So since I’m changing out my whole valvetrain I figured I would need new pushrods. The stock pushrods were like 7 1/8 but when I measured it seems like it needs 7 inch pushrods. Is it normal for the pushrod length to get shorter when having higher lift springs? I just want to make sure before I screw anything up because i just thought in my head they would get bigger.

 

Tell more about this "measure".

No in fact, springs have nothing whatsoever to do with push rod length requirements.

What are we doing here today?

That's happened to me sometimes before... not valve springs, you understand, but, other things... but it goes away in a little while.

​​​​​​​Details REALLY matter.
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I measured it like this video said although I’m not sure how accurate I was
and all the articles and what not I’ve read have said that when you change things like rocker arms and valve springs and lifters your pushrod length could change. Honestly this whole pushrod length part is the most confusing thing I’ve encountered in my entire engine build by far.

 

Push rod length depends on the dimensions of the parts in the valve train. Valve springs don't change that; they only change how hard things push against each other. The SIZE of the parts is unaffected by valve springs in and of themselves.

Push rod length should be selected such that the rocker makes as near to ZERO as possible sweep across the valve stem over the course of its travel. So, cam design, lifters, rocker arms, valve lengths, and even the amount of work that's been done to the heads, can alter the ideal length. Springs, by themselves, do not.

Can't comment one way or the other about "articles" you've perhaps "read". Maybe they apply to what you're doing, maybe not.

Tell us what you're doing instead of posting links to random unrelated videos involving a big block with a solid cam as appears to be what you're wanting us to watch (yes I've had any number of big blocks, and small blocks, with solid cams, myself), and maybe we can help.

Someone will post that stuuuupid little plastic "tool" presently. Ignore that. If the "tool" is not ABSOLUTELY IDENTICAL IN EVERY WAY to the rocker arms YOU ACTUALLY HAVE, which of course it is NOT, it will give you GARBAGE for results.

 

Ok well I’ve changed out the entire valve train and had the heads worked on with a valve job. I only linked that video because you asked how I was measuring pushrod length and I don’t see how that’s unrelated, I directly answered the question and the video is directly related to measuring pushrod length, and that was my answer. If that’s not how I should measure it, how should I do it? And if it’s helpful the motor is a Vortec 350 and if it’s also helpful I can list all the different parts I’ve put on the valve train

 

The correct way to measure for push rod length is to "solidify" one of the lifters you'll be using, or at least one identical to them (not all lifters are the same height internally) at the height you want them preloaded to; install some lightweight substitutes for valve springs; assemble the valve train with an adjustable length push rod such as this one https://www.summitracing.com/parts/cca-7702-1, set to a reasonable "guess" at the correct length; put some sort of marking on the valve stem tip (gear marking compound, white lithium grease, color it with a Sharpie or Magic Marker, all sorts of things work great); and turn the motor over a couple of revs to create a witness mark. Measure the width of the mark; adjust the push rod one way or the other by an increment equal to ones you can buy; measure again. If the mark gets narrower, rinse and repeat, and keep adjusting in that direction. Find the length that gives the narrowest mark. Buy push rods that are shorter than that by 1 or maybe 2 increments, because that will put the greater error during the part of the valve's motion that there is less spring pressure; i.e. the error will be biased toward lowest lift when it's not as critical, and the geometry will be more nearly perfect at higher lift, when the spring is more compressed and making everything line up straight matters more.

It DOES NOT MATTER where on the valve stem tip the witness mark falls. That's a mistake people make all the time: trying to get the witness mark centered on the valve stem tip. That doesn't matter. You're looking ONLY for the narrowest possible sweep, which is the indication of the least "scrubbing" of the rocker across the valve stem. That condition occurs when the rocker YOU'RE using is nearest to being exactly in the middle of its sweep (keeping in mind that it moves in a circle, it doesn't move in a straight line) at the middle of the lift range of the cam YOU'RE using on the valve YOU'RE measuring. Which incidentally, not all valves will necessarily be the same; it wouldn't hurt to measure several, especially some intakes and some exhausts, and to lay a straightedge on the valves, and check any that deviate from the height of the others, to try to catch any that will have "unique" needs.

Someone will likely come on and tell you to just get the little plastic toy tool. Don't bother. It's unreliable because: (a) doesn't take into account the range of sweep needed for YOUR PARTICULAR CAM; (b) doesn't take into account the exact geometric relationship of YOUR rockers (the relative locations of the push rod seat, valve tip, and pivot point), which is DIFFERENT for EVERY rocker, but the plastic tool only matches, AT MOST, one rocker; and (c) doesn't take into account how far your valve may be from its tip being in the "stock" expected location, such as from seat grinding, the use of longer or shorter valves, work that's been done on the valves., etc. In a word, it's a JOKE.

The way he shows in the video is kind of the long way around, but of course will work fine. I find it's much easier to just use the witness mark method though. That way you don't really have to measure anything at all, or try to find the exact center of the rocker pivot, or lots of other things he shows that an experienced engine builder can get right but somebody posting "how to" on the Internet is likely to introduce error into. Instead, you just ask the engine to show you when the rocker's sweep is exactly split in half, as installed. The narrowest witness mark occurs at exactly that push rod length. The adjustable push rod comes with 20 threads per inch, making one full turn a .050" increment. All you have to do is count the turns. No measuring required. So all you do is start out with it set to your first "guess", let's say 7.300" (if you have an adj that's 6.800" when fully screwed together, then that's 10 turns out from there), and see what it does; then lengthen to 7.350" (or 7.325" if you're using push rods available in .025" increments such as these https://www.texas-speed.com/p-8292-t...set-of-16.aspx) or shorten it by the same amount, your choice, and measure the new witness mark. Keep doing that until you find the PR length that gives the narrowest mark.

More often than not you'll end up with longer push rods than stock with an aftermarket cam. But there's no guarantee of that, it's just ... more likely ... NOT a certainty. The reason is because to make a cam "bigger", they DO NOT make the lobes "taller". Rather, cams are all made with the TIP of the lobe already just about as tall as it can be and still fit into the bearings, and then since lift is THE DIFFERENCE BETWEEN the "tip" and the "heel", they make the heel SMALLER (less diameter) so that the DIFFERENCE (lift) is greater. If all else is equal, which of course it never is, one would expect to need push rods that are about half as much longer than the new cam's peak lift is greater than a stock one; except of course, stock push rods aren't necessarily "perfect" in this regard anyway, not least because of all the tolerances in everything (casting dimensions, machining operations, valve length, rocker tolerances, and on and on). The more things you change that can affect any of these details, the murkier it gets trying to predict the right push rod length without measuring it.