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-
http://i1151.photobucket.com/albums/...pswvo1xcis.png

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.

Mild, think about this- your e-bay rod's spot face isn't flat but you don't know that, so you torque it to 100# once. Well guess what now the spot face is flat and normal to the bolt axis, luckily the cap material was India finest bubblegum. But hey! You torqued that SOB!


Did you introduce any preload? Nope. You're hosed and won't know until it's too late.

EDIT- we were typing similar cases at the same time. Torque lies.

Think about this. Earlier we got 160,000psi stress in 0.0055". That equates to some torque, say 120 ft/lb. If the spot face deforms .001, and the cap/rod crush a .001 during our 120# torque we've only imparted .0035 elongation - 45% less

But hey! the torque wrench read the same!

Can anyone tell that I like fasteners?

Very true. I recently rigged up a dial indicator on a poly lock with hardware in place (rocker, spring, pushrod, etc). Just rolling the engine over by hand, I had about .002 of stud deflection while valve went thru its lift cycle. With just my thumb pushing against the polylock, I was able to get about .004 deflection.

I think I just pizzed my pants LOLOLOL

sure glad their is no over thinking going on here,lol.

u sure

You saying the bolt cycling is necessary, or that theirs been failures ?

You're absolutely right and to be even more accurate, they're called cap screws, not just screws.

If any of you are really interested in learning about fasteners, there is a book that is my Bible on this stuff and it's written by a guy named Carroll Smith, called Nuts, Bolts, Fasteners and Plumbing. If any of you have an ARP catalog, you'll see a tribute to John Carroll Smith somewhere in the beginning. He was one of the early contributing engineers at ARP and worked throughout the racing industry. This book is a great read for anyone interested in this sort of thing. It's out of publication but I'm sure you can find copies on EBay and such.

This is the reason for stud girdles, to minimize the deflection and help stabilize the valve train, I'll tell a little story that BS told me and if it wouldn't have came from his mouth in person I doubt I would of believed it. So he's building a pretty healthy small block for one of his customers, made a couple pulls and it made good power, then the guy shows up with some girdles and they get bolted on, another dyno pull and gained 15-20hp, he didn't believe the numbers so he pulled them back off and the HP gain was gone, True story from the man himself with his father Bob as a witness.

It's available in new print. I paid $22 about a year ago.

http://www.carrollsmith.com/books/

I wouldn't even consider an off-shore type build without a stud girdle.

Thanks Cole, that's good to know. He's got some really great books.

Me ether

they look like webster industry,s.did you get them from bob?

Can one assume that cam specs, spring pressures, and intended RPM range play into this decision? and rocker stud materials? Just asking where you think the Cut-off or jump up point might be? Refering to Offshore type build. Serious question not intended to be confrontational, I think it is good information for the budget minded builds.

Nice looking engines and no offense, but I would not run a Professional Products Damper on an off shore engine (or any other engine for that matter).

Yes, their nice pieces

I appreciate the question and IMO, it's just something that should be considered in any budget for any off shore build...but valve train stability is one of my pet peeves. Are there thousands of engines out there running without one? Sure. Without issues? Well that could only be determined with a before and after comparison, I guess. The factory "automotive" valve train wasn't designed to run at 5000 rpm and at or near peak torque, max loading and peak cylinder pressures for extended periods like we see in off shore engines. Again, IMO, anything you can do to make the engine "happier" at those rpm's and loads is well worth the investment.

No salt water close to me............so I guess I'll keep em

I edited my post...and I wasn't trying to be funny.

Why did you make that statement

Thanks for the reply! In my perticular case running mild non-offshore engines, I will leave them as is. Good information!

Those webster girdles look nice for their price. Do they come with the nuts for 269.00?

I think they did, but its been a while........... these were specific to the AFR head, so I was told

I see they make them for dart heads as well.

Not to hijack the thread but IMO the damper is no place to go "budget" on an off shore engine, let alone bottom of the barrel budget, which is what those dampers are. Looks like you spent good money on some other really nice parts for your builds...I think you would be well advised to replace them but again, JMO.

Dammit... now I gotta buy girdles too? You guys are killing me!

This is the only Girdle I wanna buy!

http://g03.a.alicdn.com/kf/HTB1MGWnI...en-girdles.jpg

Using stretch is not overkill, its just more involved, and you have to have the gauge to do it with. but don't get confussed between stretch and Tq.You only need to know the one that you are using. ARP gives you both on bolt directions. Their Tq spec is what their testing has shown that it should take to stretch the bolt where it needs to be. On their Wave Loc 7/16 BB bolts, they say .006-.0065 stretch OR 80 lbs Tq. (with their lube wihich is VERY important. If you use regular oil ts a whole different world). Some people will take a static reading of the stretch and then Tq to spec or close and read stretch and go from there. Most will tighten the bolt with a box wrench until it reaches stretch and be done. I've seen a lot of stretch where its no where near the Tq spec. but not usually by much. 20 lbs like you saw would make me question the Tq wrench. I trust the stretch gauge more than I trust most Tq wrenches unless its been calibrated recently.

You know what, maybe you should think before throwing out insults about the parts people choose to use, personally it kinda sticks in my A** abit, I don't care about your opinions on my builds their running just fine.