Why can’t I use my stock pushrods?

Wear, length and strength are three reasons not to use your stock pushrods.

Wear on the ends of the pushrod where it contacts the rocker or lifter is always a concern. Because of the high loads, it’s important that the radius on the end of the pushrod match the radius of the seat in the rocker or lifter and that it be free of scoring. A used pushrod end may not be worn round and may have score marks on it.

Length is important on a stud mount rocker to assure proper rocker geometry. On a non-adjustable pedestal mount rocker, length is critical for proper lifter preload. Whenever you change anything in the valvetrain pushrod length must be checked.

Strength is always a concern as stock pushrods are marginal for stock cam profiles and spring loads. When higher load springs, faster cam profiles and higher rpm are involved the increased pushrod flexing (pushrod bends then springs back) disrupts valve timing, rpm stability and component durability.

What should I look for in a new pushrod?

Absolute minimum wall thickness of .080″ to reduce pushrod deflection or flex, this also includes the ends (radius) of the pushrod. If you’re running pushrod guideplates the pushrod must be hardened and smooth to be compatible. The radius on the pushrod ends must be correctly sized and with a smooth finish free of tool marks to prevent galling of the pushrod seats in the rocker and lifter. Scorpion Racing Products pushrods have an .083″ wall thickness on the 5/16″ diameter units and .120″ wall on the 3/8″ diameter ones. Our radius ends are billet CNC machined for radius accuracy, finish and maximum thickness. SRP pushrods are hardened and centerless ground smooth for compatibility with guideplates.

I’ve never had a pushrod failure, why change them?

Most people think if they haven’t had a visible problem (broken, bent or galling) with their pushrods that nothing better is needed. This is wrong! Even on mild performance applications, a pushrod that is flexing can cause problems from power loss to component failures elsewhere in the valvetrain and there may be no actual pushrod failures.

Should I use a 5/16" or a 3/8" diameter pushrods?

Usually the cylinder head design limits the diameter you can use. Without having sophisticated testing equipment, the general rule is to use the most rigid pushrod that will fit. Naturally the 3/8″ diameter is more rigid than the 5/16″ diameter, so if it will clear the cylinder head, use the larger diameter.

Will the extra weight of the larger diameter/thicker wall pushrod affect rpm?

Added weight on the "pushrod side" of the valvetrain is not as important as on the "valve side". The added power, rpm and valvetrain durability from the lack of pushrod flex is more important.

What is pushrod flex and how does it affect power and durability?

The pushrod deflects or flexes upon valve opening and then again just prior to valve closing. As the cam tries to move the valvetrain to open the valve, it has the weight of the valvetrain against it and in the case of the exhaust valve, cylinder pressure also. Upon valve closing, the cam is trying to slow the valvetrain down. So on valve opening and valve closing the pushrod flexes and as rpm increases it gets worse. Here’s how it effects rpm, power and durability;

Rpm is reduced when the pushrod flexes, and then rebounds like a pole vault. It throws the rocker and valve out of control. As rpm increases, the more the pushrod flexes, the more out of control the valvetrain becomes and the more valve "float" or "bounce" to the point that the engine rpm no longer increases.

Power is naturally affected by the loss of valvetrain control at rpm, but something else happens. Remember the engine responds to valve timing not cam timing. Even if the valvetrain stays in control. When the pushrod flexes upon valve opening and valve closing, the valve opens later and closes earlier. The engine "sees" less valve open time (like a smaller duration cam) and as rpm increases it gets worse. Loosing 6 to 8 degrees of valve timing is not unheard of!

Durability is affected when the valvetrain parts are "thrown" out of control by the rebounding pushrod. When this happens the valvetrain parts are jerked apart from each other, and then slammed back against each other by the valve spring pressure. This "slam" can break parts anyplace in the valvetrain and at the very least break the lubricating oil film causing excessive wear. All of the above can happen without a pushrod failure!

Do higher ratio rockers affect the pushrods?

Higher ratio rockers will increase the load on the pushrod because the rocker is moving the valve side faster. This means more pushrod flex as the valve starts to open. To get the most performance from higher ratio rockers a rigid pushrod is a must.

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