What Are Oil Support Rails And Do They Hurt Performance?

May 9, 2018 / by John Baechtel

Oil rail supports are a common feature of short compression height pistons. Despite some builder's concerns, they work fantastic and allow wrist pin placement and stroker crankshafts that would otherwise be impossible. 

When the crankshaft stroke grows, the piston is often tasked with shrinking in height to accommodate it. This causes the wrist pin position to move up and often bisect the oil ring. Oil rail supports come to the rescue, supporting the ring and allowing ultra-short compression height pistons.

Among the more obscure details of high performance engine building, oil ring support rails have never risen to the glitz and glamour of more esoteric marvels such as copper beryllium valve seats or custom guided lifter bores for high-end applications such as Pro Stock engines. Support rails have been around for many years, performing the basic function of providing a solid, oil-ring-landing surface for pistons with reduced compression height that moves the pin bore high enough that it interferes with the oil ring groove.

Typically, the pin bore passes directly through all or part of the oil ring groove, interrupting the support surface the ring land provides for the oil ring. It’s an installation problem caused by the need to pass the pin through a part of the oil ring groove to install it. When the pin is positioned higher in the piston, the pin bore must be drilled through the oil ring land, removing part of the land’s supporting surface. Once the rod has been installed with the pin passing through the pin bore and pin locks installed, the pin is no longer in the way, but the ring land gap on either end of the pin axis remains.

This diagram illustrates the proper orientation of the anti-rotation detent or dimple in an oil rail support ring. The detent must face downward in the opening for the pin bore. Wiseco recommended placement for the locking detent is centered on the wristpin axis in the pin bore.

Consensus suggest that a slowly rotating lower oil ring rail could dip into the gap, lodge in the pin bore and begin to unravel, first by losing its clearance with the cylinder bore and then jamming until friction and engine speed cause a complete failure. Oil ring support rails are designed to remedy this situation by providing a thicker, snug fitting support ring that covers the gap and provides a firm foundation for the oil ring.

Oil ring support rails are a different breed form your typical piston ring. They are not designed to seal against the cylinder wall, in fact they should never even come close to contacting the cylinder wall. They don’t have to seal against the bottom land either since the oil ring is not a compression ring, but rather a control mechanism that cools and lubricates the piston by collecting oil and distributing it evenly on the cylinder walls. The oil ring is also the primary stabilizing component of the ring pack.

An oil support rail doesn't change the thickness of the ring pack as the rail itself does not touch the cylinder wall.

Support rails are wound tighter with their tension focused inward to snugly grip the back of the oil ring groove. Their only function is to stabilize the oil ring pack and prevent it from coming undone. Support rings often overlap and may have to be ground to fit. In some cases, they will have a large gap built in. The gap may be perhaps .100-inch or so and this is not seen as a concern by piston engineers since the lower oil ring rail cannot fall into such a small gap.

The primary concern is to prevent the oil ring support rail from rotating. Although research is inconclusive whether a rotating support ring is detrimental, most support rings are manufactured with a dimple or anti-rotation detent on the bottom side. The dimple is designed to prevent rotation by catching against the gap in the pin bore before it can move very far. Racers and engine builders have debated whether it really matters, but piston manufacturers would not go to the extra trouble to add the dimples if it didn’t.

Pistons that utilize pin buttons have a built-in groove that provides support for the oil ring where the high pin bore passes through the oil ring groove. A support rail is not required in this application because the pin button fills the gap. Oil rail supports are the choice for the majority of pistons. 

Some racing pistons avoid the need for an oil ring support by using aluminum pin bushings or buttons. These replace the traditional pin locks and are used in pairs with one fitted at either end of the piston pin. They are designed to float with some degree of movement. They are also machined with a flat surface on one side that corresponds to the oil ring groove when installed. The flat surface matches the bottom of the oil ring groove and supports the oil ring without requiring a support rail.

Recommended practice for installing an oil ring support rail is to lightly deburr both edges of the gap and then carefully wind the ring down into the oil ring groove taking care not to scratch the piston. The dimple should be positioned toward the bottom, so it can stub its rotational travel against the cut in the ring groove. It will have freedom of movement in either direction until it contacts the cut. Under no circumstance can the support rail end gap overlap, or it will jam the ring in the bore. A typical gap runs from .020 to .100-inch. A larger gap is inconsequential in most cases.

If the oil support rail gap overlaps the pin bore, it can collapse. Because of this, a dimple in the rail support prevents it from rotating.

The support rail must lay completely flat to provide the best support for the oil ring. Wiseco recommends that users exercise care in not expanding or twisting the support ring too much while working it down to the oil ring groove. Once in position, it should lay perfectly flat all the way around the groove. A final check after the oil ring is installed is to confirm that the oil ring can rotate freely. A word of caution reminds us that the oil ring expander must be installed with the ends butted against each other and not overlapped. Overlapping will cause the oil ring to seize. Do not remove the tiny plastic tips on the expander ring. That will change the tension of the ring and may result in oil consumption problems.

The support rail has radial tension that causes it to hug the back of the ring land, and its radial depth, or clearance, is designed so that it can never protrude past the ring land diameter like the oil ring does. The oil ring itself will always have radial back clearance so it can fit completely into the ring groove if compressed. Once the support ring and the complete oil ring are properly installed, the oil ring prevents the spacer from ever overlapping itself because it has nowhere to go. Circumstances prevailing in every case, whether an oil support rail is necessary or not depends heavily on the application, stroke, deck-height, rod-length, and a multitude of other factors.

Shown is a ring pack installed without an oil ring support– a cardinal sin in engine building. As the piston changes direction the ring pack slams from the top of the groove into the bottom due to the extra clearance. This ruins the rings as well as the ring lands.

Topics: ENGINE BUILDS, featured, ENGINE TECH

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Written by John Baechtel

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