When engaged in building Dodge Mopar engines for use in USAC’s National Midget series, Bob Wirth of Hayward, California relies on pushrods from Smith Bros of Bend, Oregon.
He works with the raised port (RP) head and relates, “The RP cylinder head layout is really hard on parts. The intake rocker is offset 3/4-inch and that exacerbates the stress at the adjuster and at the end of the pushrod, because you’ve got a rocking couple across the span of the rocker. This ‘ball on ball’ pushrod has an undercut on the ball and on the rocker end that allows for a better sweet at high lift.”
In building these engines, “The most important thing to keep the pushrods functioning in the engine is lubrication. You need the oil to lubricate and pull the heat away from the pushrod. We gave ourselves a good demonstration of this when I decided to upgrade the W9 engine to bigger lifters. I tightened up the lifter clearance,” he related.
“Without enough clearance, the oil couldn’t get past the lifter to the pushrods. We welded the pushrods to the adjusters in the first 30 seconds on the spin fixture,” he said. After Wirth increased the lifter clearance, put the lifter, new pushrod and adjuster back in the engine, he noted the same pushrod and adjuster were still in the engine – a year later.
Wirth started with a 7/16 x 0.065 wall pushrod. That worked well with the aluminium rockers and the valve spring he had chosen, but as the demand for more rpm increased from 8500 to 9500, the valvetrain parts began to fail. Steel rockers and a new valve spring brought stability back and increased the power output.
However, Wirth noticed a substantial power loss in the sixth harmonic range (about 8800-8900 rpm). “We found that increasing the wall thickness of the pushrod from 0.065 to 0.090 reduced the loss,” he related. “We then went to 0.120 and finally to 0.165 wall with each increase in wall thickness reducing the power loss further.”
According to Wirth, “The pushrods right now are bullet-proof. We’ve gone from using 5/16-inch five years ago to 7/16 and even 9/16- and 5/8-inch in NHRA Pro Stock,” he noted. “I firmly believe that you cannot make a pushrod too stiff.
Smith Bros’ Greg Tanner noted his company is “toying with making some ends out of tool steel (A10) instead of 86L20. A10 is an air hardened tool steel that has fair machining properties and very good durability when heat treated in a vacuum furnace to maximum hardness (58-60 RC). The cost for the material is 2-3 times that of 86L20, which has good machining properties and good durability. Our testing is just about to begin,” he confirmed.
Written by Anne Proffit.