I've seen and read various instructions for determining the required pushrod length, but this method by the Lunati Camshaft company is by far the best I've ever seen.....(just thought I'd share).....

http://www.holley.com/data/TechService/Technical/valvetrain_tech.pdf

They show it in the picture, but failed to put it in the procedure to use checker springs. You won't be able to open the valves with an adjustable pushrod when real valve springs are in there.

They show it in the picture, but failed to put it in the procedure to use checker springs. You won't be able to open the valves with an adjustable pushrod when real valve springs are in there.

Really?

I didn't seem to have any problems with 120 psi Hydraulic Roller cam valve springs.

Well, maybe at 120 psi, but do not try it with 250/650 springs. Not sure what the maximum is for an adjustable checker, but it may bend/break with a stout solid roller spring. They do show checker springs in that picture.

1. They don't discuss the pushrod cup height differences that can exist when checking pushrod length using lifters of a different style than what is going to be run in the engine.

2. They don't tell you how to measure the length of the checking pushrod once you get the proper contact pattern on the valve tip.

3. They don't tell you to add enough length to compensate for hydraulic lifter preload.

1. They don't discuss the pushrod cup height differences that can exist when checking pushrod length using lifters of a different style than what is going to be run in the engine.And why would you even use different lifters than what you would be using when running the engine? Perhaps I'm missing something.

2. They don't tell you how to measure the length of the checking pushrod once you get the proper contact pattern on the valve tip. So what's the big deal with measuring a pushrod? You measure it from tip to tip. What's so hard about that?

3. They don't tell you to add enough length to compensate for hydraulic lifter preload. I don't mean to offend you my friend, but it sounds to me like you're making it more complicated than it needs to be. The three things you've brought up have never been mentioned in any of the instructions I've ever seen anywhere for checking valvetrain geometry. So maybe you can supply us with a link to what you believe are better instructions for this procedure than the ones that I've supplied a link to. Eh?



I mean hey, I was just trying to be of help to other board members here who might need this info, and who might be in the middle of a custom hi-perf engine build. But if you think that you can be of more help, than by all means supply us with some better instructions than the ones I've shared. Otherwise, I really don't see any point in your post. And if you've already suppled such information on this board elsewhere, then please let me in on it by showing me a link to that.

What Shurkey has said is all correct...

He is not trying to make it more complicated. All these differences are not usually critical on their own..BUT when you have a cumulative amount of error it can cause a problem like early valve guide and valve/valve stem wear.

Comp Cams website talks about three different pushrod lengths and the length of the radiused tip...here is the link

http://www.compcams.com/information/Products/Pushrods/

Make sure you don't depress the hydraulic part of your hyd roller lifter,

(As a side note) this is especially important in checking piston-to-valve clearance. use a SOLID Roller lifter and longer pushrods to check piston to valve clearance.


and Lastly - this website has it's share of information and not everyone is correct in what they post...Your intentions we very good and the Lunati setup should work well, but they are eliminating some very important information necessary to do the job right. Some of us already know that.

I was thinking of buying that setup as well, Since I already have the checker springs it didn't even dawn on me that the instructions didn't mention using them....

The Checks and Balances on this site are excellent. And we all need to take it as a lesson learned and lighten up about it...it is all POSITIVE so stop being so DEFENSIVE

Shurkey's Post was a jab at the limited Infor provided by Lunati - NOT directed at you personally Billy

I am the new guy here and i don't want to step into anything but Shurkey has some good points.

The site listed is a excellent way to do push rod length in a mild engine, And that is what most home builders build. But the rules will change with .100" longer valves, killer springs, and so on....

This is one of those deals where you cannot even get 3 good engine builders to agree on... I have done a buch on R&D on this subject and there are about as many theorys as ways to do it...I do what works for me and have no issues with it ,the next guy might do it another way and also have no issues... This is all reguarding competition engines not the normal street performance pieces.....

Keith

Thankyou for your replies. I think that K-star has a good point about there being a lot of disagreements about this. I don't like the method that Comp Cams uses at all. I think that part of it is all wrong. Not all valvetrain set-ups will have the rocker arm tip starting off on one side of the valve stem tip, and ending up on the other as the Comp Cams illustration shows. So that part of it is just plain inaccurate. But I didn't mean to start a war with this thread. Like I said, I honestly thought I was being of help. Perhaps we will have to agree to disargee on these things. Sorry. I wasn't trying to stir up any trouble here.

I was told by my machinist that you want the rocker to ride on the center of the valve and to have no more than 1/8" of travel on the top of the valve. This is how I did mine, and I had no problems...btw....I used my pushrod checker on 300lb seat pressure / 750lbs open pressure @ .714" with no problems. It came out the same as a regular pushrod

I don't like the method that Comp Cams uses at all. I think that part of it is all wrong. Not all valvetrain set-ups will have the rocker arm tip starting off on one side of the valve stem tip, and ending up on the other as the Comp Cams illustration shows. So that part of it is just plain inaccurate.

Maybe with those new .050 lift cams for excellent fuel economy....:rolleyes:

I was told by my machinist that you want the rocker to ride on the center of the valve and to have no more than 1/8" of travel on the top of the valve. This is how I did mine, and I had no problems...btw....I used my pushrod checker on 300lb seat pressure / 750lbs open pressure @ .714" with no problems. It came out the same as a regular pushrod


Yes, Center of the valve at mid-lift...and barely moving from the center at no lift (base circle) and max lift.

Divide the valve stem into four equal sections and stay within the inner 2 spaces...and you will be fine....just like the figure on CC's Site.

Remember..pushrods come in .050" increments unless you are running Warren Johnson of course..

Remember..pushrods come in .050" increments Which is precisely why a .017" difference in the "Gauge length" and the "actual length" of the pushrod is meaningless. An .017" difference in length means nothing. This is just another example of some instructions making this more complicated than it needs to be. :rolleyes:

What Shurkey has said is all correct...

Thanks. You covered almost everything I'd have said. One additional thing I'd like to stress: If you plan to use hydraulic lifters, but you measure your pushrod length using solids like Lunati says to do, you better be sure that the solids have the pushrod cup at the same height as the hydraulics you are actually going to use in the engine. EVEN DIFFERENT BRANDS of similar lifters may not put the cup in the same place. There is the same problem at the top end of the pushrod--different rocker arms may have the pushrod cup in different locations.

You are MUCH BETTER OFF using lightweight springs and the lifters (and rockers) you plan to run in the engine.

I can see both sides here, but Billy has a point. If your method results in a measurement that is significantly under the increments in which you can purchase the pushrods then you may be over thinking the situation a bit. While accuracy never hurts anything attention to detail might be better spent some where else.

That said im not a master engine builder. I have had some success and failures. Im sure at the highest level it is precisely this attention to detail that makes the difference between failed project and their successful ones.

Shawn

If you plan to use hydraulic lifters, but you measure your pushrod length using solids like Lunati says to do, you better be sure that the solids have the pushrod cup at the same height as the hydraulics you are actually going to use in the engine. EVEN DIFFERENT BRANDS of similar lifters may not put the cup in the same place. YES!! I agree. That's a very good point. And I also agree with you about the point of it being best to use the lifters that you'll be running in the engine to check for proper pushrod length requirements. I have no argument with you there. I particualrly like the method that Lunati uses to mark the valvestem tip to see where the roller of the rocker arms are making contact. That's really what I wanted to focus on, as well as the fact that the travel of the roller along the valve tip needs to be minimal like they're stating.

Which is precisely why a .017" difference in the "Gauge length" and the "actual length" of the pushrod is meaningless. An .017" difference in length means nothing. This is just another example of some instructions making this more complicated than it needs to be. :rolleyes:


Billy - Apples and oranges...

The reason Comp Mentions the different lengths used in the industry is because there are different ways to measure..Their Hi-Tech pushrods are based on Gauge Length

You will not be able to see a .017" difference in pushrod length reflected on the valve stem when examining the pattern on the valve stem..That is where the marginal differences in between the 0.050" increments do not matter.

If you order pushrods from another builder make sure you tell them the length from the points you measured and ask how they measure theirs

If you need from tip to tip with an oil hole (theoretical length), they will know how much length there is in each tip when pressed on and can adjust the shaft length accordingly when they are cut to length.

If they say they go by gauge length theoretical or actual length you'll at least know what they are talking about

The Whole Point of the different ways lengths are measured are to make you aware and how to properly measure when ordering pushrods from any source

The instructions on Comps website are not a "Catch All" by any means but do give you good information and awareness.

I can see both sides here, but Billy has a point. If your method results in a measurement that is significantly under the increments in which you can purchase the pushrods then you may be over thinking the situation a bit. While accuracy never hurts anything attention to detail might be better spent some where else.

That said im not a master engine builder. I have had some success and failures. Im sure at the highest level it is precisely this attention to detail that makes the difference between failed project and their successful ones.

Shawn


Shawn,

there are no two sides here I don't know where that is coming from

My responses are to Billy's claim that Comps instructions are all wrong and Shurkey was wrong.

By Billy -

"I don't like the method that Comp Cams uses at all. I think that part of it is all wrong. Not all valvetrain set-ups will have the rocker arm tip starting off on one side of the valve stem tip, and ending up on the other as the Comp Cams illustration shows. So that part of it is just plain inaccurate."
If Billy actually reads the WHOLE sectiuon on Comps website he will see that those instructions are applicable to COMP CAMS. Knowledge of how valvetrain geometry works would help him to weed out the information that is NOT pertinent to other manufacturers pushrods and only pertinent to COMP Cams.

By Billy -

Not all valvetrain set-ups will have the rocker arm tip starting off on one side of the valve stem tip, and ending up on the other as the Comp Cams illustration shows. So that part of it is just plain inaccurate.


These statements show that he should not be making statements rebutting information from people that may have just a few years more experience under their belt.


Certainly the .050" incrementals are sufficient and it was never said that it had to be more accurate....unless you are Warren Johnson and running Pro-Stock...Similar to your statement of the necessity of a higher precision at certain levels, and most certainly not necessary by most of us.

I certainly agree that these things can be overthought and nobody is saying it needs to be that precise....

The whole gauge length thing is just there because CC uses that for their custom made pushrods measured for with their Hi-tech length checker..

But if the only point in having to check for pushrod length is to know which of the .050" length increments are required, then there isn't any need to get into, nor to be concerned with, measuring differences that merely equate to an .017" variation in pushrod length. A pushrod that's merely .017" too long, or .017" too short will not throw the valvetrain geometry off enough to cause premature wear in any of the valvetrain parts. Right or wrong? You can have all the technical know how in the world, but if you're majoring in the minors for no apparent applicable reasons, then it's merely for some type of bragging rights, which are of no importance in this topic.

And BTW Tom, the brand of camshaft you have is irrelavent to what the best methods are for determining proper pushrod length. Valvetrain geometry is valvetrain geometry. So your claim that Comp cams method for this procedure applies to their cams only is inaccurate. So maybe we should just agree to disagree before this gets ugly ( like I've previously suggested).

Shawn,

there are no two sides here I don't know where that is coming from

.Tom, what Shawn is talking about, and what you're failing to consider, is the fact that different cam companies give us different methods for checking pushrod length requirements. And that hasn't anything to do with camshaft brands being different as you've suggested. So there's obviosly many sides to this story as Shawn has mentioned. That's all.

Man this subject gets ugly all the time....on ever site i am on...

I can tell you i have had 400hp-ish engines in my shop from other builders that the geometry was all wrong. I fixed it and the engine ran the same as it did when it was wrong... And there was not excessive guide wear...... Take that for what it's worth..

Again the difference between a 350hp hyd cam engine and a 800 hp solid roller cam engine are like night and day when it comes to finding the correct geometry....And again 5 builders will have 5 different ways of getting it.

Keith

Tom, what Shawn is talking about, and what you're failing to consider, is the fact that different cam companies give us different methods for checking pushrod length requirements. And that hasn't anything to do with camshaft brands being different as you've suggested. So there's obviosly many sides to this story as Shawn has mentioned. That's all.


I am not saying there aren't many ways to do it, of course there is...

My only point was in regard to the statements you made that Comps method and roller tip figure were incorrect.

Just be careful saying things are wrong as CC's method is not "wrong" and the roller tip does vary slightly off center in both directions as their figure shows.

Lunati uses a specific rocker arm supplied with their kit, and what you measure using their kit is based on a (if I remember correctly) .700 lift cam. If your net lift is less than or geater than .700 then you need to add or subtract to the measurement you get based on what the lift and lash your cam is...That is another way to do it..

You can also go to mid lift and change your pushrod length to make sure the roller tip is dead in the center and call it a day...(using checker springs of course.

:)

mc71454......You really dont see that you and Billy have a difference of opinion? or there is a argument concerning the proper method to select pushrod length? and to just what extent one should even concern himself with it?

2 different opinions = 2 sides = i can see both ;)

Im actually interested in this thread because when i get my heads back from AFR ill be needing to select one myself...

Before i knew close to what was required and through various engines i had upwards of 3-4 sets to choose from. But im in new territory with big inch bbc's


This is a great forum.

But if the only point in having to check for pushrod length is to know which of the .050" length increments are required, then there isn't any need to get into, nor to be concerned with, measuring differences that merely equate to an .017" variation in pushrod length. A pushrod that's merely .017" too long, or .017" too short will not throw the valvetrain geometry off enough to cause premature wear in any of the valvetrain parts. Right or wrong? You can have all the technical know how in the world, but if you're majoring in the minors for no apparent applicable reasons, then it's merely for some type of bragging rights, which are of no importance in this topic.

And BTW Tom, the brand of camshaft you have is irrelavent to what the best methods are for determining proper pushrod length. Valvetrain geometry is valvetrain geometry. So your claim that Comp cams method for this procedure applies to their cams only is inaccurate. So maybe we should just agree to disagree before this gets ugly ( like I've previously suggested).


I give Up ----> Billy instead of going back and editing your posts just continue on in the next post...

Once again you do not read....I never said anything about cams. Who mentioned Brand of Camshaft ???? ...it is about Comps Specific Hi-Tech pushrods....Please Read...

If you are NOT BUYING Comps Hi-Tech Line Than I agree who cares about the Gauge length measurement...I know I don't..

Once Again - My only point was not to say another method is wrong when it is not, it may be too exact for what WE need but it is not wrong using the correct definition of wrong..

mc71454......You really dont see that you and Billy have a difference of opinion? or there is a argument concerning the proper method to select pushrod length? and to just what extent one should even concern himself with it?

2 different opinions = 2 sides = i can see both ;)

Im actually interested in this thread because when i get my heads back from AFR ill be needing to select one myself...

Before i knew close to what was required and through various engines i had upwards of 3-4 sets to choose from. But im in new territory with big inch bbc's


This is a great forum.


My opinion is Comp Cams is not wrong - Billy says they are Wrong...

It may be too precise for our needs, but it is not wrong...That is..and always has been what this big waste of time is about...

The pattern should be centered on the valve tip and ideally should be .060" wide. Most checking pushrods (at least the three brands I've used) have a line on the adjuster and on the fixed side of the checking pushrod. With those two marks in alignment the checking pushrod is at it's shortest advertised dimension. The checking pushrods usually have a thread pitch of 20 (you can verify with a thread pitch gauge). That means that each full turn is .050". Install the checking pushrod and set the valve lash to your cam card spec (allowing for material expansion). 9 out of 10 times this will put the pattern where it need to be on the valve stem. Remove the checking push rod and count the number of turns needed to bring the mark on the adjustable portion of the pushrod back into alignment with fixed portion of the pushrod. Multiply the number of turns by .050" and add the base dimension of the checking pushrod. This is the length of pushrod you need to order. Another way to check is when the valve is at mid lift (as measured with a dial indicator) the rocker arm should be perpendicular to the valve. By this I mean draw an imaginary line between the center of the rocker tip and the center of the trunion bearing. At mid lift the valve should be perpendicular with this line. Hope that makes sense.

you know i would think this should come down to math. You have 250lbs of seat pressure and 650 lbs open. You would think that you would sacrifice optimal geometry from the seated position to attain a perfect linear relationship to the valve tip at full lift where the most pressure is applied to everything.....

curious....

FYI -- while .017" isn't critical by any means, if you have luck like mine you will find yourself needing a pushrod length smack dab in the middle of what is available. Knowing the pushrods you order will come in .017" short or long helps make the call when in this situation.

Good info IMO

oh yea trust me i have luck like yours :)

FYI -- while .017" isn't critical by any means, if you have luck like mine you will find yourself needing a pushrod length smack dab in the middle of what is available. Knowing the pushrods you order will come in .017" short or long helps make the call when in this situation.

Good info IMO

i hear you. when i went thru this on my 468 build last year i wasnt right in the middle but i was further away than i thought was optimal. however, when i ordered the push rods (closest fit available) which were, by the way comp cams rods with comp cams roller rockers and a lunati roller cam, i found the pattern positioned right in the sweet spot and with even a tighter spread than i got with the checking push rod. go figure. i guess i should have bought a lottery ticket that day :)

FYI -- while .017" isn't critical by any means, if you have luck like mine you will find yourself needing a pushrod length smack dab in the middle of what is available. Knowing the pushrods you order will come in .017" short or long helps make the call when in this situation.

Good info IMO

Off-the-shelf pushrods come in .050" increments. If you are midway between increments, sure, you order whichever is closest to your "ideal" length. Therefore, even with "shelf stock" pushrods, you should not be more than + or - .025" away from ideal.

That .017 error is a pretty good share of the .025" range all by itself. And the tendancy will be that the pushrods you receive will be SHORTER than what you have measured as your ideal length if you don't compensate for the drilled center of the pushrod when you take the measurement.

Yes, you're going to round the length up or down some when you place the order. I just feel it's important to measure (everything) accurately, so that you wind up within the optimal + or - .025 range.

Don't forget that with most Chevys, the rocker adjusting nuts are 7/16-20 thread. One inch divided by 20 threads means each thread is .050. So if you adjust your hydraulic lifters 1/2 turn past where they stop clicking, you now need a pushrod .025 longer than what Lunati tells you using their method, because you've sunk the pushrod cup .025 into the lifter body with your 1/2 turn of preload.

I particualrly like the method that Lunati uses to mark the valvestem tip to see where the roller of the rocker arms are making contact. That's really what I wanted to focus on, as well as the fact that the travel of the roller along the valve tip needs to be minimal like they're stating.

Agreed. You should have seen me do this the first time. I'm in there with flashlights and mirrors trying to see where the roller is riding on the valve tip. It was a nightmare. The magic-marker-on-the-valve-tip makes it a lot easier.

you know i would think this should come down to math. You have 250lbs of seat pressure and 650 lbs open. You would think that you would sacrifice optimal geometry from the seated position to attain a perfect linear relationship to the valve tip at full lift where the most pressure is applied to everything.....

curious.... That's^ a very thought provoking question, and I had to stop and think about that for awhile....but after thinking about it, i realized that if you set it up like that, then the roller tip of the rocker arm would be traveling from the far side of the valve to the middle, and would be always located on the outside edge of the valve stem tip all the time. And that would probably put undue lateral stress on the valves, which might very lead to accelerated valve guide wear, and maybe even accelerated valve seat wear too. That was an excellent question though.

BTW, not only do I have the Comp cams catalogue, and have read ALL their drool about valvetrain geometry and pushrod length requirements more than once, but because the various major camshaft companies ALL have more experience at this than most of us here do, and yet still disagree on their suggested methods for checking for pushrod length requirements, then it isn't neccessarily out of line to state that one is method is better than the other.

Just because a cam company has been doing what they do for many years doesn't neccessarily make them correct about everything they tell their customers. Because if that were the case, then all of them that have many years of experience would basically tell you the same thing about this pushrod length checking method. However, they don't agree on this at all. So one method has to be better than the other. And it's up to each one of us to decide which once it is. I've made my choice based on my pushrod checking tasks in the past, keeping in mind which way each cam company tells you to perform the procedure.

Here's another interesting link......

http://www.mid-lift.com/MASTER-INDEX.htm (http://www.mid-lift.com/MASTER-INDEX.htm)

http://www.mid-lift.com/TECH/TECH-Installed-G1.htm

I have always tried to adhere to the theories suggested in the enclosed post, something left over from my motorcycle days. Accepted practice was to have the fulcrum-to-tip axis at 90 degrees to the valve centerline at midlift. Unfortunately, the aftermarket people don't seem to be able to get together on some sort of standard dimensions for valve geometry. I can't understand why the head compamies can't tell you what rocker will work best with their particular head setup or have dedicated rockers. (I know some shaft mounts are.)

Just because the roller rides in the middle of the valve stem does not mean rocker geometry is correct. But that is where the roller SHOULD be when the rocker geometry is correct. (I also disagree with the CompCam method, but who am I? They are also in the pushrod selling business and they pretty much tell you that you WILL NEED different pushrods. Even if they are only .050" different.)

When I do valve train mockup assembly, I use a piece of nylon that I insert in place of the cam that has a 1.400 diameter in the area where the lifters sit. With a 1.5 rocker ratio, this gives me a reference point of a 1" base circle cam@ .300 valve lift, or about half lift of the average cam. The nylon gives me a standard that I add to or subtract from to come up with the dimensions I need. This makes working with rocker arm stud height, pushrod length, and rocker angles easier for ME. I'm not saying everyone else should adopt my method or it's better than anyone else's method. This is what works for me with an engine on a stand. (With this, roller lifter orientation is not necessary. It just sits on the nylon and takes up space.)

Another thing that needs to be addressed on SB's is: the lifters, rocker studs, and valve centers all being different. That makes the roller rockers skate back and forth while they are working. This is another reason for going to shaft mounted rockers.

In the spirit of this post I decided to have a little fun last night...

First I checked piston-to-valve clearance...nothing exciting, but a slightly anxious situation as to what follows if you don't have enough..:(...But I did :hurray:

Next,

I used three (3) different methods to determine the pushrod lengths I need for my 350+ HP motor with canted valves, angle milled heads and longer valves than stock...

Ya know what, (as expected) all the answers came out the same in the end when rounded to the nearest .050"...

the biggest pain in the neck was the intake valve..there was a max spread of 0.031 under 8.500" to 0.022" longer than an 8.500" pushrod...So for the Grand Finally I used a part of all three of the methods and what came of that was that 8.500 to 8.520 would be to quote Goldilocks .....just right

I was happy to hear today that a wide pattern on the intake side of the AFR's is not uncommon, it is not a problem, just wasn't expected and just didn't happen with previous heads I have used.

Ken and Tom,

thankyou both for your input on this. And Tom, that sounds like you had an interesting experiment there with your engine. Again, thanks for sharing.

Just curious why hydraulic lifter preload would have anything to do with adding length to the pushrod? The rocker doesn't really start moving until the bleed hole in the lifter is covered by the plunger anyway regardless of preload . . . 1/4 turn, a full turn preload, whatever, I don't think preload matters -- on the other hand, how much the lifter compresses before opening the valve seems like it WOULD matter.

Personally, I like checking pushrod length in a hydraulic cam motor with the lifters to be used installed (and full of oil of course). As far as I can tell this would most closely simulate operating conditions.

Not trying to argue, just thought this point would be worth discussing :)

I down loaded the instructions and tried this method. Very easy. I like it! :D

Just curious why hydraulic lifter preload would have anything to do with adding length to the pushrod? The rocker doesn't really start moving until the bleed hole in the lifter is covered by the plunger anyway regardless of preload . . . 1/4 turn, a full turn preload, whatever, I don't think preload matters -- on the other hand, how much the lifter compresses before opening the valve seems like it WOULD matter.

Personally, I like checking pushrod length in a hydraulic cam motor with the lifters to be used installed (and full of oil of course). As far as I can tell this would most closely simulate operating conditions.

Not trying to argue, just thought this point would be worth discussing :)
=
Hi Eric,
Simply put, as you preload the lifter you effectively shorten the lifter because the plunger is lower in the body.
Just like a small base circle lowers the whole lifter requiring a longer pushrod, lowering the plunger does the same thing.
And you are correct, you need to check length with the parts you are going to use, lifter, rocker,,, all can change needed length.

=
Hi Eric,
Simply put, as you preload the lifter you effectively shorten the lifter because the plunger is lower in the body.
Just like a small base circle lowers the whole lifter requiring a longer pushrod, lowering the plunger does the same thing.
Yes, but if you're going to be using hydraulic lifters when the engine is being run in the vehicle, then the valvetrain geometry is going to be the same as it would be while you turn the engine over manually with hydraulic lifters. So it isn't going to change the pushrod length requirement since the plungers in the lifters will be compressed to the same degree and will be the same height while the engine is running as they are when you're checking for pushrod length. Right?

Yes, but if you're going to be using hydraulic lifters when the engine is being run in the vehicle, then the valvetrain geometry is going to be the same as it would be while you turn the engine over manually with hydraulic lifters. So it isn't going to change the pushrod length requirement since the plungers in the lifters will be compressed to the same degree and will be the same height while the engine is running as they are when you're checking for pushrod length. Right?
=
The question was the difference between a quarter turn & one turn.
With a 7/16 stud this is .038"
One turn is .050"