Cup/ball pushrods are usually measured from the bottom of the cup to the tip of the ball. If you're using the OE adjusters, you'll want 2 or 3 threads showing to ensure the cup won't hit the rocker arm. If you're using the longer Crane-style locknut adjusters that have the ball at the end of a longish neck, the threads should be approximately flush, but one thread showing won't kill anything.
With your chosen lifter(s) on the base circle of the cam, install and torque the rocker shaft retainer bolts on either side of the rocker(s) you're using. Install your adjustable pushrod, opening it by hand until you feel resistance. Tighten the locknuts so the length can't change, then double-check that you can
spin the pushrod freely but cannot feel any play. Loosen the rocker shaft enough to remove the pushrod, and measure it...
...but how? There are a couple of ways, but most manufacturers want the
effective length of the pushrod as described above. There are two ways to do this:
- Measure the overall length from the ball tip to the edge of the pushrod cup. Now use the depth-measuring part of your caliper to measure the pushrod cup's depth. Subtract the second number from the first and you've got your effective length within a few thousandths.
- Set a 5/16"-diameter individual bearing ball in the adjuster cup. Measure overall length between ball tips. Using your caliper, get a precise measurement of the ball's diameter, and subtract that number from your OAL. That's you're effective length, and about as accurate as you're going to get.
I usually check at least four pushrods, one near each corner (two intake, two exhaust) and average those numbers. That's to allow for production tolerances, since your deck may not be perfectly parallel to the crank or either head could have a slight angle to its deck surface. On the W2 engine I did eight, simply because of the rocker-arm offset on the intakes (in case I needed two different-length pushrods) even though I know the block was decked to parallel and the heads were straight.
By the way, this process is actually easier with your final valvesprings installed, simply because threading the pushrod longer easily overcomes most checking springs. You ain't doin' that with your fingers on the real deal, although with a hydraulic cam you
could start depressing the lifter cup (but you'd feel it). Also, once you've got the first one done, the rest go pretty quickly since your adjustable pushrod's already in the neighborhood lengthwise.
If you find an off-the-shelf pushrod within, say, .040" of your average number, hit the "order" button
after you've verified they measure by effective length (and ensured cup-to-rocker clearance). Check that information on the manufacturer's website; don't trust Summit/Jeg's/Mancini/Hughes to get it right.
As a wild-ass guess, I'm going to say you need something between 7.050"-7.250" effective length depending on the adjusters you have (longer for OE). The Comp 7821s are too long despite being advertised for this application, and near as I can tell they don't make one that fits. Edelbrock shows a 6.900" length (9623); the next I could find was Melling MPR-157 @ 7.234". Melling uses effective length. Smith Bros and OE Pushrods come to mind for custom (the latter being less expensive), but it's possible Mike @ B3 has a line on something that'll work.
