Since we're discussing rods and torque, a little clamp theory primmer may be germane.

You want the force of the preload to exceed any other forces exerted on the screws. The idea is to preload the screws so they will not deform (stretch) when the rod/piston assembly tries to separate from the cap, such as at TDC on the exhaust stroke. To do this under the extreme conditions of the rod/cap mate you have to do a couple things correctly. Perfect axial loading is probably the most important and that's why having a spot face normal to the bolt's axis is critical, it makes sure the only force the screw sees is direct tension where steel performs best. It also needs a homogenous composition, smooth surface and well formed threads. All those things contribute to loading the fastener evenly. Uneven loading means uneven stresses and that is not good.

Since we have evenly loaded the stud it becomes a simple equation- stress (PSI) / strain (stretch) = 29,000,000psi (Young's modulus, the ratio of stress to strain in steel)

We want to load our studs to say 80% of the YTS of 200,000psi, or 160,000 psi and that sets up the equation (160,000/29,000,000) = strain = preload = 0.0055in

Hey, that sounds pretty reasonable. The downside is that you can only use that method in limited circumstances, those where you can directly measure fastener elongation. The indirect way to do this is torque + angle. The torque gets you lined up tight and the angle insures proper fastener elongation.


Here's a pretty good diagram to explain it-


HP1- do you prefer straight blots/studs or necked? Interested to hear if you have any direct experience between the two.

YES! Now mine doesn`t make any sense! Way to leave me out there buddy

??? Me?

im not sure i understand what you mean,when you are torquing the rod bolt it is stretching no?

I think he's differentiating between how you measure your preload. Torque means you need to wear the bolts in so to speak, and stretch(elongation) doesn't lie so it's not as critical to wear things in.

Thats how I read it.

In otherwords, simply sticking a bolt (screw) in, and torquing it to a certain spec, leaves many variables, which may not, allow the fastener, to reach its proper "stretch", therefore, also not reaching its proper clamping force.

Three basic elements that contribute to the friction factor:

Most importantly - The fastener assembly lubricant
The condition of the receiving threads
The surface finish of the fastener

Because of these variables, a phenomenon known as "preload scatter" or preload error occurs. This is basically the difference between the amount of preload achieved on the first installation of the fastener and the amount of preload achieved on subsequent torque/loosen/re-torque cycles. It's not uncommon to see "preload scatter" in the range of 4,000-8,000 pounds between the first and tenth pull on a new fastener depending on the lubricant used.

I`d prefer Mike to stick to whatever he`s doing cause it`s working just fiiiine!

Looking at the above stretch vs preload chart in post 166, you can see how much clamping force can vary, by as little as .001 of stretch.

Now, say you have some chinese special rods. You don't have a stretch gauge, so you go off the torque spec. Lets say its 75ft lbs with arp moly lube. You make one pass with your torque wrench (which you hope is accurate), and tighten the screw to 75ft lbs. Now, it SHOULD stretch to the recommended .0064 value. Key word SHOULD. BUT, being that its a new fastener that hasnt been cycled (remember these are china rods), you may have only stretched the bolt to .005. So now, your clamping force is decreased by say 2,000 psi per screw. Will that cause failure, no, not unless you're running operation exceeds the clamping force of the rod cap. But, overtorquing a fastener, certainly can!

I think its safe to say, using a stretch gauge, is the way to go. Not only do you assure you get the proper clamping force, you also get to see if the screw itself, has "taken a set", which means it goes in the trash bin . Whose to say you don't get a screw right out of the box? Will you catch that with your torque wrench?

Absolutely. Torque is a poorly derived and crude representation of preload, unfortunately sometimes it's the only method. Luckily, 9 times of 10 we only need crude numbers.

Rocker studs are the ones that weird me out, they have no shank length in a highly stressed environment.

How about when a guy bolts a set of heads on an old block, and doesnt chase the rusty chitty burred up threads in the block. Then proceeds to do a quick slap together one pull torque on the head bolts, then wonders why his head gaskets are pizzing water. Another bonus for head "studs" in my opinion.