HPBG: Don - Engine Builder Magazine


In that discussion we hashed over the disparity in coefficients of expansion between cast iron and aluminum (that caused a revolution in head gaskets around the time of the Cadillac 4.1L). We discussed surface conditions and how that affects gaskets and scrub.

We discussed gasket coatings and how they remain plastic while not scrubbing and allow differing expansion rates to coexist without so many problems. We discussed multiple layer construction and embossing.

We discussed how modern light-weight castings present more challenging sealing issues for gaskets – an eye-opener to say the least. I must say I was impressed by the obvious scientific approach to solutions to current engine building issues.

However, not much was said about fasteners. Not that these guys did not have answers, or did not know their stuff, but like so many of us they just made the assumption that builders everywhere understand all they need to and will follow proper procedure.

They don’t and it is an unfortunate assumption. It’s always been a problem and frankly in my long years playing with engines I have seen far too many failures caused by improper fastener installation.  You’d think this would change as technology got more sophisticated and we got more educated, but it hasn’t.

The same Neanderthal “crank it ’til it squeals” or “close enough is good enough” techniques are still used. About the only change is now those techniques are even more critical than ever.

All gaskets have requirements for surface condition. All. Some just need to be flat and clean. Others need to be machined to a specific Ra (roughness quotient) or they will fail.  Head gaskets are often like this. If too coarse, the machined surfaces will act like files and scrub away the gasket as expansion and contraction during heat/cold cycles occur.  

This was far less a concern in days past when thick cast iron heads were bolted to thick cast iron blocks. Now we have lightweight aluminum castings bolted to cast iron or aluminum to aluminum and it’s nowhere near forgiving. 

Many of the anti-scrub coatings used now will fail if the surface is machined too rough. Now add in the fact that these coatings are intended to adhere but remain pliable (plastic) so they move without scrubbing and you can see problems.  

The new fastener designs are carefully calculated to make sure the proper amount of clamping pressure is exerted exactly where the pressure needs to be. Embossing and fire rings actually redirect clamping pressure to focus it at the points most needed. So ask yourself what happens if the torque you apply is not what the design specs call for and the clamping pressure is either incorrect or inconsistent.  Yep, gasket failure.

Here are some facts that may help understand why this is important stuff.  One is that while we tend to think the components and fasteners are rigid once tightened but they are NOT. You’d be better served if you considered all of these items to be springs in compression. They are, and will, act that way, so why not?

Clamping forces required to keep gaskets sealed are dependent on having the spring pre-load right and having all the fasteners involved exerting the same amount of spring pressure. Changing the torque applied to tighten hardware changes this ‘spring’ pressure and the clamping pressure as well – up to the point where the fastener breaks and you have no spring at all.

Both too little clamping pressure and too much (when gaskets are damaged and embossing is crushed) pressure, as well as wide variations in pressure, cause gaskets to fail.

Thread condition is very important and often ignored.  The K-factor (a constant indicating friction) for threads that are clean and in proper condition can be wildly different than rusted, abraded, deformed, or galled threads as you can imagine.  What may not be so obvious is that when this friction factor changes so does the amount of torque required to create the same clamping pressure.  

Whereas the torque to get the proper clamping pressure might be 100 ft.lbs. on clean and good-condition fasteners and threads, you may discover that the fasteners will break far before the same clamping pressure is achieved on bad threads. I clean and chase all critical threads on every engine I build because I know this. I can’t remember the last time I had a head gasket fail.

The same K-factor issue comes up with thread compounds.  These include oil, anti-seize, sealers, assembly lube and moly thread compounds. Most OEM specs are determined using clean, lightly-oiled threads. As long as you use torque specs that reflect this and adhere to these conditions you should be in good shape.

However, adding other compounds radically change the torque specs. For example, if you start with grade 8 carbon steel 1/2? head bolts that have a factory torque spec of 100 ft.lbs., you get 12,000 psi clamping pressure if you use a K-factor of .20 – which is typical for lightly-oiled steel.

Substitute the oil for ARP lube and recalculate to get the same clamping pressure and your new torque spec is 55 ft.lbs. By the time you get to 100 ft.lbs. you will probably destroy the gasket and strip the threads out. Know what your torque spec uses for a K-factor and recalculate it if you change anything.

Remember that while those trick super-alloy bolts may be stronger than the gates of hell, the cast iron they are threaded into didn’t get any tougher! This is why ARP not only specifies a torque value for their kits according to application, but makes it clear that they only work on clean threads in good shape using their compounds. Wise approach!

Finally, to wrap this up neatly, you’ll want to know just why there is so much difference in torque values when K-factors change. The friction from threads rubbing against each other accounts for at least 40 percent of the torque applied to the bolt. The under-head friction also accounts for at least 40 percent of the torque applied. That means under the best of conditions only 20 percent of the torque you apply goes to create clamping pressure.  

If you think about this you quickly realize that changing the friction involved radically changes torque specs.  The conclusion you are forced to is that the most important aspect of applying proper clamping pressure is friction and not only knowing what conditions you have but how to adjust for any changes.  

Hardware installation appears almost too simple to bother with, but these days getting it wrong can mean the difference between a successful build and one that comes limping back into your shop chewing up your profits all the way.

Doc Frohmader is the founder and president of Webrodder.com, a website dedicated to hot rods and vintage engines.

The Changing Demand For Thread Repair   

When fasteners are selected, installed and used correctly, good things happen – or, perhaps more precisely, bad things DON’T happen. However, sometimes thread repair is necessary, and understanding why threaded holes can be stripped and how they can be repaired is necessary.

To discuss some of the parameters surrounding thread repairs, we asked Gary Reed, president of Lock-N-Stitch for thoughts on the evolution of damaged threads and what engine builders should know about the repair process.

Has the demand for thread repair technology changed?

Yes, new engine designs that call for lower emissions and higher fuel economy have resulted in thinner castings (higher horsepower to weight ratio). Thinner casting walls have led to more cracked and stripped threaded bolt holes. More aluminum engine parts have resulted in more stripped threaded holes than ever before. Some manufacturers like GM use capsulized thread locker on bolts in aluminum parts which leads to stripped holes when bolts or studs are removed.

What are the primary causes of damaged threads?

In cast iron parts, torquing of standard threaded fasteners produces overpowering outward radial stress that frequently results in cracks emanating from the threaded holes. Insufficient thread engagement or depth of the threaded holes due to poor design can also result in damaged threaded holes. (See Doc Frohmader’s article above for information about proper torquing techniques.)

What has been the impact of more powerful engines or power adders?

This can add to the strain on threaded holes. Especially head bolt holes and main bearing cap bolt holes. In order to maximize the clamping force to improve head gasket seal for higher compression, higher strength head studs are available. Higher torque can cause bolt hole failures especially in aluminum parts.


Are thread failures ever caused by installation errors?

The increased use of aluminum cylinder heads has resulted in a much higher thread failure rate in spark plug holes. Over and under torquing are major factors. Cross threading is also a contributing factor. Many bolt holes get damaged during installation due to miss alignment and uneven tightening.

In addition, poor quality components fall under the category of under-designed parts that can’t withstand  the loads required.

How does changing fastener technology impact thread repair options?

Increased fastener strength can increase the strain on the threaded holes which can lead to hole failures. Increasing the strength of the fastener increases the radial outward force which is easily transferred through a repair insert with V style threads to the surrounding metal often leading to a crack. The Full-Torque insert with Spiralhook external threads can contain the spreading force

What should engine builders know about thread repair technology that they perhaps don’t understand?

Most parts were not designed to accept a thread repair insert. When a repair insert is installed it requires making the original hole larger to accept the insert. Enlarging the hole moves it closer to the edge of the part in many cases.

The radial spreading forces created by torqueing all standard fasteners with V shaped threads are often significant enough to cause a crack to form at the weakest or closest point to an edge adjacent to the threaded hole. If a thread repair insert is not installed perfectly perpendicular to the machined surface the bolt will not engage the hole at the correct angle.

This usually results in a failure of the hole or the fastener. Without using a drill and tap alignment device or machine to assure perpendicularity is a very big mistake and prevents correct clamping pressure. Any type of eyeball alignment to install a thread repair insert or attempting to use the old worn, cracked or stripped threaded holes will result in misaligned installation.The new fastener designs are carefully calculated to make sure the proper amount of clamping pressure is exerted exactly where the pressure needs to be.


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