I have a lm7 gen 3 5.3 . Stock lifters , summit cam, milled heads , pac valvespeings that came with the cam, measurments puts me at a 7.475 pushrod for about .050 of preload . I measured with the comp cams tool. Most of the pushrods I've seen say they're for use with guideplates. Question is is it a necessity to use the guideplates? If so does the plate bolt on under the stock rocker cradles? Would I Need to measure again after since the rocker will be sitting higher? Or am I overthinking this?

 

LS engines do not need or use guide plates

 

That entirely depends on what type of rocker you use. I'm using Lunati stud mount rockers with guideplates

 

I believe OP is using stock rockers and mounting hardware

 

If you are talking about the rocker stands you have to have them there to get the correct measurement.

 

7.475" seems kinda long-ish. Sounds like maybe you used the "twist the push rod" "method" (I use the word loosely) which ALWAYS screws up and results in a too-long estimate?

But no, you don't need guide plates. Push rods that say that aren't implying that they're REQUIRED, only, that they're COMPATIBLE with them.

 

I agree OP needs to re-measure the pushrod length and with the heads surface machined that doesn't sound right. What ever pushrod length you go with verify using the turn method. Yeah most 5/16" pushrods are considered "universal" as they'll fit different engine platforms so the "guide plate compatible" covers more than just the LS engine family.

 

The correct way to measure is, adjust the push rod to some length that's probably too short (say, 7.250"), by screwing it all the way together and then unscrewing it the correct # of turns; set the engine to where the valve you want to measure is ABSOLUTELY FOR SURE UNQUESTIONABLY known to be on the base circle, such as by the valve 4 cyls away in the firing order being fully open, or the EOIC method; install it, install the rocker arm on top of it; see if the rocker arm has up-down play; if so, remove the rocker, remove the adj PR, unscrew it another half-turn (or full turn if there's LOTS of play); reinstall the rocker and check for up-down play; continue until the ROCKER has no play. Note how many turns out the adj PR is and calculate its length accordingly; order PRs that are longer than that by your desired preload.

The Comp 7702 has ¼-20 threads, therefore each turn is EXACTLY .050". It is QUITE ACCURATE and needs no further measurement with calipers or any of that, regardless of what you might have "heard". So for example, if you're using that one, you turn it all the way in, and it will be 6.800" by the standard aftermarket method of specifying PR length which is NOT what a caliper says, and NOT what GM says (not that either of those things is "wrong", just, that's not how it's done in the aftermarket world); if you back it out 9 turns from there, it will be 7.250"; if you have to back it out 1½ turns (.075" further) to eliminate the UP-DOWN play in the ROCKER ARM (NOT anything about the push rod itself, you're looking for play in the ROCKER ARM) then the length of the PR is 7.325" at that point; if you want, say, .050" of preload, then you order PRs that are .050" longer than that, which would be 7.375". Push rods are available in .025" increments for these motors, which makes .050" per turn very convenient: ½ turn = the next size up. Way too easy.

Most often, PRs for a motor with the work done to it that you describe, will end up wanting to be somewhere between 7.300" and 7.375", depending on how much was cut off of the heads and what gaskets you use. Note that I said "most often", NOT "they will", NOT "always", NOT "never anything else", NOT NOT NOT NOT NOT. The actual words I used were "most often". Stock is 7.400" by the aftermarket measuring and specification method, and a cam swap and head mill MOST OFTEN shortens the required length from there. Yours would be expected to want to be something nearer 7.350" if I were to give a guess at the "most likely", and therefore "expected", length you'll need. But you have to measure it yourself to know for sure, and your measurement has to be RIGHT. 7.475" sounds to me like you "twisted the push rod" until the lifter plunger bottomed out, that is, before it reached a point you couldn't twist it anymore. Which is EXACTLY how that "method" (I use the word loosely) goes south. It's been consistently failing people since the overhead valve engine was popularized 75 yrs ago, in the direction of ending up with PRs that are too long, or valve adjustment that's too tight in motors with adjustable valve trains like the traditional SBC and BBC.

I'm not a big fan of the "tighten the rocker bolt and count the turns" method myself. It seems to work for some people, but maybe those people have developed a better feel for it than I have, or maybe they're just luckier than me (which just about everybody is). For me, it produces results that are too vague and unrepeatable. The method I just gave you gives accurate, repeatable (if you do the same thing multiple times, you get the EXACT same result), reliable, dependable results, EVERY TIME. Same for the idea of measuring the adj PR length with calipers; the standard aftermarket lengths are specified as if the end was a complete sphere, which of course they're not, they have the hole in them, which shortens them somewhat from that, but YOU HAVE NO WAY OF KNOWING by how much. Best you can do is measure the hole size and guesstimate from there, assuming things like no chamfer around it and so forth. Again, NOT PRECISE. The method of simply counting the turns on the adjustable push rod however is EXTREMELY accurate, EXTREMELY precise, and MATCHES the way the lengths of push rods that YOU CAN BUY are given, so that's the way to do it.

 

Great post RB. What I will define as "most" circumstances, I agree with everything you said. Such as-- stock or stock-ish lifters with normal travel ranges, typical street or street/strip cams, stock or stock-ish rockers, Stock castings.

I find most cam grinds tend to have smaller base circles than stock, so slight increase in PR length. Milled heads almost always offsets the reduction in cam base circle. Plenty of slop in the lifter preloads, so you can be off by .010 in stacked tolerances with no detriment, and you're ordering in .025 increments anyway. End result, what you posted is dead on for most applications - exactly as you said.

I tend to find that:
* roller tip rockers require shimming up to get the wipe pattern correct, results in longer PR length
* AM castings tend to have raised ports and/or thicker decks resulting in longer PR length
* Lifter cup heights can vary quite a lot. I have seen swapping lifters change the PR length by nearly 1.5" first hand.
* Short travel lifters require pushrods to be within .005" of perfect, so you must measure all 16, and you must be more precise than with the turns on the checker.

Under these kinds of "Specialized" conditions, I think we are better off putting teflon tape on the checker threads, and using a caliper to directly measure the pushrod lengths. When ordering from Manton, for example, they prefer the caliper measurement over any other method. Just need to specify that is how you measured them. The manufacturer will know how much difference there is between the gauge length and the caliper length.

Also, I 100% agree that tightening the bolt and then trying to adjust the pushrod to fit the gap is the worst way to do it. It is better to start small, install the rockers, see how loose it is, and sneak up on the measurement. I like to use a feeler gauge under the rocker tip to help reduce the number of attempts to get it right.

Example using stock-style stuff:
I set the PR at 7.300, and with feeler gauges, I find there is 0.100 slop at the valve tip, so I remove the rocker and PR, adjust the checker .050, reinstall, and now the gap is .015. At this point I would use a caliper. But if not using one, I would rotate 1/5 turn to ballpark .010, and reinstall. then the rocker will either be just barely tight or just barely wiggle. That puts me at 7.360. Then, I add my preload at .070, which gets me 7.430. I would then reinstall with the checker at 7.430. Find dead soft touch with the rocker bolt. Then mark the rocker bolt at 12:00, put a wrench on it and rotate until the bolt hits home, but do not torque. With .070 preload, this should put the bolt at almost exactly one full turn. Maybe slightly less. This is simply a verification step and not to be done as the primary measurement. Then, I'll order 7.425 pushrods and call it good.

Example using aftermarket heads, very tall solid roller lifters, milled deck, milled heads, .750 lift cam, and adjustable roller tip rockers:
Every single time I adjusted, I removed the rocker pair and adjusted the pushrod outside the engine. I started with the 6.8-7.8 checker tool and found it was too tight. Went to the 5.8-6.8 tool and started at 6.5". I used a caliper the whole time. I found I had too much lash to measure, so I went to 6.6" on the caliper. Found I had approx. .080 lash. Went to 6.640" on the caliper. Found I had .020 lash still at the tip. Went to 6.652, and it was slightly too tight. Pulled everything back apart, and remeasured the pushrod. It was 6.652 going in and 6.650 coming out. I wrote down 6.645 (split difference between 6.640 and 6.650) for the #1 exhaust, and i will use the adjuster to close that last .010. Repeat for all 16 cylinders. Here's how I ended up. You'll notice cylinder 2 intake, I kept finding something wasn't right, and it turned out it wasn't fully on the base circle:


I then called Manton and ordered 2@6.633, 8@6.642, 5@6.660, and 1@6.686. I simply don't think I would be able to do this type of set up counting the turns, personally. But this is afar from a "most of the time" set up also.

 

Thanks Darth, that's exactly what I meant by "most"... there are always exceptions, but yerbasic typical AM hyd cam with LS7 lifters and stock rockers and stock heads is what I was aiming at. Once there's deviation from that, it's anybody's guess what's gonna happen.