yes 5/16
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yes 5/16
Not sure how critical it is, and you guys seem to be doing OK with what you are doing using the cast slots (no guide plates), but some think it is an advantage to stabilize a roller rocker as high up the pushrod as you can get without hitting it. Reasoning seems to be that rollers don't often have as wide a contact surface with the tip of the valve stem as a non-roller (stock type) rocker, and it is therefore not as forgiving of being off center as a stock rocker is.
Hard to say if that .75" of height on a 7.800" long pushrod would make much of a difference in a motor that sees less than 8 grand. Taking in account any flex you could assume that the guideplate would work better but the chromoly pushrods are pretty stout. The slot in the heads have worked great since they were first made after all arent the greater percent of chevy heads the same way?
Like all of this splitting hairs stuff, guess it depends on how crazy you are getting with spring pressures and such. If I was going to start pushing over 6000rpm on a regular basis I would probably go with 3/8 pushrods - requiring that the slots be drilled out and guide plates be installed anyhow. Does seem to make sense to grab the pushrod as high as possible but .75" is likely a non-issue for most applications. For those that don't know, 502 and other bridged heads do not have slots