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Wondering why GM used headbolts on the C5 knowing that there are problems with locking, instead of going with threaded stud posts with a threaded end and nut? Below is something to live by with aluminum engines.

I. Engine Mechanical
E. Dealing with Aluminum Heads and Blocks
Aluminum is a wonderful metal. Made from plentiful bauxite ore, it's strong, dissipates heat well, and even looks nice milled or polished. But, of course, its main virtue is lightness they make airplanes out of it, after all. And that's why a 1.5L Honda Civic engine, which has both its head and block cast of this stuff, weighs a mere 223 lbs. wet.

Engineers have long lusted after this advantage in mass. There were many early applications, but the first I remember was the Ford flathead V8 60 with its aluminum heads. Terrible warpage and corrosion problems (they used straight alcohol as anti-freeze back then) made this a short-lived experiment. If you've been into cars for a while, maybe you've encountered the little 215 cube V8 that Buick and Olds put out in the early sixties. Its aluminum block with iron sleeves was so light, a strong man could lift the whole motor. This was pretty successful (if expensive to manufacture) and survives today in an upscale manifestation the $50K Range Rover.

If that's before your time, how about the Vega with its aluminum block/iron head OHC four? No sleeves this time. Instead, the wear surface of the cylinder walls was made up of "proud" silicon exposed by an electrochemical etching process. Lots of problems here.

Cadillac's once sterling reputation was damaged by the first version of its sleeved aluminum block/iron head V8, the notorious HT4100. But serious reengineering has resulted in laudably improved dependability in later iterations.

Population penetration
Now, of course, it's becoming pretty hard to find a new engine, whether imported or homegrown, with an iron head, and the number of light alloy blocks out there is growing more rapidly than you may realize every Honda power-plant is now aluminum top and bottom, and so is the Saturn's 1.9L four, to cite two prominent examples of the trend. Ergo, you're going to have to learn to live with this metal.

Those of us who actually work on cars can be excused for not being ecstatic about that because iron, the traditional material of engine castings, has some undeniable advantages. It's stable, easy to service, doesn't present any unusual thread or corrosion problems, and, on blocks, you can bore deeper. But it's just too heavy to be acceptable to today's ruthlessly efficient engineers. Also, when used for the block in a bimetal unit, differing expansion rates cause trouble. Until plastics and ceramics take over (if you take very good care of yourself, you may live that long), aluminum's going to be it.

Basic nature
I'll start with some notes on this shiny metal's basic characteristics. First, there's its arch enemy: corrosion. All you really need to know here is that regular coolant changes are absolutely critical. You might even want to do this annually instead of every two years to keep that additive package up. Use antifreeze (an unfortunate term since it does a lot more than keep icebergs from forming) that contains the right formulation for aluminum, which is to say buy brand name. The regular 50/50 blend is still appropriate.

Aluminum isn't exactly bulletproof, either. One slip with that scraper can make a nick no gasket will seal. And if you like to go over sealing surfaces with a Scotch Brite pad on a sanding disk, you may find yourself having to buy some new castings because of the waves you've caused.

As far as chemical cleaning is concerned, solvents blended especially for this delicate metal will keep it from dissolving in the hot tank like an Alka Seltzer. Now, oven cleaning is becoming popular because it doesn't require any chemicals that are costly to buy, keep hot, and dispose of (they're considered hazardous waste).

How do you find out if that casting is a potential leaker before lavishing time and money on it? There are a couple of ways. As the machine shop training manager of a major aftermarket parts company told me, "Pressure testing remains the most reliable way because it catches stress and fatigue cracks which might not otherwise be visible, and also tests for porosity. But it's relatively expensive because it involves sealing the water jacket, pressurizing it with 4060 psi of air, then immersing the head in water or spraying it with soapy water. Bubbles indicate the location of a problem."

Aerosol fluorescent dye can also detect cracks in aluminum components. A penetrant is applied, then a developer, and an ultraviolet light is passed over the suspect area. Fissures stand out like lines on a black light poster.

Cracks are not necessarily a death knell. Castings are often resurrected using TIG welding, but you've got to find somebody who knows what he's doing or you're risking a disaster. If the engine is common, you might very well do better shopping for a used unit at your favorite wrecking yard. If you trust your machinist, ask his advice when he calls with the bad news. If he says he has no problems with his aluminum welding repairs, maybe he's right, but ask how much it costs before giving the go-ahead.

Then there's porosity. Especially on lost foam, low-pressure cast components, coolant and oil seepage right through the material is common. At the factory, the pores are plugged with silicones and resins, then the cooling system is filled with specially formulated antifreeze. But after a block or head is cleaned (hot tanked, washed, or baked) during major repairs and regular coolant is used, the original sealing characteristics are lost. And, if welding has been done to fix cracks or eroded areas, serious seepage is even more likely.

The solution? Try to find a machinist who's invested in aftermarket equipment and materials designed to reestablish that fluid barrier. This will be a circulator setup that permeates the metal with a silicon based ceramic sealant at about 180 deg. F. Air pressure is used to force the substance into the pores, then it's cured into a tough, perfectlybonded glaze. You may have to travel to a metropolis to find such a machinist.

Fastener deathgrip
I'm sure you've encountered evidence of the electrolytic action that occurs wherever dissimilar metals meet. That's what makes steel bolts grip aluminum threads so tenaciously. Anti-seize compound is the right stuff for preventing problems, although anaerobic thread locking chemicals also help.

Which brings me to thread repair, something you'll be doing a lot more frequently with aluminum than you're used to with iron. A good set of screw extractors, metric taps, and a suitable thread insert system in the most common sizes would be great. Working with this metal is less frustrating if you have what you're bound to need on hand.

With aluminum heads, the first thing to remember is not to cause trouble. Wait until the engine's cold before you twist out those head bolts or the casting will warp as it comes down to room temperature without the rigid support of the block.

But warpage is a fact of life anyway. It's common for the casting to develop a hump in the middle on bimetal power-plants, it's been trying to expand over twice as much as the block through all those cycles of heating and cooling.

If your straightedge and feeler gauges have revealed excessive undulations, you'll need the services of a machinist (unless you're wealthy enough to afford a new casting). Since aluminum tends to clog that big, expensive grindstone, the more involved operation of setting the head up on a broach for milling is generally used. In some cases, however, such as the VW diesel head with its hard steel precombustion chambers, a grinder is the easiest way to go even though the stone will have to be dressed often.

There are limits. Go too far and compression will rise to the detonation point (extra thick aftermarket head gaskets are available in some cases to compensate for this). If the engine's a V6 or V8, a corresponding amount has to be taken off the intake manifold sealing surface. Cam sprocket timing marks can become meaningless, and it's even possible to make the casting too thin to be dependable.

A related point is that a cam can't spin freely if its bearings aren't aligned, and that's exactly what happens when an OHC head warps. The cam bores at either end are lower than those amidships. On designs using bolt-on cam bearing retainers, it's often possible to shim them up as necessary to achieve straightness. Otherwise, align boring and the installation of bearings with thicker shells becomes necessary. But, as with machining of the head's sealing surface, there are limits, and the operation is expensive in any case.

Cook 'em
Even if sealing surface flatness, cam saddle straightness, or both are beyond retrieval by traditional means (our machine shop instructor says .008 in. is generally the max aluminum heads can be cut, yet he often sees them with more than .020 in. of warpage), there's a relatively new procedure that can often save it: heat straightening. I've talked about it elsewhere in this encyclopedia, but it bears repeating.

Shims are placed under the casting where necessary, it's bolted to a heavy, absolutely flat block of steel, then the assembly is baked in an oven. The heat relieves the stress and makes everything fall into place. You can get results in half an hour at 400 degrees F. Since straightening is usually done in conjunction with heat cleaning, however, the parts are often left for hours, or even overnight. In most cases, you wind up with a head that needs nothing but a light cut to be perfect.

Guides and seats
Guide service is important. Most of the automotive machine shops I deal with like to replace them, but there are the usual alternatives: knurling, or reaming and the installation of valves with oversize stems. Do whatever fits the economics of the specific case.

If you go with new inserts, measure the protrusion of the originals, and make damn sure you support the area around the bore properly before applying pressure or impact. Some manufacturers say you should oil the new guides, and others want you to heat the head, so consult the manual. You should also know that reaming after installation is necessary to achieve the right oil clearance.

Valve seats are another consideration. In the majority of cases, the seats can be ground into decent shape. Sometimes, however, a loose or damaged seat must be replaced, which will require a delicate operation best left to a specialist.

Erosion of the aluminum under a valve seat can create an alarming looking pocket. This may mean the head has a future as an impractically large doorstop, but there are intrepid machinists who actually repair this type of damage providing it hasn't gone too far. As Charlie Bennett, owner of AIM Machining in N.J., told me, "I TIG weld them, then remachine. It works most of the time." So, ask your head man what he can do before junking the part.

I didn't dig up as much on light alloy cylinder blocks as I'd expected. It seems there just aren't enough of them out there in the aftermarket repair cycle yet to have created a substantial body of real world service experience.

But I can still mention a few essential points. To begin with, most specimens have pressedin iron cylinder liners. Some are meaty enough to allow boring, others aren't. For example, Honda lets you go .020 in. over, but Toyota's big bux Lexus 400 V8 has sleeves only .08 thick, so all you can do is a little honing.

Liners that leak at either the top or bottom are a potential problem. Commonly, they can be satisfactorily sealed by the ceramic glazing process mentioned above.

Cadillac doesn't allow you to bore its V8, but you can replace the liners, which sit in Orings.

Then there are aluminum bores that rely on a silicon wear surface, ala the Vega. You'll find a much more highly evolved version in Mercedes V8's. If they get scored, the only fix is a bore and sleeve job. Or, you could buy a new short block how does $20,000 for the part alone strike you? Fortunately, MB does an outstanding job engineering, manufacturing, and inspecting its engines, so cylinder problems are rare.

Freeze plugs are a critical area. In order to avoid leaks, many authorities are now recommending that you use the rubber expansion type instead of the plain steel dish variety. They'll seal perfectly against aluminum, they're easy to install, and modern synthetic rubbers last a long, long time.

Finally, if paint or an accumulation of oil and dirt makes you unsure of that block's material, just try a magnet.


568 Posts
Wondering why GM used headbolts on the C5 knowing that there are problems with locking, instead of going with threaded stud posts with a threaded end and nut?
Would studs still allow the heads to be removed with the engine in the car?

Many were fond of replacing the head bolts with studs on the small block Chevy. Funny thing is, serious race engines shops I knew who did Chevy and Ford engines said it was a Chevy problem. The Ford blocks were stronger and and the threads held up okay.

Regardless, I agree that studs are a better choice.

Shims are placed under the casting where necessary, it's bolted to a heavy, absolutely flat block of steel, then the assembly is baked in an oven. The heat relieves the stress and makes everything fall into place. You can get results in half an hour at 400 degrees F.
That's a fine way to go.. But the epoxy in the intake ports of my race heads would hate me for that! :eek:
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