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Pushrod clearances count

pushrodsToyota Racing Development (TRD) of Costa Mesa, California maintains a long-term relationship with pushrod manufacturer Trend Performance of Warren, Michigan. "We've worked with them on many other projects as well as the NASCAR Sprint Cup developments," notes David Currier, vice president of engine engineering at TRD.

Although the service life of intake and exhaust pushrods is, conceivably, longer than the 700 miles that make up each Sprint Cup race weekend, TRD elects to replace them after each race. "Generally they are in a condition where we probably could reuse them, but we just don't," Currier told me. "We'll use them again for something else, like in development or performance engines or in dyno engines for testing other components, because they are in good shape."

The pushrods used by TRD are part of the TP (Trend Performance) line of double-tapered rods, using 7/16-inch diameter and a 165 wall. The manufacturer places a 100-thousandth wall down the centre, which is, according to Trend's John Williams, a standard hole. Cases are hardened to Rc 60 and there is a black oxide finish with laser-etched length. The pushrod, 7/16-inch at its widest point, tapers to 3/8-inch. Lengths on both intake and exhaust pushrods runs in the 7-8-inch range, Currier said.

The material for TRD's pushrods is 4130 chrome moly and the 5/16-inch ball end is CNC-lathe machined onto the rod during manufacturing; it is a single-piece item. "There used to be some tip wear issues, but we don't have any of those issues anymore," Currier said.

"Configurations are changed when we do something different to the valve train; then we change the length of the pushrod; sometimes we'll do a little other work with it, but it just fits into the process. We might change the diameter or something like that," Currier explained.

The challenge, he noted, "is to fit them into the head so it's a little bit tricky because we have to work to make it fit. We're trying to get the biggest pushrod we can fit into the head and to accommodate the bigger rod depends on what kind of machining we can do around the head for clearance where the pushrod goes. This depends on what we're doing with the rocker arm geometry. That can affect where the rod is placed, relative to the head."

Of course there are times when you can't compromise on placement, so Currier might need to go with a different size pushrod in order to get it to work. "That is where we have to compromise," he shrugged. "Sometimes we might have to go with a less hefty pushrod for fitment purposes, but we try to go with a stiffer, more responsive pushrod in all instances."

Fig. 1 - Toyota Racing Development uses double-tapered pushrods from Trend Performance (Photo courtesy of Toyota Racing Development)

Written by Anne Proffit

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