Pushrods in a high-performance engine build are always items that need considering. Many older OEM engines were equipped with heavy soft-steel pushrods of 5⁄16-inch diameter—definitely not performance valvetrain stuff. Virtually all aftermarket adjustable valvetrains will require aftermarket or custom pushrods. There are several factors to decide in choosing pushrods for a specific engine build, notably the length, diameter, tube material, and types and sizes of ends.

The load capacity of the pushrod is determined by the material it’s made of, the wall thickness, the diameter, and overall length. If the pushrod construction is not up to the load put against it, the result will be pushrod deflection and valvetrain instability, or outright failure. Increasing the material strength, wall thickness, diameter, or all of these variables will increase the stiffness of the rod. Longer pushrods have less load capacity, all else being equal, than shorter ones. We spoke to the pushrod experts at Smith Brothers for some basic rule-of-thumb guidelines on pushrod material selection. The accompanying chart gives some of the materials and general max spring loads recommended by Smith Brothers.

Ordering custom pushrods requires that you know or can find out a few basic specs. First is the required length, best measured by installing an adjustable checking pushrod, and directly determining the length requirement. Most adjustable rockers work best with the adjuster set nearly all the way up—that is with only about one thread showing out of the bottom of the rocker. Stud-mounted rocker arrangements depend on the correct pushrod length to optimize the geometry of the rocker sweep across the valve tip. Some rockers require the length to be set so the oiling hole to the pushrod is in the range of the oiling band in the rocker adjuster. The checking pushrod allows the pushrod length to be adjusted until the perfect length is determined. With that done, all that’s required is measuring it, and to give the length spec to the pushrod supplier.

Pushrods are easily measured if they are of the more common ball/ball end configuration. For an exact dimension with oil through pushrods, the small flat at each end created by the oil hole can be taken into account. Many engines, however, use a ball/cup design. The overall length measured from end to end of the pushrod is not an accurate way to measure a typical ball/cup pushrod, since the design of the pushrod cup can vary considerably. The preferred method is to measure the length from the ball end to inside the bottom of the cup. This is the true effective length. Just measure the overall length and then subtract the distance from the cup’s top to the bottom of the cup.

Besides the length, the type of end required needs to be identified. The two most common possibilities here are ball or cup end, with the two most common sizes of 5⁄16 or 3⁄8 inch. Whether a ball or cup is required is pretty obvious, but the size is often not considered; it has nothing to do with the tube size of the pushrod. Some lifters or rocker adjusters are machined for a 3⁄8-inch ball, while others are machined for a 5⁄16-inch. The only way to know for sure which you need is to measure it. Likewise, determining the size at the rocker end is a matter of taking out the calipers and checking the diameter at the rocker adjuster.

Pushrod Max Recommended Load*
Tube Material Heat Treat Max Recommended Load**
5/16” 0.049” wall No 400 pounds
5/16” 0.049” wall Yes 500 pounds
5/16” 0.083” wall Yes 750 pounds
3/8” 0.049” wall No 400+ pounds
3/8” 0.049” wall Yes 500+ pounds
3/8” 0.065” wall Yes 750 pounds
3/8” 0.083” wall Yes 900 pounds
*General recommended max open valvespring load for a 9-inch pushrod. Subtract 100 pounds capacity for every 1-inch over 9 inches effective length. Add 100 pounds capacity for every 1 inch shorter than 9 inches. **Maximum valvespring open load

SOURCE
Comp Cams
3406 Democrat Road
Memphis
TN  38118
800-999-0853
http://www.compcams.com
Powerhouse Products
3402 Democrat Rd.
Memphis
TN  38118
800-872-7223
http://www.powerhouseproducts.co
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Smith Brothers Pushrods
800-367-1533
http://www.pushrods.net
Crane Cams
1830 Holsonback Drive
Daytona Beach
FL  32117
866-388-5120
http://www.cranecams.com/
Manley Performance Products
1960 Swarthmore Avenue
Lakewood
NJ  08701
732-905-3366
http://www.manleyperformance.com
Probe Industries
2555 West 237th Street
Torrance
CA  90505
310-784-2977
http://www.probeindustries.com