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If you’ve ever driven a car that experiences regular piston slap, you’re probably wondering how to fix it. In general, this problem requires the purchase of a new piston or re-bore the existing piston. But it’s not always necessary to replace the piston immediately. You can run the engine for thousands of miles before you notice the slap. And if the piston slap has occurred only a few times, it may not even be worth the repair.
GM hydraulic roller lifters have solid rollers
The most important benefit of a GM hydraulic roller lifter is its reliability. These lifters are standard on millions of engines and can go years between adjustments. You can even go years without changing them if you’re driving a high-revving Holden 6 with a high-revving valvetrain. However, high-revving Holden engines will require more frequent adjustments than lower-revving V8s.
Another important factor in quiet piston slap is proper valvespring pressure. High-quality lifters are made to handle higher valvespring pressure without noise. In some engines, such as small-blocks, Dart heads use single springs with a 110-pound seat load and 280 pounds over-nose seat load. Higher spring loads can also be achieved with good-quality lifters. Performance cams, on the other hand, typically produce noise because they have aggressive lobe designs.
GM pistons are barrel-shaped
A barrel-shaped piston has one or more elliptical features, such as an expanding skirt at the top of the piston. The shape is also known as an ovality, and is determined by the piston’s application, thermal properties, skirt thickness, and overall size. The ovality of a piston helps to minimize friction. Generally, the piston’s skirt is shaped like an oval in order to provide an even distribution of thrust loads and minimize the amount of friction.
GM pistons are barrel-shaped, as they were originally designed. They have a ring belt and skirt that extend peripherally from the crown portion 18, and two generally opposed skirt portions. The ring belt portion and the pin boss portion are generally cylindrical, and both converge toward the cylinder bore wall. Generally, the second skirt portion of the piston is barrel-shaped as well, extending from the crown portion 18.
The skirt of a piston is designed to reduce friction and maximize stability during secondary motion. The skirt shape is an important factor for maximizing stability, but is often compromised in the name of reducing friction. Pistons with short skirts are more efficient at reducing friction, but can also compromise on stability. For increased stability, a piston with a long skirt may be a better choice. The skirt’s surface area can be optimized to reduce friction, and the ring seal is improved. Advanced piston coatings have become the norm in racing circles and are designed with a thermal heat barrier on the crown and friction-reducing coating on the skirts.
GM hydraulic roller lifters have pounds of pressure pushing on them constantly
GM hydraulic roller lifters have to handle many pounds of pressure, but the modern designs have better overall performance. Factory grinds were popular during the 1960s, but most were single-pattern designs. GMPP hydraulic roller lifter cams work with Gen VI engines with single-roller timing sets. Mark IV engines can also use tie-bar hydraulic roller lifters. These cams can also be retrofitted with Gen V engines.
The inertia in the valvetrain helps overcome the weight of the return spring. This forces the lifter to bounce instead of following the cam profile. This bounce wears the valve seat and causes harmonics to travel through the system. This decreases volumetric efficiency and robs horsepower. GM has not installed flat-tappet cams in a production small-block in decades.
To repair a GM hydraulic roller lifter, remove the valve spring. The spring is the most likely source of valve float. A hydraulic lifter piston assembly is only able to handle so much pressure before it starts to wear out and experience valve float between six and seven thousand RPM. Consequently, the valve spring may experience some valve float. If this happens, the piston assembly has to pump up more fluid and is subject to higher RPM. The high-RPM operation of a car engine requires a solid roller lifter, but it is also expensive and needs routine lash adjustments.