valve train geometry

[grumpyvette]

IF Im changing just the cam and lifters do I need to recheck the pushrod length?

JUST WONDERING,IF YOU HAVE STOCK LENGTH PUSHRODS,YOU MILL THE HEADS,USE THIN METAL HEAD GASKETS,WOULDN'T THIS PUT THE ROCKER ARM CLOSER TO THE CAM?THEN WHY DO YOU NEED LONGER PUSHRODS WHEN YOU INSTALL A BIGGER CAM THAN STOCK?YOU WOULD THINK YOU WOULD NEED A SHORTER ONE SINCE THE LOBE IS TALLER THAN STOCK AN THE ROCKER ARM IS CLOSER TO THE CAM BECAUSE OF MILLING & THINNER HEADGASKETS.

 

 

while your correct! the answers not always that simple because the cam base circle dia. on the new cam is frequently a differant dia. than the stock cam and the lifter seat height varies also, it will require verifyiong the correct valve train geometry.

you want the rocker to exert it force as much as possiable on the valve centerline to reduce friction and valve guide wear

YOULL NEED TO CAREFULLY CHECK YOUR PARTICULAR ENGINE!

heres a simple way to get close to the correct length

BUY ONE OF THESE

http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=3567&prmenbr=361

 

after making sure the valve springs are correctly installed you drop the checker in place on the rocker stud and install your adjustable pushrod

adjust the length to fit and measure the resulting length if its within twenty thousands of the stock length its fine for most applications, if its more than 30 thousands long or short get the closest length set available

 

btw, if your one of the people that still does not own an adjustable push rod! you can easily make your own by cutting a stock pushrod in 1/2 (2 pieces), removeing 1 inch from the total length an then with about 2 " of a 4 inch section of 3/16 or 1/4" thread rod installed and (in one section epoxy it in place leaving about 2" sticking out thread two nuts onto the thread rod and slip on the other end of the cut pushrod,(no epoxy) use the two nuts to adjust to stock length and let the epoxy harded in the one section[/color] now you can easily measure and order custom push rods useing the pushrod checker and adjustable push rod as tools

AND YEAH IT ONLY WORKS WITH THE CYLINDER HEADS ON AND THE INTAKE REMOVED BECAUSE THE HOLE IN THE CYLINDER HEAD that GUIDEs THE PUSH ROD WON,T ALLOW THE NUTS ON THE ADJUSTABLE PUSHROD TO PASS THRU, UNLESS YOU PLACE THE CUT ABOUT 1" from the UPPER END OF THE ADJUSTABLE TEST PUSHROD BUT I prefer to place the adjusting nuts centered as I like to watch for all clearances with the intake manifold removed while manually checking as I turn the engine over by hand durring assembly, and at that point, while checking all the clearances, I use test springs which apply very little load on the push rod

 

http://www.jegs.com/cgi-bin/ncommerce3/ProductDisplay?prrfnbr=3272&prmenbr=361

 

HERES OTHER TOOLS YOU MIGHT NEED

 

 

 

 

SOMETHING TO READ

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

CCA-7705 5.800 in. to 9.800 in. adjustment range, Master pushrod length checker 4 piece kit ... $78.69

 

here

 

http://www.circletrack.com/techarticles/ctrp_0611_rocker_arm_valvetrain_geometry/index.html

 

http://www.circletrack.com/techarticles/ctrp_0611_rocker_arm_valvetrain_geometry/roller_tip_rockers.html

 

http://www.compcams.com/Technical/FAQ/ValvetrainGeometry.asp

 

http://www.babcox.com/editorial/ar/eb50232.htm

 

http://www.customspeedparts.de/content/en/technic/kipphebelgeometrie.php

 

http://www.hotrod.com/techarticles/camshaft_tips_definitions/index.html

 

http://www.eatonbalancing.com/blog/2007/12/10/rocker-arm-geometry/

[pparaska]

Grumpyvette,

Are we SURE that "Diagram A" is correct?

 

 

My investigation reveals that they have the words next to the numbers all mixed up and they should be relisted as:

1. Base ciricle AND Max lift

2. 1/4 and 3/4 lift (approximately)

3. Mid Lift (opening and closing)

 

In other words, during an opening and closing cycle, with the optimal length pushrod, the rocker tip contact line on the valve tip will:

 

A) start at 1,

go through 2 at about 1/4 lift (opening),

C) hit 3 at mid lift (opening) and start back towards 2,

D) go through 2 at about 3/4 lift (opening),

E) hit 1 at max lift and start back towards 2,,

F) go through 2 at about 3/4 lift (during closing),

G) hit 3 again at mid lift (during closing), and start back towards 2,

H) go through 2 at about 1/4 lift (closing), and

I) end up back at 1 when fully closed.

 

That's a lot different than what they are showing. But I really do think they have this wrong. In fact, I think what they are showing is the result of a too-short pushrod!

 

I've set up valvetrains with adjustable pushrods, a "Ferry" type tool to measure actual roller movement across the valve tip, and have found the sequence above (A-I) to be what happens when I get the pushrod adjusted to a length that minimizes the actual roller movement extent across the valve tip.

 

BTW some builders think that it is more important to have the roller movement extent across the valve to be minimal, not trying to get it centered over the tip Centering the roller contact pattern on the valve tipe actually has more to do with the tip height, distance between the stud centerline, and other geometry. Sure it's best to have it centered, but if you are rolling the tip across a smaller area, you are also pushing it sideways less, meaning less friction with the valve guide/stem, and less wear. If it never comes too close to the edge (less likely if you adjust the pushrod length optimally to get minimal roller movement extent). And if you measure actual lift, it will always be maximum when the pushrod is adjusted to give the smallest roller movement extent (smallest pattern) on the tip.

[dr_hunt]

Your right Pete especially with really high lift roller cams. At least that has been my expeience.

[grumpyvette]

One little known bit of engine trivia, the thread pitch on the rocker studs is a known value, and therefore you can use that info to get a close approximate lash clearance.

THAT INFO IS GOOD TO KNOW WITH SOLID LIFTERS

 

ok, HOW you ask?

If the rocker studs have a comon NF thread theres a pitch or number of threads per inch, now obviously youll need to carefully verify what the thread pitch, and what the clearances are with a feeler gauge on your engine, while its not running and take into account the rocker ratio but lets assume

3/8" 24 Threads Per Inch

7/16" 20 Threads Per Inch

from that info you can adjust the rockers remarkably close to thier correct lash value, in a similar manor too that used for hydraulic lifters , but instead of adding preload by running the rocker nut in slowly just to the point the lifter stops clicking at idle, then adding a 1/2 turn to add preload, you run the rocker in slowly at idle just to the point it stops clicking and BACK IT OUT the approximate lash distance, using the known thread pitch as a guide.

lets say the lash should be .016-.018

youve got a 3/8" stud with 24 threads per inch, with a bit of math we find that each full rotation of the rocker nut advances or reverses the nut .041 time 1.5 (or your rocker ratio),per inch, so youll get the approximate lash with backing the nut off about 2/5ths of a full turn, or a bit less than 1/2 turn but more than 3/4rs of a 1/2 turn

naturally youll have verified exactly what your engine requires before doing this, so youve got the correct math and value

yeah! I know your thinking its not exact! well from YEARS of doing it I can tell you that after awhile youll get a feel for the engines sound and responce and youll find that you can get very good results

 

this infos very handy with self alighing rockers that make setting lash with a feeler gauge a P.I.T.A.