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Thu Feb 09, 2006 8:22 pm

I'm installing a new set of CB Performance Uni-Tech rods that have nuts without the collar that OEM rods had. This means there is nothing on the nut to peen into that little groove on the rod. Should I just torque them down and forget about it or use some threadlock on them before installation? Makes me nervous not being able to peen them.

Thu Feb 09, 2006 8:48 pm

Put some thread locker on the threads. Also, put some anti-sieze around the base of the nut for tourqe purposes.

Thu Feb 09, 2006 10:57 pm

up to a certain year, vw quit installing and peening the nuts.torque it good and recheck torque.

Thu Feb 09, 2006 11:54 pm

It would sound that you have a rod with an after market rod bolt and nut. Most rod bolts are torqued to an bolt stretched length. You would benefit from finding out what bolt and nut are used to determine proper torque. I don't believe the use of thread locker would be recomended through.

Fri Feb 10, 2006 6:02 am

Use the threadlocker! Just don't go bananas with it. I used it on my Uni-Tech rods. Very cheap insurance.

Fri Feb 10, 2006 7:20 am

As with any application of thread-sealant, clean the threads off the bolt and within the nut before installation using appropriate cleaner/degreaser spray such as a brake cleaner spray (use goggles), letting the threads dry before you apply the sealant.

Sun Nov 12, 2006 11:33 am

Is there a thread locker that is recommended, or best to use for cam bolts, rods, ...etc

Sun Nov 12, 2006 11:35 am

should thread locker be used on H-beam rods?

Sun Nov 12, 2006 12:24 pm

The rod bolt torque is the most critical torque spec in the engine.

You need to carefully follow the instructions of the manufacture.

If you do not use the proper lube on the bolt, then you will either over or under torque the bolt. bad things happen.

To save me from typing, read this-

Theory of Rod Bolts and Other Prestressed Bolts
Vance Jaqua
Originally published May 1997

With a recent dramatic demonstration of what a connecting rod bolt failure can do to an engine, and with a AD dealing with poor rod bolts out on the market, it seems like a subject many of us might be interested in.

The connecting rod is one of the first areas of concern when ever one feels that he has overstressed his engine. "It threw a rod" is a common cry after a disastrous engine failure, or as our British cousins would say in their often more descriptive way "I ran a big end and retired with expensive noises". We seldom truly "throw" a rod, the usual failure is that the rod to crank bearing material (the "big end") breaks down from over stress or more commonly lack of lubrication. Accelerated wear then makes a large clearance, and the remaining soft bearing material is pounded out with large clanking noises.

The highest tensile force on the rod and bolts happens when the throttle is closed at high rpm. The rod and piston are being accelerated downward at close to 700 "Gs" and the crankcase pressure (ambient or greater) is pushing upward against inlet pressure which is near zero absolute from manifold vacuum. In an engine like the Lycoming O-360 this load is probably on the order of 6000 lbs (I don't have a good number for the weight of the piston or rod, but this is probably a pretty good estimate). The highest compressive load is near top dead center in a full throttle power stroke where the pressure can peak over 1000 psi. The acceleration of the piston and rod compensate partly for this pressure loading, but it probably peaks at a about 10000 pounds compressive load.

Now the rod bolts on an engine like this are not particularly impressive, being about a 5/16 inch thread, and a reduced section about 1/4 inch diameter (to match the thread root diameter). The load of 6000 pounds spread between the 2 bolts would correspond to a stress level, a bit over 61,000 psi. Well that is pretty scary, but isn't that bolt good for over 150,000 psi? Well, yes and no. For steady state loading that would show a safety factor of over 2, but there is that little thing called "endurance limit" for cyclic loads. This is the stress level for repetitive loads that will let you last a million cycles (ten to the sixth for you scientific notation types). For most steel alloys this value is about half the steady state value (or 150,000 is now down to 75,000). And on top of that the stress concentration factor for a standard thread form is about 3 to 1 (oops! we're down to 25,000 and in big trouble). It also might be worth mentioning that a million cycles at 2700 rpm is just a little over 6 hours (that won't get me half way to Oshkosh).

Well, it obviously works better than that--so what is the secret? The answer is preload. If a bolted joint is preloaded by bolt pretension torque to a load greater than the cyclic load IT WILL NOT SEE ANY ALTERNATING LOAD AT ALL. For example if we preload the connecting rod joint to 10,000 pounds, the stress on the stretched bolt cannot increase until the tension load overcomes the locked up compression in the joint, and the surfaces start to separate. Until that point is reached, the forces will just reduce the effective compressive forces on the surface, and the bolt load stays the same. This is hard to visualize, but it is definitely true. For example if a 200 pound man stands on a box on the floor, and you try 100 pounds worth to pick up the box. You have reduced how hard the box is pressing on the floor but you have not increased the 200 pounds force that his feet are putting on the box. Now replace that man with a clamped down spring with 200 pounds of force, the situation is the same, as you lift on the box you do not change the force until you have lifted the box off the floor (exceeded the 200 pounds force) and are further compressing the spring. The bolts in the rod are really springs as you will see in a bit.

With a bolt it is very difficult to know how much preload you have applied. Just whip out your calibrated torque wrench you say? BZZZZZZZZ WRONG! The torque wrench is directly dependent upon the friction in the threads, and on the thrust faces of the bolt or nut, and this friction is widely influenced by a whole bag full of things - surface finish, smooth or rough - lubrication, amount or lack of it - fit of the thread - you name it and it can vary all over the map. To truly work the bolt to it's maximum potential a preload as high as 80 percent of yield is required, and trying to work to that criteria with a torque wrench can easily lead to permanent stretching and even cracking in the thread roots. The best method, and one that is quite common for aircraft engines is torquing to an elastic elongation value. If you have access to both ends of the bolt, and the surfaces are flat, you can measure the length of the bolt before final tightening, and measure the few thousandths of an inch of stretching that indicates the desired amount of prestress. All steel bolts have virtually the same elastic modulus - 30,000,000 pounds per inch per inch. If your bolt has a working length of 2 inches a 100,000 psi prestress will lengthen it 0.0067 inches - a small number but easily read on a good micrometer. The bolts frequently have the major length reduced to roughly the same diameter as the thread root, it is just as strong (the weakest link is still the thread root diameter), and the amount of stretch is increased at the same load (because the stress is more evenly distributed along the length) for a better indication with the micrometer. If the bolt has been properly hardened, such that this load is below yield, it will act like the very stiff spring that it really is, and relax back to it's original length when unloaded, just like any other spring. If it is too soft (wrong material or improper heat treat) it will yield to this dimension, and may have a crack, just as a spring made from a wire that is not properly hardened does not return to the same length if you stretch it too much.

Many fancy systems are used to properly prestress bolts and studs which are not so easily measured as a rod bolt. Properly designed bolts can be read for length by ultrasound in blind locations. Special load measuring washers are available where an outer ring is free to spin until the design load is reached. Special bolts are available with a pin indicator built into the head. However, in most cases the designer just doubles the bolt strength (goes to the next size up) and preloads with a torque wrench to a fraction of the available working load, and just accepts the scatter in actual strength.

(Vance is an honorary member of Chapter 1000 and near full-time engineering advisor for High Tech Composites and Tri-R Technologies. Be sure to catch his excellent article in the March 97 Experimenter titled "Engine Myths & Old Mechanic's Tales" - ed)

Sun Nov 12, 2006 1:11 pm

Good post, Jamestwo.

Sun Nov 12, 2006 1:29 pm

Thanks, but credit the engineer that wrote the article.

I should add, if you don't know the proper lube, torque, then ask the good folks who made them.

Sun Nov 12, 2006 4:23 pm

adding to what jamestwo wrote... the bolt stretch method doesnt lie, and is the only way to know what the preload really is. if you arent going to use that method, it becomes so important to maintain the correct relation between the torque and the preload with the specified lube-SAE 30. loctite wont provide the factory intended preload at the specified torque. nor would synthetic oil, gear oil, WD-40 etc.

if you read ARP literature, you find that there are catastrophic failures that come from underloading the rod bolts- without the nuts ever loosening! so the bottom line is- you could prevent the nut from coming off with loctite and specified torque, but the loctite could cause the specified torque to translate to a preload that falls short of that intended by the factory (if you dont use the bolt stretch method), causing it to break, due to cyclic fatigue failure. game over, you lose.

heres the ARP link: http://www.arp-bolts.com/Tech/TechWhy.html from there you must click "common failures" then click #6 "cyclic fatigue cracks initiated by incorrect installation preload". great picture of twisted metal along with their explanation of how that works.

or it is possible to get too much preload at the specified torque if you arent using the specified lube- depending on what you choose. and the resulting metallurgical failure should be obvious to all.

Mon Nov 13, 2006 3:14 pm

hey , where did all you loctite proponents go? did i say something wrong ? :lol:

Mon Nov 13, 2006 7:15 pm

dansvans wrote: hey , where did all you loctite proponents go? did i say something wrong ? :lol:
Locktite is secondary to proper bolt torque. Most of the world's engine don't use it. It's the lock ring/tab/groove thing on the VW bolts, which is fairly unique to ACVWs, that makes people think threadlocker MUST be used on the rod nuts. Not that that's a bad thing, it's just not the most important thing.

Mon Nov 13, 2006 8:56 pm

I actually said to use lock tite before. after reading ARP's site and the article above, I changed my mine.

Tue Nov 14, 2006 12:56 am

Another passage from the good book of Douglas (aircraft that is). This is from ch. 20-50 standard practices. Bolt torque data. "Unless use of thread lubricant or antiseize compounds is specified in applicable maintenance instructions, threads should be free of lubricants other than those applied by manufacturer, check dyes, identification dyes, etc. Use of lubricants will increase preloading applied to bolt, and foreign material on threads will reduce amount of preload" . There you have it. Always follow the manufacturer's instructions when dealing with "special" fasteners. And refer back to the good ol' VW manual when dealing with they're stuff.

Sat Oct 17, 2015 11:06 pm

So do I have this right?

First take my used rods and pass a die over the threads to make sure their clean. Wash them up real good and then do them in 30 grade oil. Put new hardened nuts on them. Then torque to spec and just hope they'll never come apart?

What about green Loctite after assembly?

Sun Oct 18, 2015 1:23 am

1. Don't use a die on the threads - good rod bolts have rolled not cut threads. If the threads are munged up, get new bolt.

2. I reuse rod nuts on stock and mild performance engines with perfect results.

Sun Oct 18, 2015 7:46 am

Dr OnHolliday wrote: 1. Don't use a die on the threads - good rod bolts have rolled not cut threads. If the threads are munged up, get new bolt.

2. I reuse rod nuts on stock and mild performance engines with perfect results.

I have the original peenable nuts. But a couple feel a bit hard to turn, even lathered in 30 grade oil. I was thinking of taking a die on the studs and a tap in the nuts. But if not, now what? Do I need new studs and nuts now? This is for a rather mild, near stock engine.

I guess I should trade around nuts first to see if it's the nuts or the studs that are messed up. If it's hard to turn wouldn't that mess up the torque reading? From what I'm seeing the peening isn't necessary. So new VW rod nuts without the collar might fix the problem.

Sun Oct 18, 2015 2:51 pm

Volkswagen Official Service Manual, Super Beetle, Beetle & Karmann Ghia 1970-1979, by Bentley Publishers wrote: CAUTION -
Do not reuse any nuts that have been locked by peening. Always install new nuts of the later type to avoid having them come loose. The old-type nuts can be reused, however, if they have never been peened.

Mmm... Maybe something's telling me I need new nuts, peenable or not.