Auto Services - Maintenance
Maintenance Topics
  1. Difference between low-tension and ordinary piston rings.
  2. Picking the Right Perfomance Camshaft.
  3. What kind of replacement pistons are best?
  4. What things should be included in a complete valve job?
  5. When replacing a cam, what other parts are recommended?
  6. Why 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.

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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.

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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.

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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:

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.

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When replacing a cam, what other parts are recommended

Engine cross-section for cam replacementNew 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).

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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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