone that is interested in doing some reading on the subject, I'd recommend downloading a copy of MIL-HDBK-60.

 

[MikeCaruso]

Thanks guy's good reading. Book ISBN 0-8247-8167-8 by John H. Bickford which is pretty good too DATED. Love the old v12's


Mike Caruso

http://www.aera.org

 

[Lou Scannon]

Here an interesting article comparing liquid-cooled (inline) and air-cooled (radial) engines while the jury was still out.
Air-Cooled vs Liquid Cooled Aircraft

"Schiefgehen will, was schiefgehen kann" - das Murphygesetz