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FAQ: Engine
FAQ (Frequently Asked Questions) >Engine
Difference between low-tension and ordinary piston ringsHow often should oil and filter be changedPicking the Right Perfomance CamshaftThe crank seal has been replaced but it still leaks
What is the difference between asbestos and non-asbestos gasketsWhat kind of replacement pistons are bestWhat things should be included in a complete valve jobWhat type of motor oil is recommended
When replacing a cam, what other parts are recommendedWhy do some engines use RTV sealer instead of gasketsWhy shouldn't torque-to-yield head bolts be re-used
Difference between low-tension and ordinary piston rings

Low tension rings are thinner and exert less pressure against cylinder walls than conventional rings. This reduces friction, improving fuel economy and cylinder sealing. Low tension rings are used in most engines today.

Ring tension is described two ways. One is tangential tension, which is the amount of force needed to squeeze the ends of the ring together. The other is unit pressure, or the amount of pressure exerted by the face of the ring against the cylinder wall.

In the 70s, conventional piston rings had tangential tensions of up to 30 pounds. Compression ring tension specs for a Ford 302 V-8 used to be 22 to 26 pounds. It is 14 to 16 pounds on later model versions of the same engine. On some applications today, compression rings are rated at 5 to 7 pounds.

The amount of force the ring exerts against the cylinder wall (unit pressure) depends on tangential tension as well as ring thickness and cylinder bore diameter. Conventional oil rings exert pressures in the range of 180 to 240 psi. Low tension rings fall in the 90 to 160 psi range.

Most aftermarket low tension rings have a somewhat higher tension than the OE rings they replace. If an OE ring specification calls for 6 to 12 pounds, an aftermarket ring may have as much as 12 to 16 pounds. Higher tension is needed because rings are often installed in oversized cylinders. Cylinder bores may also have more distortion than a new engine, so extra loading improves sealing.

Low-tension rings require rounder cylinder bores, which may require the use of torque plates when honing certain engines. When heads are torqued, cylinder bores can distort up to 0.0015" or more near the bolt holes, throwing cylinders out of round.

This obviously makes it more difficult for rings to seal properly. Simulating bore distortion by bolting a torque plate to the block allows cylinders to be honed so they will maintain their shape when the engine is assembled.

It is essential that correct replacement rings be used. Conventional rings designed for standard grooves must not be used in shallow groove pistons designed for low- tension rings. Narrow, low-tension rings must not be used in deep groove pistons.

How often should oil and filter be changed

Change oil and filter often enough to protect the engine from premature wear and viscosity breakdown. For most cars and light trucks, the standard recommendation is to change oil and filter every six months or 3,000 miles, whichever comes first.

Most late model owner's manuals say that except for "Severe Service" applications, oil change interval can be safely stretched to once a year or every 7,500 miles, with filter changes at every other oil change.

When auto makers make such recommendations, one assumes they are based on extensive durability testing. After all, auto makers themselves would have to bear the warranty costs should their maintenance recommendations prove inadequate.

Except for Chrysler's 7/70 powertrain warranty, and a few others that go up to 5/50 or 6/60, most new car powertrain warranties don't go beyond 3/36. So where's the risk? There isn't any.

With proper maintenance, there is no reason an engine shouldn't go 100,000 miles or more without developing a thirst for oil. That is why most oil companies, as well as aftermarket service professionals, recommend changing oil and filter every six months or 3,000 miles.

They also make such recommendations because many motorists are not aware that they should follow the "Severe Service" maintenance schedule in their owner's manual, calling for oil and filter change intervals of three to six months or 3,000 miles. Severe service (as defined by auto makers themselves) includes:


  • Making frequent short trips (less than five miles)

  • Making frequent short trips (less than 10 miles) when temperatures are below freezing

  • Driving in hot weather stop-and-go traffic

  • Extensive idling and/or low speed driving for long periods of time (taxi, police, door-to-door delivery, etc.)

  • Driving at sustained high speeds during hot weather

  • Towing a trailer

  • Driving in areas with heavy dust (gravel roads, construction zones, etc.)

Protective additives in a motor oil do not hold up as well under such driving conditions for several reasons. If the engine is not running long enough to get the oil hot, condensation and fuel vapors will not boil off. Contaminants will accumulate in the crankcase, leading to formation of corrosive acids and sludge.

Excessive idling and high operating temperatures from towing and high speed driving during hot weather accelerate viscosity breakdown. Exposure to dust can put dirt particles in the crankcase.

The filter also needs to be changed every time for two reasons. Today's pint-sized filters do not contain as much filter material as their quart-sized counterparts. The filter contains dirty oil that can contaminate fresh oil added during an oil change.

Considering what four quarts of oil and a filter cost, versus the cost of replacing an engine, it is better to change oil and filter a little more often than might be absolutely necessary rather than risk not changing it often enough.

Picking the Right Perfomance Camshaft

Choosing a performance cam is never easy because there are so many variables involved and so many cam grinds from which to choose. Finding the one that's right for a given application requires some serious communication between you and your customer.

First and foremost, the cam must match the application. The cam determines the engine's power curve and personality. More specifically, the valve timing created by the cam determines where the engine's peak torque and horsepower will be developed along the rpm scale.

Ideally, an engine should develop its peak power output within the rpm range where it will spend most of its time working. For a street engine, this would be 1,500 to 4,000 rpm. For a competition engine, things do not start to get interesting until you are on the high side of 5,000 rpm.

To match cam to application, you have to ask your customer if he wants more top end power, mid-range torque, or low speed pulling power. Will it be an all-out competition engine, a combination street/strip performer, or a daily driver? Will the engine be turbocharged, supercharged or naturally aspirated? Will it be mated to a manual or automatic transmission? How about the final gear ratio? How much weight will the engine have to push?

Once these have been defined, there are two ways to go. One is to pick a cam and build the engine around it. With this approach, carburetion, compression ratio, cylinder heads and gearing are all matched to the cam to achieve the desired results.

The other way is to match the cam to an existing engine. In other words, given a certain combination of parts, a cam is selected that works with the stock or modified carburetion, compression, heads, and gearing.

Either way, the key is to end up with a combination of parts that work well together. This is where many a novice goes astray when picking a cam. They're seduced by the "bigger is better" trap and insist on the hottest cam in the catalog. They often end up dissatisfied because they have too much cam and not enough engine.

As a rule of thumb, the longer the duration, the shorter and higher the useful power band of the engine. A radical drag strip grind that comes on strong above 5,000 rpm and requires lots of compression, carburetion and gearing is not going to work on the street because the power band is in the wrong range.

High duration cams have other serious drawbacks that make them impractical for the street: they reduce intake vacuum and idle quality (which can upset computerized engine control systems), and they increase emissions (which makes them technically illegal in areas requiring tailpipe emission inspections).

In a typical 300 cubic inch V-8, 215>1| (measured at 0.050" cam lift) is about all the duration the computer can handle before a recalibrated PROM chip becomes necessary. A 350 V-8 can handle up to 220 degrees of duration before intake vacuum is reduced enough to affect the computer.

There is more to picking a cam than comparing a list of duration and lift specs. Another consideration is the amount of spread or separation between cam lobes. This determines valve overlap and how the opening and closing of the valve relates to the pistons.

According to the experts, the intake valve should be at least halfway open by the time the piston reaches maximum acceleration away from top dead center. This is the point at which a piston generates the strongest pull on air in the intake port.

Choosing the right cam takes some thought. It does not have to be a "hit-or-miss" proposition. As long as you understand why the cam has to be matched to the application, you can usually pick the right cam by following the guidelines outlined in most catalogs, or by using one of the new "cam selection" computer programs or hotlines that are available. By entering the appropriate application data (engine displacement, compression ratio, rpm range, vehicle weight and gearing, tire size, etc.), the program will give you a specific cam recommendation by part number.

The crank seal has been replaced but it still leaks

Try a crankshaft repair sleeve. When a groove is worn in the crankshaft seal area, a new seal will not have a smooth surface to seal against. It will either leak immediately or wear prematurely. Replacing the crankshaft will solve the problem, but it is an expensive fix. An economical and effective alternative is installing a slip-on repair sleeve.

What is the difference between asbestos and non-asbestos gaskets

One gasket type contains asbestos as a reinforcing fiber while the other does not. As a reinforcing fiber, asbestos is strong, can withstand high temperatures and chemical attack, and is relatively cheap compared to many other materials. The physical properties that make asbestos an excellent gasket fiber also make it a hazardous substance to work with.

Asbestos fibers are long, thin and extremely small. Exposed fibers easily shred into thin needle-like strands that can drift in the air and be inhaled. The fibers lodge deep in the lungs where their sharp needle-like presence becomes a source of constant irritation. Over time, the accumulation of asbestos fibers can lead to a variety of lung ailments, including cancer.

Because of that, the government has tried to ban asbestos. A court ruling has put the proposed ban on temporary hold. Even so, all domestic gasket manufacturers are now using non-asbestos materials in their U.S. gasket plants. Asbestos is still being used by offshore manufacturers and is still found in many import applications.

The asbestos hazard is only a concern to those who work in the asbestos handling and processing industries, including companies that supply gasket manufacturers with rolls of gasket facing material and paper.

Once fibers are encapsulated in the filler material used to make gaskets, they cannot escape and pose no significant health hazard to those who work in gasket manufacturing plants, the distribution system, or installers.

Most domestic gasket manufacturers today use a proprietary mix of non-asbestos reinforcing fibers to produce gaskets equal to or better than asbestos gaskets they used to make. One type of fiber being used is aramid fiber (Kevlar), which is two to nine times stronger than asbestos.

Kevlar lacks the temperature resistance of asbestos and cannot be used on exhaust manifolds or certain head gasket applications. Kevlar also costs a lot more than asbestos. The amount of Kevlar typically used in an asbestos-free gasket material is usually no more than 10%.

Expanded graphite is another material being used in place of asbestos. Graphite is an excellent conductor of heat and can easily handle temperatures as high as 1,800 degrees F. It is ideal for high heat applications such as exhaust manifolds and head gaskets in diesel and high output engines. Graphite is also a natural lubricant, making it well suited to engines with cast iron blocks and aluminum heads. Like Kevlar, graphite is expensive.

What kind of replacement pistons are best

Ordinary cast pistons are usually adequate for most passenger car applications. When it comes to modified or high output engines, racing engines, marine engines, and severe-duty applications, ordinary cast pistons may not be adequate in terms of strength, durability and longevity.

Higher operating speeds, horsepower output, torque loads, and thermal stress create an environment calling for a "performance" piston. For most applications, that means forged pistons made from forged aluminum slugs. Ordinary cast pistons are made by pouring molten aluminum into a mold. The forging process increases metal density, significantly improving strength, ductility and thermal characteristics.

Hypereutectic pistons (which are also cast) are a low-cost alternative to forged pistons for certain original equipment engine applications requiring something better than an ordinary cast piston.

Hypereutectic alloys contain a higher level of silicon (16% to 22% versus 8% to 11% in a typical cast piston). This increases hardness to reduce ring groove, pin boss and skirt wear. Most performance engine builders use the forged variety. Pistons in high output, racing, marine, or severe service applications are subjected to forces far exceeding those encountered in everyday passenger car engines.

In a 350 Chevy passenger car engine with a compression ratio of 8:1, combustion pressures generated at wide open throttle typically peak out at around 700 psi. This yields a total force of about 8,800 pounds pushing down on the top of the piston.

By comparison, a high performance 350 engine with a compression ratio of 12:1 can generate upwards of 1,200 psi of combustion pressure at wide open throttle. This translates into a downward force of 15,000 pounds on each piston (nearly twice that of the passenger car engine). Although a cast piston might be able to survive this kind of punishment for a while, a forged piston is preferred for long-term reliability.

The difference in strength between forged and cast pistons is considerable. Cast pistons made of F-132 alloy (commonly used for many OE pistons) typically shatter when their maximum yield strength of around 27,000 psi at room temperature is exceeded. Hypereutectic alloys stand up a little better than standard cast alloys, but are also brittle and shatter when their yield limits are exceeded.

Forged, on the other hand, can withstand loads anywhere from 40% to almost 100% higher without failing. When they do fail, forged pistons tend to deform rather than shatter because the forging process makes the metal up to 600% more ductile.

What things should be included in a complete valve job

Everything needed to restore a cylinder head to like-new condition, including basics such as:


  • Cleaning cylinder head and valve train components

  • Inspecting head for cracks, leaks, warpage, or other damage

  • Inspecting valves, seats, springs, guides, and other valve train components for wear and damage (and replacing parts that cannot be reused)

  • Grinding or cutting valves and seats

  • Restoring valve guides by knurling, relining or replacing, or reaming guides and using valves with oversized stems

  • Checking installed valve stem height, spring height and tension, etc., and reassembling the head with new valve guide seals and any other required parts

  • Resurfacing head to assure flatness and a proper seal

  • The following "extras" might be required, especially on overhead cam aluminum heads:

  • Repairing leaks or cracks

  • Straightening

  • Overhead cam line boring

  • Repairing threads

  • Replacing seats

There is no such thing as a "standard" valve job. Every job is different. An overhead cam aluminum head may require a lot more time and effort than a cast iron head off a pushrod engine. There is often no way to tell what a head will need in terms of repairs until it has been cleaned, disassembled and inspected.

Exhaust valves and springs often need to be replaced. Valve guides and/or seats may have to be replaced. The head may be warped or cracked, requiring additional repairs. The list of things included in a "complete" valve job will vary from job to job.

What type of motor oil is recommended

Use the type of motor oil specified in your customer's owners manual. Most manuals say it's okay to use a variety of viscosity grades depending on temperature conditions. Generally speaking, the following holds true:


  • 10W-30 is best for all engines for year-round driving. 10W-40 is more popular in the aftermarket, but 10W-30 is actually a better oil because the additive package in it holds up better over the long haul. This is why General Motors does not recommend 10W-40 motor oils for any of its cars.

  • 5W-30 is approved for most late-model four-cylinder, V-6 and V-8 engines on a year-round basis. It is not approved for many turbocharged or diesel applications, some high output V-8s, or applications involving driving at sustained highway speeds or towing in hot weather. It may not be the best choice for older, high mileage engines.

  • 5W-30 is used as the factory fill oil on most new cars because it pumps through the engine more quickly after start-up (important for keeping overhead cams properly lubed). It also makes cold weather starting easier and reduces fuel consumption.

  • Straight viscosity oils have limited temperature ranges and lack the versatility of multiviscosity oils. They can be safely used as long as their temperature limits are observed.

  • Straight 10W is okay for cold weather starting and driving, but too thin for warm weather driving.

  • Straight 20W is okay for all around driving, but doesn't provide the temperature protection of straight 30W (which gets too thick at low temperatures for easy cold starting).

  • Straight 40W and 50W oils are primarily for heavy-duty applications.


  • Special multiviscosity oils such as 2OW-50 are typically formulated for racing or severe duty applications such as towing.

  • Synthetics are a good alternative for any of the above because most provide extended temperature protection and service life.

When replacing a cam, what other parts are recommended

New lifters should always be installed. Reusing worn lifters on a new cam is asking for trouble. With flat tappet cams, lobes are ground with a slight taper so they will spin the lifters as they push them open. The lifters are also slightly convex on the bottom to reduce the point of contact and to promote rotation. If lifters are worn flat or concave, the mismatch between lobes and lifters will accelerate wear and probably ruin the new cam in short order.

New valve springs are also recommended in many instances, especially if the cam is a performance grind with increased lift and duration.

Pushrods should be checked for straightness and replaced if any are bent. Rocker arms should also be inspected and replaced if found to be worn or damaged.

Other parts that might be needed include a cam drive gear and chain set (or belt on overhead cam applications), a timing cover gasket set, and assembly lube (to prevent lobe wear during cam break-in).

Why do some engines use RTV sealer instead of gaskets

For a period in the 1980s, domestic auto makers thought they could lower production costs and improve sealing by using RTV (Room Temperature Vulcanizing) silicone instead of conventional cut gaskets to seal valve covers, oil pans, timing covers, transmission pans, and other parts. In theory, the idea made sense. By applying a thin bead of RTV to such parts, they could be assembled and sealed in one step.

Engineers liked RTV because it does not take a set like a conventional cork/rubber cut gasket. Heat causes a cork/rubber gasket to harden and become brittle with age.

To seal properly, RTV requires both surfaces to be clean, dry and oil-free. Though installers use RTV all the time with no problems, car makers apparently could not keep their parts clean enough on the assembly line to produce a lasting seal with RTV. They found they were having more oil leaks, not less, with RTV.

Eventually, domestic auto makers dropped RTV in favor of molded silicone gaskets which combine the installation ease of a conventional gasket with the sealing properties and durability of silicone.

When working on an engine that has RTV instead of gaskets, the installer can either use RTV to reseal the engine, or replace the RTV with conventional cut gaskets. In some applications, longer bolts may be necessary to compensate for added gasket thickness.

Some prefer to use RTV because it eliminates the need to stock a lot of different gaskets. Others prefer to substitute a cut gasket because of RTV's limitations. Care must be taken when using RTV so excess sealer does not seep out from between the seating surfaces and end up someplace where it does not belong.

RTV takes 30 minutes to an hour to set up (full cure takes about 24 hours). The vehicle should not be driven during this time.

Why shouldn't torque-to-yield head bolts be re-used


Torque-to-yield (TTY) head bolts are designed to stretch when used. Once stretched, they are not as strong as before. Consequently, they cannot provide the same amount of clamping force and may break or shear off if reused.

The TTY bolt-tightening procedure is designed to provide a better, more uniform seal. On the factory assembly line, sophisticated torquing equipment is used to tighten head bolts beyond their yield point. This stretches the bolt slightly and evens out the loading so each bolt provides almost exactly the same amount of clamping force on the head gasket.

Because the tightening procedure permanently stretches the bolts, there is a risk of breakage if reused. Since there is no way to tell how many times such a bolt has been reused, most aftermarket gasket manufacturers say the risks of reusing TTY head bolts far outweigh replacement cost. For that reason, new TTY head bolts are often included in head gasket sets.

Applications where new TTY head bolts are recommended include Chrysler's 2.2L and 2.5L engines, Ford's 1.6L and 1.9L Escort engines, General Motor's 1.8L, 2.0L and 2.5L fours, 3.0L V-6 and 381 diesel V-8.

When new TTY head bolts are installed, a special tightening procedure must be used to achieve proper results. After bolts have been tightened to the recommended torque, each bolt must be given an additional twist. The amount of twist may be 1/4 turn or more, or specified as so many degrees of rotation. Using a simple "torque-to-angle indicator" tool when making the final twist ensures uniform loading and prevents overtightening.



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