Click on a thumbnail to see the full-size image
The Task of Sealing Gaskets, Bill McKnight
By Bill McKnight
Gasket sealing technology is a big issue with today’s modern engines. Hours of work may be involved in replacing the head gasket. New types and styles of gaskets, different engine materials, and changes in tightening technology increase the chance of an installation problem – something no engine rebuilder wants to have happen. Fortunately, sticking to some basic rules and guidelines will greatly reduce the possibility of these problems occurring. In this article, we’ll go through nine key issues in an effort to cover all the bases.
It’s important to feel confident in the gasket you’re installing. When the stakes are high, you want a well-engineered gasket, made of modern materials from a reputable manufacturer who backs up its product. Multi-layer-steel designs are quite common on newer engines, and more than a few engines utilize separate right and left head gaskets, increasing the odds that something could go wrong. Be diligent when installing the gaskets to be sure you’ve got the left and right, top and bottom sorted out.
Surface conditions boil down to two issues; surface finish and flatness. In the most basic sense, flatness is the measurement we’ve all made with a straight edge and feeler gauge. A rule of thumb for determining flatness is a maximum deviation of .002˝ from level in any 6˝ span across the casting.
Some engine manufacturers are also specifying waviness today. A relatively expensive measuring tool is required for the waviness (Wt) measurements. Upwards of $10,000 can be spent for a profilometer capable of measuring waviness. This is probably overkill for all but the largest rebuilders, or those who have OE contracts. So, what can the small rebuilder do? Keep the cutting tools sharp; use solid, quality-made fixturing to hold the heads and blocks being machined; and machine any heads and blocks that are questionable in regard to flatness.
Surface finish (Ra) is also critical for proper gasket sealing and engine longevity. In many cases, the gasket material is more accommodating of finish variations than the engine is. In other words, the gasket will handle a rougher finish that can be tolerated in the engine because of concerns about gasket fretting. Gasket fretting is a very real concern in bi-metal head and block design. Take two dissimilar metals with different expansion rates and add a gasket sandwiched between the two, and you’ve got an invitation for problems. If the gasket sticks to, or is embedded into the finish of, and/or both surfaces and the relative motion of one surface to the other differs, we’ve got gasket fretting and eventual failure.
Victor Reinz has outlined conservative recommendations for the different types of engines and the different types of gaskets that we see today. That isn’t to say that finishes are limited to the parameters in the chart, only that we feel very confident that if you adhere to these parameters and follow the other recommendations in this article, you’ll have excellent gasket reliability.
Fastener Quality, Condition
Bolt load, for example, means more than "torque to 70 ft. lbs." It’s a science in today’s modern engines. OEMs have fastener labs and devote much time and technology to perfecting the tightening methods and procedures used on their engines. Rebuilders need to take as much care as possible to ensure that they duplicate OE conditions.
As fasteners are tightened in the engine, they stretch. Because of the elastic properties of steel, the fastener wants to return to its original length (tension). Load on the gasket surface is created by the elasticity of the fastener and this desire to return to the original length.
Fastener threads deteriorate with use and also can become damaged by handling and cleaning, negatively affecting the ability of the fastener to deliver the desired load on the gasket. There’s also the potential for fasteners to fatigue, to work-harden and to become at least partially yielded during normal engine operation.
All of these potential problems are reasons to replace fasteners during the engine rebuild. So, try to think beyond the small block Chevy days, in which you could seal virtually any gasket with 17 head bolts. Times have changed and so have engine dynamics. We just don’t have the same margin for error. Engine builders need to give very serious consideration to replacing head bolts when they rebuild an engine.
Quality, Condition of the Threaded Hole
Often neglected, but equally important as the fastener, is the condition of the threaded hole the fastener screws into. Thread chasers should be employed to chase every head bolt hole. In the traditional method of tightening fasteners, using a torque wrench and tightening to a specified point, thread friction accounts for about 45 percent of the tightening effort applied to the fastener. Rough threads in the block can increase that percentage, robbing effort that would have, under normal conditions, been used to stretch the fastener and load the gasket.
In the engine business, the standard lubricant for threaded fasteners is 30-weight motor oil. Both the threads and underside of the head of the bolt should be lubricated because these are the two major friction sources. There are, however, a couple of exceptions to this general rule. One is for fasteners that screw into contact with the water jacket. Their threads should be coated with a thread sealer rather than 30-weight oil. Another case is when the fastener comes with lubricant or sealant pre-applied. Finally, of course, is the case in which the engine manufacturer specifically recommends something other than 30-weight oil as a lubricant.
The use of so called "super lubricants" can actually be detrimental, reducing the friction so much that the fastener is inadvertently yielded in the normal tightening process. The logic in this is if the bolt is stretched the desired amount using 30-weight oil as a lubricant, then using a lubricant that reduces friction more than 30-weight oil will result in more stretch at the same torque. This could potentially cause a fastener or a gasket failure.
On average, 90 percent of the effort applied to tighten a head bolt is used overcoming friction. This fastener friction is spread almost equally between thread friction (45 percent) and under head friction (45 percent). The 10 percent of the remaining effort goes to stretch the fastener and to load the head gasket. It should be obvious then that anything you do in your operation to increase the friction — like not using lubricant, damaging the threads of the fastener, not cleaning the threaded hole in the block — reduces the amount of load applied to the gasket.
With conventional tightening strategies, measuring resistance to turn with a torque wrench, this is a major issue in terms of gasket reliability. Realizing this is a very real concern with both gasket survival as well as block and bore distortion, manufactures have been quick to adopt a tightening method that eliminates the effect of friction variations when a head bolt is tightened.
The method of measuring the tightness of a fastener by the distance it has been turned instead of by how hard it is to turn is widely used today. If you think for a minute, tightening a bolt 180 degrees could be easy or difficult depending on the amount of fastener friction. The net result though, is the same amount of stretch and, consequently, the same amount of load no matter what the friction. Using this process, manufacturers build engines with very even load on the gasket and provide us with a tightening methodology that allows us to very accurately duplicate their results when we reassemble the engine.
As an engine builder, your stakes are very high when you rely on tightening tools. A torque wrench that is off by 10 percent can be the difference between success and failure. Torque wrenches are still used for lots of conventional tightening jobs, and they need to be checked and calibrated. We suggest annual inspections for a small shop and daily or weekly inspections for the big rebuilder.
Measuring the amount of turn accurately is also important. If the OE specs call for 62 degrees of turn, then by all means, that’s what you need to apply! Make sure you’ve got a tool that’s accurate and a solid process to achieve it!
Accuracy of the Operator
Almost everything we’ve talked about in this article is done by human hands, controlled by someone who makes a decision on what to do or not to do. It’s one thing for you to be aware of and understand the importance of these processes, and another to be sure your employees do too.
Ask yourself these questions: Have your employees been educated and trained in proper procedures? Is their performance monitored? Do they "buy into" the need for quality? These three things are as important as anything we’ve discussed in this article and will result in failures when they’re not under control.
Most of you reading this article could recite the tightening specs for a 350 Chevy by memory. In the old days, specs didn’t change very often; sometimes remaining the same for years. That’s not the case today.
Today, specs can change frequently, sometimes even during the model year or after the engine has been put in use. As rebuilders, we have to make every effort to have the absolute most reliable specs possible, then make sure our employees use them. That means reliable sources, frequent rechecking to make sure they haven’t changed, and monitoring employees to ensure they use current specs.
We’ve discussed nine things that are important when it comes to sealing the head gasket. Think of them as variables that need to be controlled. For instance, you can do something about the condition of the bolt hole or not; it’s up to you. If you choose to control that variable, it’s unlikely to cause you a problem.
Look at the other eight factors the same way. Can you identify processes in place in your operation to control each one? The problems you have with head gasket sealing will relate directly to how many of these critical processes you have under control.
Bill McKnight, Contributing editor