Main Stud advantage over Bolts?

[Tim Cucci]

What are the advantages of studs over bolts on the mains?
I realize the turning verses pulling effect when torquing but are there other advantages such as holding the caps true? Car not used for hardcore racing but I do love drive it hard. You know. Can't keep my foot out of it.
Thanks
Tim

 

[Intercooler]

More clamping force.

 

[The Radius Kid]

Originally posted by Intercooler
More clamping force.

Exactly.The load exerted on the threads is spread to two sets of threads instead of one.More stability.

 

[tom h]

From strictly a physics/mechanics analysis of stud vs bolt, the stud does NOT provide "more" clamp force.

The tension force in the bolts the same, stud or bolt. Also, the female threads (say in the block), has to pull the bolt/stud inward with the same force as the fastener's other end is pulling outward.

The female threads in the block don't "know", or care, whether the force on the other end is being exerted by a nut on a stud, or by the head of at bolt.

I don't know if this analogy helps, but when you standing on the ground, your force (ie, "weight") pushes down with the exactly the same force the ground is pushing up on you! Else you would fall through the ground to the center of the earth. This is a fairly typical of an assigned problem in an introductory physics classes.

So what are the stud advantages? As I see it:

- studs help locate the object & its gaskets while you wrestle with it (some foreign cars use wheel bolts & not studs, it's a minor PITA to put a wheel back on).

- a more accurate fastening torque can be applied to the well machined, well lubricated end of the stud, instead of to the unknown hole in the block, which might be encrusted with sealant , or might encounter rough spots and friction in the female thread.

 

[Intercooler]

Tom,

Only way I can describe it is with normal bolts going into the block the load and force you can apply is on the cap/bolt interface only. When you use studs the force is pulled from two directions block/stud and cap/stud. Makes for less walk or movement also.

 

[REDS HOT AIR]

if studs are better then why do people prefer cap screw rods over bolt and nut ??

on a v6 buick motor i will stick with bolts in the mains since all the mains that i have seen fail it wont the bolt it was the block that gave way.

the way i see it the only way to make th bootom end stronger is to put a girdle on it and studs with that to locate everything as it is assembled.

 

[tom h]

Intercooler,

guess I'd have to disagree ... easier to explain my point of view with drawings but that's not feasible.

After the wrenching & torquing are done, and the whole assembly is static, a nut on a stud is indistinguishable from the head of a bolt, as far as all the forces and tensions are concerned.

Here is an excerpt I pulled out of the ARP fastener catalog.
NO WHERE do they claim that studs provide "more force". The advantages cited are moe accurate torque/preload, easier parts alignment, and less torsional/"twisting" stresses on the fastener ((I hadn't thought of this last feature).

HEAD STUDS vs. BOLTS...
A TECHNICAL DISCUSSION

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 ... 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.

Note in the last paragraph, "..more even clamping force .." refers to undercut SHORT studs vs non-undercut LONGER studs, and is not a comparison between studs and bolts.

"Undercut" is a technique ARP uses so the preload on studs is same when both short & long studs are necessary on, say, a cyl head.

 

[Tim Cucci]

I have my engine apart and the thrust bearing has worn into the crank. I have yet to positively identify what caused this from the torque conv. to detonation to main caps moving. This is a stock GM bottom end on the car with 60 k miles. I don't want this to happen again. That is why as a precaution I was going to use the main studs to put it back together .
Any suggestions?
Thanks

 

[tazgn]

I personally had a similar problem and went ahead and put a girdle on it..... But I love the studs. Another advantage of the studs I "think" is that they can engage more of the threads in the block. A bolt will only go into the block as far as the head of the bolt will let it as it pushed against the cap. The stud can be bottomed out in the bolt hole of the block and then you put the nut on it. And yes, I found assembly a LOT easier. Disassembly of the mains is a bit more difficult though since it's a pain to get the studs out, and not possible (really) to rock the cap back and forth to get it loose from the block.

Derrick

 

[The Radius Kid]

Interesting topic,this one.
I can tell you that when we install heavy duty equipment,we use studs exclusively.We don't do it for location purposes,it's easy enough to locate the units with bolts.
I've also seen where it says if you're using studs to clamp a head instead of bolts,to use ten lbs. less torque than with a bolt.
As to the bolts on the rods instead of studs,bolts for any given thread are always larger in the hex,taking away meat on the cap around the head where the bolt,nut would seat.
Studs are better for dimensional stability[maintaining their loaded length]because there is two sets of threads to bear the preload[less thread stretch].
However,they can also be a pain in some areas where access is limited[head removal, in car].
As to what caused your thrust bearing to fail,I doubt if studs would have helped that situation.Sounds like an axial thrust load on the bearing caused the problem from what you're describing.Not a radial load.If it's not converter problem,perhaps it's time to investigate modifying the oiling to the thrust surface of the bearing.This could get interesting.

 

[ijames]

Originally posted by Tim Cucci
I have my engine apart and the thrust bearing has worn into the crank. I have yet to positively identify what caused this from the torque conv. to detonation to main caps moving. This is a stock GM bottom end on the car with 60 k miles. I don't want this to happen again. That is why as a precaution I was going to use the main studs to put it back together .
Any suggestions?
Thanks

Interesting article on crank thrust failure modes at: http://www.atra-gears.com/crankshaft/

They also give a little procedure to make sure the cap is aligned with the block so both halves of the thrust bearing get loaded equally - I'm no expert engine builder but I've never had that pointed out to me before.

 

[The Radius Kid]

Interesting article.The bit about the ground connection is right on.You can never have too good of a ground.bad grounds can cause arcing in the interface between the bearing and crank,eventually leading to welding of the two surfaces.That's why we never weld a job when it's mounted on a machine.Could screw up the precision, machined ways.Seating the thrust bearing properly,is a time honored tradition as well as "sparking out" the grinding wheel when finishing the thrust surface of the crank[I do this every day on parts I grind].Gives a much smoother finish.
I don't like the grooves shown in the picture[thrust face of the bearing.They should NOT extend all the way to the edge of the thrust face.Oil will take the path of least resistance and flow out of the area instead of lubricating it.The method shown is less effective than if the grooves tapered off before they hit the outer edge of the flange face.
There are other ways to enhance oiling to the thrust flange,but they would be too hard for your average home engine builder to do.

 

[lazaris]

I feel their also better because of more accurate torque. The reason is, the nut on the stud is a fine thread as opposed to the coarse thread on a bolt.

 

[The Radius Kid]

Yup,lower helix angle on the threads.