getting the valve train geometry correct

[grumpyvette]

once youve installed a new cam in your engine its necessary to determine the correct valve train geometry, this is MANDATORY if you want the cam to function correctly.

yes the stock lenght may work just fine! but you must check!

 

heres a quick way described below

 

 

" to QUOTE MOTORMAN.....

you just put the lifter on the cam base circle,slip the checker over the stud,if the checker hits the valve first use a feeler gauge between the checker and the pushrod tip to see how much longer pushrod you need. if the checker hits the pushrod first use a feeler gauge to check the gap at the valve stem tip and that is the amount you need to shorten the pushrod"

 

its not that difficult to figure out, read these links also

 

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

 

http://www.powerandperformancenews.com/s...=LENGTHCHECKERS

 

yes you need to check.

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 (correct height, and youve checked for spring bind,retainer to valve guide, retainer to rocker,rocker to rocker stud,and pushrod to cylinder head slot clearance ETC, you drop the correct style pushrod 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

be aware the pushrod checkers come in 3/8" and 7/16" stud models, and bbbc and sbc models

 

btw

http://www.powerandperformancenews.com/s...;Category_Code=

 

BEEHIVE DESIGN springs , with the upper side being smaller in dia. and useing a smaller retainer,

 

available from several manufactures similar too these give much greater clearance to the rocker/retainer area and tend to control valve float harmonics if you find that your current springs won,t work

 

 

BTW DON,T assume that the intake and exhaust pushrods are the same length, yeah they usually are on a SBC engine but there are exceptions....CAREFULLY CHECK SEVERAL or ALL CYLINDERS

especially if your useing aftermarket heads and roller rockers

[BillZ260]

Just fyi for you valve train dummies like me, this is simple and straight forward!!! This, for me, was much easier to understand than the link in the other geometry thread. This is great

 

I'll be picking one of these checkers up tomorrow at the local speed shop

 

Grumpy, you will make an engine builder out of me yet!

[Some-Guy]

Thanks for more great info!

[Michael]

All well and good, but it seems to me that the ultimate arbiter of “what’s the correct valvetrain geometry†– assuming that the various clearances between the various components are within spec, and nothing binds – is how the rocker tip sweeps across the valve stem. Every cam manufacturer’s web site has a drawing akin to “diagram A†in Grumpy’s posting, but I have yet to find any consensus on what is the ideal displacement which is swept by the rocker tip across the valve stem. This is the distance between arrows “1†and “3†in the above diagram. Or to put it another way, if for a given pushrod length the rocker tip goes across the valve stem by, say, 0.070†as the respective valve goes through its lift cycle, is that displacement too much? If so, then the pushrod is too short or too long. If not, then the pushrod length is close enough to optimal, and there’s no point fuddling with pushrod length any further.

 

Use of that plastic “toolâ€, by the way, does not guarantee accurate pushrod length, unless the relation between the base of the rocker studs and the installed-height of the valve springs on your engine happens to be exactly as what the tool’s manufacturer intended.

 

BTW those beehive springs are pretty cool, but unfortunately they’re not (yet) available in spring rates suitable for mechanical roller cams.

[BillZ260]

Grumpy? Can you chime-in again? Do I need to roll the motor w/ a solid lifter and do all that rigamarole or will this plastic guage get me close enough for a street driven, sub 6k rpm enging?

[grumpyvette]

short answer!

the pushrod checker works well enought for 90% of the applications,to get you close enought to run a street combo.

but lets look at what your ideally TRYING to accomplish here first!

whats the reason we even give a crap about the rockers contact or angle to the valve?

well basically the ideal valve train would push vertically in perfect alignment with the valve to minimize any contact between the valve and the valve guide it slides thru to both reduce friction and eliminate wear, so ideally the rocker should push down exactly on the center of the valve with zero side loading.

 

thats going to occure only if the rocker pushes on the #2 location, but since the rocker swings in an arc centered on the rocker trunion or the rocker ball,the tips relation to the valve stem changes. getting the geometry correct minimizes the rocker tip sweep distance accross the valve tip.

now roller rockers generally have a roller tip that minimizes the DRAG friction as it at least in theory rolls very slightly allowing the force to remain mostly directly down in line with the valve and have a lower % of force wasted in forcing the valve stem against the valve guide wall.before the valve moves down ward in the guide.

 

http://www.eng.wayne.edu/page.php?id=758

 

http://www.mercurycapri.com/technical/engine/cam/vtg.html

 

http://www.pontiacstreetperformance.com/psp/valvetraingeom.html

 

http://www.holley.com/data/TechService/Technical/Valve%20Train%20Geometry.pdf#search='valve%20train%20%20geometry

 

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

[jakeshoe]

AS Michael correctly stated the plastic rocker checkers are marginal for checking at best. They don't take into account many of the factors of valvtrain geometry, and they are only made to check for stock ratio. i.e 1.5 on SBC applications.

 

The best method is to buy an adjustable pushrod, or make one using a stock pushrod.

 

Then you adjust the pushrod until you get ideal geometry, and order the proper length pushrods.

 

The rocker arm should be at a right angle to the valve stem at mid-lift. That is if you drew a line from the center of the rocker roller tip if you have one to the center of the fulcrum, it should form a 90* angle to the valve stem at the middle of the lift.

i.e. if you have .500 valve lift, at .250 measured, it should be centered on the valve stem, and at a right angle to it.

 

Some of this relies on the rocker arm being properly made (proper length from fuclrum to tip), the heads being properly machined (proper distance from rocker stud to valve guide, correct valve angles) and valve length, seat depth has an effect.

 

Decking the block, milling the heads, cam base circle, rocker ratio, valve length, and lifter type can all change geometry.

[grumpyvette]

"if you have .500 valve lift, at .250 measured, it should be centered on the valve stem, and at a right angle to it."

 

your correct in theory but in practice its best to have the rocker reach that point at slightly higher PERCENTAGE of the lift (example) ,with that example of a .500 total lift, Id try for center at .300 vs .250 at the 60% point vs the 50% point) simply because the valve spring pressures are much higher at the higher lifts valve lift, yeah EITHER method works and will be better that most engines that don,t get close after a cam swap

[BillZ260]

OK, I picked up the checker and measured the pushrods that were in the engine, they were right at 7.800" maybe a hair less but I'm guessing not seing that they ussually come in .05 increments right? This is measured TIP to TIP.

 

I checked the valves on the nubmer 1 cyl and verified on cyl 4. The gap was between the checker and the valve tip for both valves.

 

Measured gap at both intakes (larger), measured ~.063ish.

Measured gap at both exhausts (smaller), measured ~.093ish.

 

SO, do I need (7.707)7.700 and (7.738)7.750? HELP

 

EDIT: Grumpy I know you said usually SBC's have the same length rods for intake and exhaust. The Deck hight on my block is stock, as far as I know, heads are brand new Edelbrock RPM alum heads. I can get the part numbers if that helps. Currently, I have stock rocker arms, but am considering 1.5 rollers, dunno if this changes anything but thought I'd mention it. I also see that 7.700 and 7.750's are available but have only seen them in sets of 16 each! I'll look some more.

 

I'm new to this stuff, so I really appretiate you guy's imput! I'm learning alot doing this myself and it's fun. Thanks so much!!!

[dr_hunt]

In my experience the type of rocker arm you are using is critical. There is a big difference between using the comp pro magnum or high tech stainless roller rockers and any brand of aluminum roller rocker as the pushrod cup is obviously lower towards the cam than the steel rocker cause the steel rockers are thinner sectionally than aluminum. And if you really want to screw things up, get the new lift right rockers that have the pushrod cup even lower. So, if your even thinking of changing rockers, get the rockers first, then figure pushrod length.

[jakeshoe]

Dr. Hunt...

You are absolutely correct...

This is why I stated in my post that the best method to check the geometry was to use an adjustable pushrod.

 

I'm just a fairly experienced mechanic and machinist, not a high speed engineer type but I would be willing to bet that they teach "minimizing variables".

 

If you are not using the components you intend to use on the engine as assembled, you are wasting your time to check the geometry.

 

The little plastic checkers cannot accurately determine the geometry when you are eliminating one of the key components, the rocker arm.

 

A stock pushrod cut and threaded in such a way as to be able to adjust ( not necessarily centered), a small stud with 2 properly sized jam nuts will accurately check the pushrod lengths on many combo's... much more useful and likely cheaper than the plastic tool pictured.

[dr_hunt]

I have to agree with you jakeshoe, that's what I use, made my own with a stock pushrod, mig, two nuts and machine screw. Works like a champ!

[BillZ260]

FINE!

 

Don't have a welder but I'm sure I can figure somthing out. I'll cut one up tonight or tomorrow and see if I can expoxy somthing together. Guess I should figure out which rockers i want to roll w/ as well.

[jakeshoe]

I haven't used a welder, I have drilled the inside of the pushrod slightly and threaded it and used a small diameter stud. The jam nuts are to lock the final setting.

[BillZ260]

so did you drill it out using a lathe? or what? I have a small drill press and could possibly clamp up on it tight enought. What thread size did you find that worked easily? I'll grab a tap and some screws this weekend.

[grumpyvette]

http://www.jegs.com/webapp/wcs/stores/servlet/CategoryDisplay?lang=-1&catalogId=10002&storeId=10001&categoryId=23421

 

its a waste of time to build checking pushrods when thier dirt cheap to buy

[jakeshoe]

Grumpy is correct it is easier to just purchase than build unless you need one in short order or for an odd length.

[BillZ260]

Seeing that I'm cheap, and like to mess around, I'll try to make one. I know they are cheap but why buy when you can make. Anyway, I'll see what's what and pick one up if need be.

[dr_hunt]

That's the spirit, then it'll be your tool that is unmistakable! I would think you could do 1/4" tap in a stock pushrod. I don't know I used a 10/32 machine thread.

[BillZ260]

ok, orderd my rocker arms and an adjustable length pushrod last night.

 

Today, was messing around, and thought I'd cut one of my pushrods and try to make one myself anyway right? WRONG. Those are made out of some hard stuff!!! Ruined the first 3 threads on one of my taps. The hole in the pushrod was allready too big for the one I was using. So I figured i'd drill it out a bit and the drill just burnt up. So I'll be looking for the one coming in the mail...

 

Thought I'd see what wear pattern the 7.800 rods would put on my valve tips so I, marked the valve tips, took the slack out of the stock rockers, and got the engine about a half a rotation and it felt like there was bind! I was moving slow and didn't force it at all. I then backed off the bind, pulled the rockers and looked at the marks that had been left so far. The wear mark on the left (Exhaust valve) tip was almost un-readable, the wear mark on the right tip (Intake valve) looked like a full sweep and was on the high side of center which would read as a short push rod right?

 

SO going back to my earlier "plastic checker" lengths there was a ~.063 gap between the tool and the VALVE tip, and ~.093 on the exhaust tip (gap in same place, between the checker and the valve tip). Both telling me to get shorter push rods, so would what I ran into be bind or was I just being timid? I'm kind of freaking out. Any thoughts?

 

When i get my rocker arms and adjustable rod I'm guessing I should start way short and progress to a center mark on the valve tip for both valves correct? I read through all your links grumpy, thanks. I feel like I'm learning and I get the theory side but I'm confused as to why the mark was on the high side of center and the plasic checker measured the current rods to be too long?