Hey guys, I’m struggling hard with pushrod length. I’m dropping a vortec 350 engine into my car and it has an Xr276hr in it with roller rockers and lifters. Would this change the stock pushrod length? When I measured I got 7 inches flat when the stock is 7.2 and that seemed like too big of a difference for what’s changed so I’m just a little confused. And I thought if anything it would’ve gotten bigger, not smaller. Thanks

 

​​​​​​​Tell us how this was accomplished.

 

I used the adjustable tool. Put the rocker arm on and brought it to 0 lash, turned the engine over with a breaker bar a few times and looked at where it was wearing on the valve stem by seeing where it rubbed the marker off. And the wear mark was right in the center.

 

*smh*

Not sure how many times it has to be posted:

That is NOT how to "measure" for correct push rod length. On SEVERAL levels. It's NO WONDER you have problems with the result.

First off, the goal is to make the angle that the rocker pushes on the valve stem, to be as close to perfectly aligned with the valve, as possible. Keep in mind that the rocker tip "swings' in an arc; this means then, that the rocker tip's arc motion should be more or less centered (approximately) on the valve over the course of its lift; although, since the push rod and the valve stem aren't parallel, it's not quite that simple. As the valve reaches some point near halfway open, the line between the rocker tip and the push rod seat will be exactly perpendicular to a line between the push rod and the valve stem; at that point, the rocker tip will be at its farthest from the stud. At the ends of the arc, i.e. at zero lift and full lift, the rocker tip will be closer to the stud. But, the valve stem isn't parallel to the push rod; that is, if you made the valve and the push rod longer, eventually they'll TOUCH. As the valve moves, the tip of the stem MOVES AWAY FROM the push rod. So, there's all sorts of geometric reasons for the rocker tip to "scrub" across the valve stem tip. This "scrubbing" action is what you need to minimize.

The CORRECT way to "measure" for push rod length is to "solidify" a lifter IDENTICAL to one you're actually using, preferably at the height it will arrive at when you give it your desired preload. For a typical SBC with stock ratio (1.5:1) rockers and stock 3/8"-24 studs, 1 full turn on the rocker nut gives about .070" of lifter preload (1 turn = .0417" of nut movement; the lever arm of the rocker on the push rod is 2.5 ÷ 1.5 times the nut movement; 1 ÷ 24 × 2.5 ÷ 1.5 = .0694), therefore if you want, say, ½ turn of preload on the nut, that means the lifter preload you are targeting will be half of that, or about .035". Take one of your lifters apart and stack washers (or something) up inside of it, to where the push rod seat is that far below the retainer ring. Mark the valve stem tip (Sharpie works good, as does gear marking compound or white lithium grease), assemble that valve and set your adjustable push rod to some known length. I suggest the stock length as a starting point, but it doesn't really matter. Rotate the engine several revolutions, minimum of 4. Take off the rocker and measure the width of the witness mark. Adjust your push rod in the increment you can buy (for example if you're using Texas Speed push rods, they're available in .025" increments), clean off and replace your marking setup, and repeat. See whether the mark got wider or narrower. Repeat until you arrive at the NARROWEST POSSIBLE mark, with NO REGARD for where it happens to be on the tip of the valve. That length is then your "ideal", geometrically. BUTT: the force on the valve stem is ZERO at zero lift, but is at its MAXIMUM at full lift; the side force on the valve stem (i.e. the force trying to destroy the valve guide) is some fraction of the force the rocker is exerting, and thus is also at its MAXIMUM at full lift; therefore having the rocker pushing straight down on the valve is MORE IMPORTANT at full lift, than at zero lift. Therefore, the "ideal" push rod length is SLIGHTLY shorter than the pure geometric "ideal" length, since that biases the side load on the valve stem more toward being minimized at full lift, when it matters the most.

Remember then, 3 things:
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1. You must "solidify" a lifter, or, you must know EXACTLY where it is in its travel as you make your "measurement", so that you can account properly for lifter preload
2. You are looking for MINIMUM SWEEP of the rocker tip across the valve stem, NOT "centering" the mark
3. Once you find the PR length that gives MINIMUM SWEEP, make any adjustments to account for lifter preload if necessary, and purchase PRs that are SLIGHTLY shorter than that, let's say, the next lower incremental length that's available
4. There can be ENORMOUS variations from one valve to another in the same head. It's a good idea to "measure" SEVERAL valves in each head, and make a decision as to how to deal with whatever variance you encounter. In most builds the PR length isn't all that critical ANYWAY, I'm guessing yours probably fits that description, so you can get away with using the shortest length for any valve, on all valves. (as opposed to buying 16 individual custom lengths for example)

Someone will come on here soon enough and tell you all about this little plastic "gauge" thing you can buy, and you just set it on your head, and it magically gives the right PR length. Maybe even post up a photo of them using it, and a link to a writeup from around 50 years ago by someone named Miller. In my experience, that's a JOKE. There's altogether too many things that affect the "ideal" PR length that it FAILS to take into account. So, the little plastic toy POS might get you somewhere close. However it's nowhere near "reliable" let alone "right" or "perfect", for a number of reasons:
.

1. It doesn't EXACTLY match any rocker; or maybe I should say, it matches, AT MOST, ONE rocker model, and is WRONG for ALL OTHERS; you just have absolutely no way to know which kind of rocker that happens to be, although for reasons I won't go into, I think it's probably a certain model of Crane rocker that was made back in the early 70s
2. It doesn't account for different valve lifts; since the ideal point is somewhere near HALF of full lift, then as lift changes, so does the "ideal" geometry; butt you can only buy ONE plastic toy, therefore even if it matches your rockers, it's still ONLY good for EXACTLY ONE possible combination of rocker model and valve lift, AT BEST. Again, you have absolutely no way to know what lift that happens to be, although again, for reasons beyond this discussion, I suspect it would be a common lift for street cams back in the late 60s / early 70s, i.e. .460 - .475" or so
3. It doesn't account for different rocker ratios: the stock spec is 1.5, butt in reality most stock rockers "measure" a little less than that at rest, and they're SO FLEXIBLE that the valve lift is usually measures anywhere from 1.38 or so up to 1.45 times the lobe lift, with an average of about 1.43 for ones I've measured in typical engines with typical springs and so on
4. It doesn't account for different valve stem heights: different valves, different numbers of valve jobs, different valve seats, etc. all change the height of the valve stem by MOVING THE VALVE, on top of the already ENORMOUS variance in stock production tolerances. If you use a different rocker ratio then it's GUARANTEED WRONG.

For those reasons and others, I strongly advise against the "plastic toy" garbage shortcut that only leads, AT BEST, to "maybe".

You'll find, overall, that higher valve lifts, lower rocker ratios, more desired lifter preload, new or longer valves, and new valve seats, tend to LENGTHEN the needed push rod, and vice-versa. Butt there's SO MUCH variation from one head to another, and even from one valve to another in one casting, that those are only generalizations, not any kind of absolutes. The cam you're using has low enough lift that you'll probably end up discovering that PRs within a notch or 2 of stock are what you'll need, once you forget about "centering" the mark and instead do the measurement correctly.