In an earlier post of mine about rocker arms and pushrods, (Erson FE Rocker Arms), Brent attached a diagram of a rocker arm and valve tip to demonstrate proper valve train geometry. I have also seen this diagram in Comp Cams catalogs. They state, “What is the correct length pushrod for your application? The one that produces correct valve train geometry. What is correct valve train geometry? When the rocker arm roller tip rolls from the intake side of the valve tip, across the center of the tip (at approximately mid lift), to the exhaust side of the valve tip (at full lift) and back.” They then refer to the diagram to visually explain this.
When it came time for me to measure for new pushrods for my new set of rocker arms, I tried to apply this method. Problem is I could see no direct correlation between push rod length and roller to valve tip geometry. As can be seen in the picture below, there is no pushrod. The rocker roller is positioned over the valve tip. The only thing the pushrod would do is impart movement of the roller to travel over the valve tip as it pushes down on the valve and spring. How far it travels would be in direct relation to how much lift the cam imparts on the lifter and pushrod.
So what mechanism will adjust the position of the rocker arm roller to the preferred position? At first glance, it would seem that the distance between the center line of the rocker shaft and the center line of the valve stem would have to be adjusted. Or the length of the rocker arm (valve end) would have to be lengthened or shortened. In my case, the Erson assembly drops in over the studs and sits on the head. There is very minute slop or play when positioning the assembly, and the installation instructions do not address this. And of course there is no adjustment to change the position of the roller ball in the rocker arm. You get what you get, and hope the designers did their homework when they produced their product.
So what do you do to determine the correct pushrod length? Erson recommends that when selecting the length, have between one and three threads of the lash adjustment screw visible on the underside of the rocker arm. I chose two. Then with the rocker arm assembly installed, using an adjustable pushrod length checker, lengthen or shorten it while positioning a feeler gauge between the roller and valve. What you should end up with is a pushrod length that will allow you to set the valve lash as recommended by the cam maker and have from one to three threads of the adjustment screw visible. There was no mention of “valve train geometry”. When I talked to the tech guy at Smith Brothers Pushrods, he basically said the same thing.
I admit I’m no expert mechanic, and I’m throwing this out here for discussion. Maybe I’m missing something here that someone with more knowledge and understanding can educate me and others that may also be scratching their heads over this subject.
Jim Harding
#4279512 - SA452 - TKO600
#3004 - 302 - 4 speed
La Plata, Maryland
It would seem to me that in the case of an FE engine you have no adjustment available to change the pivot point of the rocker arm like you do on other Ford engines. When you adjust the valve clearance (on an FE) you are in effect lengthening or shortening the pushrod. On other engines a too long of too short pushrod can be adjusted to get proper valve lash but only by raising or lowering the pivot point to a less than ideal location.
Rod
Roseville, Ca.
289 FIA #9152 "The Flintstone Cobra"
DOH!! I've been so focused on this FE that I never even thought of pedestal mounted rockers. Your explaination makes sense out of the whole geometry thing Rod. Thanks for turning on the light
Jim Harding
#4279512 - SA452 - TKO600
#3004 - 302 - 4 speed
La Plata, Maryland
Jim, I think you just need pushrods that won't hit the rocker arms at full lift. Which the method you described should insure that.
And geometry is up to the engineer.
Taller and shorter stands can change your pivot point.
Tony Unique 427 #4279480 Precision Engine Machine Company 427 s/o 483.47 stroker close ratio toploader 3.31 jag rear
[QUOTE=Naumoff;84090]Jim, I think you just need pushrods that won't hit the rocker arms at full lift. Which the method you described should insure that.
QUOTE]
I can only hope Tony. I jury rigged a home-made pushrod length checker from one of the old pushrods, tapped it for 1/4-20 threads, screwed in about an 1 1/2" of threads donated by a long bolt, dabbed on some loc-tight, made the measurements, and mailed it off with my order. Don't know how long they'll take to make and return to me. I'll keep ya'll advised on my progress.
And by the way.... I have a bunch of spare Dove rockers if you need some to put in your tool box I was thinking of selling what's left of the assembly, but don't want to inflict anyone with my less than happy experience with them.
Jim Harding
#4279512 - SA452 - TKO600
#3004 - 302 - 4 speed
La Plata, Maryland
Jim, I think you just need pushrods that won't hit the rocker arms at full lift. Which the method you described should insure that.
QUOTE]
I can only hope Tony. I jury rigged a home-made pushrod length checker from one of the old pushrods, tapped it for 1/4-20 threads, screwed in about an 1 1/2" of threads donated by a long bolt, dabbed on some loc-tight, made the measurements, and mailed it off with my order. Don't know how long they'll take to make and return to me. I'll keep ya'll advised on my progress.
And by the way.... I have a bunch of spare Dove rockers if you need some to put in your tool box I was thinking of selling what's left of the assembly, but don't want to inflict anyone with my less than happy experience with them.
I will take you up on that if you're serious.
The set I have in my engine now are the ones that took the hit when the crank gear key gave way and all 16 valves were bent.
Tony Unique 427 #4279480 Precision Engine Machine Company 427 s/o 483.47 stroker close ratio toploader 3.31 jag rear
I have an article at home that explains how to sellect the proper length push rods. I'll make a note to get it tonight. One way to check for proper length is remove the rocker and see where it is touching on the top of the valve stem. If the length is correct, it will show a line right across the middle (like the top of a flat head screw). If it's to the top or bottom of center, rods are incorrect length. Looking at your photo, it looks like the roller is not lining up exactly in middle of stem.
Bob
With the rocker shafts on the FE you can only do that if you change the height of the rocker arm stands. The pushrod length will not change the geometry.
Tony Unique 427 #4279480 Precision Engine Machine Company 427 s/o 483.47 stroker close ratio toploader 3.31 jag rear
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Originally Posted by Naumoff
... but, hey, that would sure solve the broken rocker issue
!
