Pushrod length measuring and zero lash? What lifter preload?

[robertway]

I am in the process of measuring my pushrod length for the new blower cam I am installing and am needing a little guidance around what "zero lash" actually is. My understanding of zero lash is that it is the point at chich the rocker is tigthened down and the pushrod is lengthened JUST ENOUGH to take the play out of the rocker in that it cannot be lifted even slightly off the tip of the valve. I have my length checker set to the point that there is NO up and down play on the tip of the valve, there is no preload on the valve spring, and there is very slight play laterally on the tip of the rocker where it contacts the valve stem (you can nudge it back and forth a bit but it isn't loose). Is this "zero lash"?

The reason I am asking is because the shop I am dealing with for my parts said they typically come in around 7.450-7.800 (which includes som epreload amount) on their pushrod lengths for their cam I am installing. The problem is I am coming up with 10 turns on the pushrod checker (10x0.050=0.50) which comes out to a zero lash pushrod length of 7.300 (6.800 base checker length + 0.500 from the ten turns) Using what I have found online as well as in the two LS Rebuild How To books I am using to guide me a bit, the standard preload on the stock lifters is 0.60 and the one book even gives the 0.50-0.75 range. Even using the upper part of that preload range it puts me at a 7.375 pushrod length which falls short of the range the shop says they typically measure.

Now I did measure a couple other things and did indeed confirm that the base circle on the new cam is about 0.035 smaller on the intake lobes and 0.055 smaller on the exhaust lobes compared to stock (the stock cam has different base circle sizes for intake and exhaust while the new cam has equal for both). With the smaller base circle of the new cam one would think I would need a longer pushrod than the stock length to make up for the difference lost to the base circle and to maintain the stock preload.

So I am either not setting zero lash correctly or I am using the incorrect lifter preload amount (0.060) to come up with my final pushrod length. Taking my measured pushrod as it is now with my assumed "zero lash", I would need to add 0.150 preload just to get to the lower end of the range the shop stated.

Having said all that, I have meausred the intake and exhaust rods on both cylinders 1 and 2 to take into consideration any difference between int/ext and between two different heads that are "assumed" to be machined to the same exact height from the factory. I have rotated the engine over constantly and always made sure my timing marks were in such a position that the cylinder was in its firing position with both valves closed and lifters on the base circle.

If you have read this far, thanks you and hopefully I haven't confused anyone. Please no replies like "just use what the shop says as it is their cam so they know the pushrod lengths". I would be fine with that if my repeated measurements didn't show me falling short of their range and I tend to get overly anal about doing these types of things as I want to do it once and do it right.

Thanks in advance....

[danhr]

Are you using heads that are milled?

Stock thickness head gaskets?

You are using the correct method of measuring pushrod length. It seems like you have done it correctly.

How does the wipe pattern look on the top of the valve?

if that checks out, I would just go with the 7.375 pushrod.... I'd opt for that over the 7.35 pushrod because a little too much preload is MUCH better than a little less preload. Having a little too much, and the plunger on the lifter will just be a little further down.... too short and you will have valve float.

[robertway]

Stock heads and gaskets and they have never been off either..

I still have to verify the wipe pattern with the checker in there at what would be the length I go with

Thanks for the feedback

[Irnwkrkev]

answered while I was posting question

[danhr]

Originally Posted by robertway

Stock heads and gaskets and they have never been off either..

I still have to verify the wipe pattern with the checker in there at what would be the length I go with

Thanks for the feedback

I would go with 7.375

[SC2150]

The only way to know the proper legnth for proper preload is to slowley tighten the rocker down until the rocker tip is just touching the valve stem end (you can spin the pushrod when starting to slowly tighten until you feel resistance). Then your target preload is 50 thou from zero lash. That equals app 1/2 turn of the rocker pedestal bolt. The least I accept is 1/4 turn and the most 3/4 turn. so if you have a 7.40 pushrod and it takes 1 1/4 turns, you will need a 7.350 (I always go with custom legnths to get it exact).

There is no way to determine this ahead of time until you have your cam installed and measure for it. Any vendor telling you a certain cam needs a certain legnth push rod is just guessing. The guess may very well be close, but I choose to measure for the exact legnth needed.

[robertway]

Originally Posted by SC2150

The only way to know the proper legnth for proper preload is to slowley tighten the rocker down until the rocker tip is just touching the valve stem end (you can spin the pushrod when starting to slowly tighten until you feel resistance). Then your target preload is 50 thou from zero lash. That equals app 1/2 turn of the rocker pedestal bolt. The least I accept is 1/4 turn and the most 3/4 turn. so if you have a 7.40 pushrod and it takes 1 1/4 turns, you will need a 7.350 (I always go with custom legnths to get it exact).

There is no way to determine this ahead of time until you have your cam installed and measure for it. Any vendor telling you a certain cam needs a certain legnth push rod is just guessing. The guess may very well be close, but I choose to measure for the exact legnth needed.

Thanks for the feedback. This actually confuses me a little bit. When you say 1/4 is the least and 3/4 is the most turns you will accept, 1/4 or 3/4 turns to what. My understanding is the rocker pedestal bolt gets torqued down to 22-26ft/lbs and the preload is determined by the pushrod length. The way I am reading what you wrote is somehow the threading down of the pedestal bolt is where the preload comes from. I know it would probably take 15 seconds for you to show me what you mean but I am having a hard time visualizing it. Am I wrong to torque the pedestal bolt down to spec and continue to adjust the adjustable pushrod until the rocker tip makes contact with the stem tip?

[danhr]

sc2150 is referring to a different way of measuring pushrod length

you start off with a pushrod of any given length (NOT A STOCK ONE, some claim they measured their stock ones and they weren't exactly 7.4)

You tighten the pedestal bolt down to zero lash. So you keep tightening it until you get resistance trying to spin the pushrod, you do not have any vertical movement in the rocker tip, but you still have some horizontal movement in the tip.

You then put a torque wrench on the bolt and set it to 22 ft lbs. You want the bolt to spin anywhere from 1 1/4 to 1 3/4 turn(s), for stock lifters.

1 Full turn = ~.047"

For an example, if you use a 7.400" pushrod and come up with 3/4 of a turn, you will need at least .025" longer pushrod to get into range. If you end up with 2 1/4 turns, you will need one .025" shorter...

[robertway]

Thanks danhr, that totally clarifies it, It was what the "target" of the pedestal bolt turns after zero lash I was unsure of and now understand it is the torque setting. I think while I have things opened up still and before ordering pushrods, I can use one method to verify the other. Just some added insurance that I did it right so I only have to do it once.

Would anyone happen to know what the actual preload is on the stock LS3 setup, I know what the specs say (0.060) but I would be interested to know if anyone has actually measured what theirs was from the factory. I should have thought to do that before taking the stock cam out. My stock pushrods have measured to 7.375 (exactly 11.5 turns on the comp cam pushrod checker) and the stock camshaft that has a 0.035 larger intake base circle and 0.055 larger exhaust base circle give or take 1/1000th on each (I measured multiple times with a digitial caliper) I mention this because if I go with a 7.350 pushrod I'll be at 0.050 preload and at 0.075 with a 7.375 (ironically the stock length) keeping in mind I have measured my pushrod length to be 7.300 at zero lash on the new cam. I mention this only because the measurements tell me that after factoring in the larger base circle measurements in the stock setup and with the 7.375 pushrods it would represent and additional 0.035 and 0.055 preload on the intake/exhaust respectively, putting intake at 0.110 preload and exhaust at 0.130 (more than double the 0.060 spec) with the stock cam and pushrods. Is that much preload from the factory likely?

I know my attempt at reverse-engineering this is probably annoying for some to read but the math doesn't seem to lie. Unfortunately my factory preload is the unknown here other than what the math dictates and there is no way I am so anal that I'll be putting the stock cam back in to confirm or deny things. Painful how I try to think this stuff through but hopefully it will help keep my mind sharp later in life when I am slobbering all over myself waiting for someone to change my diaper (this silly attempt to get a chuckle was in no way written to harm or cause undue stress to anyone who is actually slobbering all over themselves and waiting on a diaper change)

[robertway]

Just a quick follow-up, I did a quick re-check using the pedestal bolt turns to 22ft/lb method and using a 7.350 length on the pushrod checker (my 7.300 measure length at zero lash plus 0.050 preload) I threaded the pedestal bolt down by hand until there was no play in the pushrod in its rocker seat and then got just past 3/4 turn out of the pedestal bolt to 22ft/lbs

Originally Posted by SC2150

Then your target preload is 50 thou from zero lash. That equals app 1/2 turn of the rocker pedestal bolt....

Originally Posted by danhr

1 Full turn = ~.047"

I just want to get some clarification on what a full turn of the pedestal bolt gets me since one statement says it is half a turn and another one full turn. I guess since I got just a hair past 3/4 turn it isn't too critical as I am "in the range" so to speak.

[danhr]

Sounds like you need a 7.375 pushrod! Both methods just verified that.

[SC2150]

Danhr gets it! One of the few out of so many that claim to know what they are doing. You have my respect Dan!

[danhr]

awwww

[CamaroSpike23]

Very informative thread. Thanks danhr and SC2150