• Hi Guest!
    You can help ensure that British Car Forum (BCF) continues to provide a great place to engage in the British car hobby! If you find BCF a beneficial community, please consider supporting our efforts with a subscription.
    There are some perks with a member upgrade!
    **Upgrade Now**
    (PS: Subscribers don't see this gawd-aweful banner
Tips
Tips

Rocker Arm and Push Rods

Share this page

V

vettedog72

Jedi Knight
Offline
What's all this talk about shorter push rods being needed when the head is shaved? Does rocker arm geometry really change when the head is milled? I don't see how the the geometry actually changes because the adjustment seems, in effect, to lengthen or as needed when the head is shaved, reduce the length of the push rod. In effect the plane that the rocker works in stays the same and the movement (arc) in degrees would be the same. OK, enlighten me before you kill me if I am wrong.
 
When a head is shaved, the rocker arm fulcrum moves closer to the camshaft. If pushrods are not shorter, the pushrod will raise the rest position of the pushrod, which causes rocker arms to rest on an angle. Although you may be able to adjust valves to within spec by backing way off adjusters, since the rocker arm is canted over tward the valve, all three (pushrod, rocker and valve) fulcrum points are now not in the designed seated area. As a rocker arm presses on a valve it moves in slightly in the end of the valve stem, incorrect rocker arm geometry causes this point to be too close to the edge, causing excessive wear on the stem and valve guide. In extreme cases, the rocker can slip off a valve and cause major carnage. Some Truimph heads are shaved 1/2 inch or so, and there is no way other than pushrods to correct. Hope this helps.
 
One alternative to shorter pushrods is to shim up the rocker arm pedestals by the amount that is milled off the head. Things to be concerned about with that, though, is clearance with the top of the valve cover, and the one pedestal with the oil feed hole. Shorter pushrods are better, imo.
 
If you shim up the pedestals then you will have the same problem with rocker angle----the rocker will have to lean farther to make contact with the valve stem. Its too easy and cheap just to order a set push rods for a fuel injection engine, they are the correct shortened length for most shaved heads. Allan
 
Hello VD,
it depends how much you machine off the head face (and how much earlier owners may already have taken some off.)
If your adjusters have enough thread to adjust the valve clearnaces correctly then all is well and the geometry will be correct. What could compromise this is the amount the valves have sunk into the seats due to wear and possible re-cutting, this will reduce your available adjustment. Asw Allan says, Triumph made a shorter push rod for the P.I. engine as this had a much thinner head. Shims are not an answer as they do compromise the geometry.
 
Now please explain how shimming the pedestals "compromises" the geometry. 35b-day is exactly right.

Consider: A 1/4" is milled off the head. Now shim the pedestals up 1/4" to give them the same distance to the cam as before milling the head. Won't the overall geometry be identical?

While resetting the adjusters will preserve the rocker to valve angle (assuming there is enough adjustment), the pushrod to rocker angle is disturbed. The pushrod sweeps an arc when it moves. The pushrod and the adjuster form a "L" shaped lever. When you run the adjusters up to compensate for a milled head the end of the pushrod follows the arc closer so there is slightly less valve lift.
 
Shimming the rocker would make the problem of rocker to valve angle even worse. You are raising the fulcrum point, bringing the valve stem contact point even a greater angle. Take three pencils and make an upside down "U" - that is your valvetrain. If you raise one side up 1/2" or so, look at the angle on the lower side. Rockers are designed to contact a valve at a precise angle that allows it to open and turn the valve slightly. Turning the valve is important in preventing seat damage and valve burn, and in evening out wear. If that point is incorrect, valveguide and stem damage, as well as premature rocker arm tip failure, will occur. PI pushrods are the way to go on shaved heads.
 
The adjuster is in effect an extension of the push rod---if there is enough adjustment and there is no bind between the rocker and the push rod-- then as Piman indicated there will be no problem with the geometry. Regarding the petastal---picture spacing it up 1 inch---- now how much angle would it take for the tip of the rocker to touch the valve stem? Alot!. ---IF YOU RAISE THE PETASTAL YOU CHANGE THE GEOMETRY. Allan
 
Ok, ok; uncle. You're right, I'm wrong. Shimming is a bad idea. It maintains the pushrod to rocker geometry angle, but not the pushrod to valve since the valves will now be lower. Sorry.
Shorter pushrods is the way to go, like I said in my first post. But, milling the head and using stock pushrods is not entirely compensated for with the screw. The adjuster is NOT an extension of the pushrod - it is a pivot point offset from the rocker axis of rotation. Just as you want the rocker to slide/roll from one side, through center, and stop at the other side of the valve stem, the pushrod should have a similar motion on other side in relation to the pivot point. The closer it gets to the rocker axis, say, when the screw is adjusted up with stock pushrods and a milled head, the more it follows the arc of the pushrod. So slightly more of the travel is to the side rather than up. I think that's the other side of the geometry issue that most miss.
 
Back
Top