• 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 pedestal Height

Share this page

texas_bugeye

texas_bugeye

Jedi Knight
Offline
I was MN. last week visiting a freind who also is an LBC fan.
He had just peice milled a little Moke togather.
Ran fairly good but some questions came up over a few beers.
The engine and the head are of unknown amout of recondition in that how much was skimmed off the head and the deck. So how does one know if the rocker is at the correct height? I'm no engine wizz.
But looking at the geomerty of the valve train I would have to guess that any change in the deck height or removel of mat. from the head would change the lift. So.. how do you figure out if pedestal height is where it needs to be? can it be corrected?
In thinking about this I now am wondering about my own engine. :crazy:
 
But it is very very minor for a skim so is not an issue.
 
Removal of material from the surface of the head will change the distance from the rocker arm to the lifter, but the adjustment on the rocker takes up exactly that distance. The angle of the rocker arm is determined by the seating of the valve and length of the valve stem. The lift is determined by the difference of the base of the cam lobe and the peak of the lobe. Times of course the rocker ratio. Skimming the head does nothing to this geometry except the valve adjustment. (Actually there's a very minor difference in the angle through the valve contact point - pivot center - pushrod contact point, but this has almost no actual affect on valve lift.) In extreem cases, this angle change would need correcting, thus shorter pushrods.
 
So the fact that the center lines of the cam and the rocker shaft coming closer togather and the push rod not changing length does not change the ratio? I would have thought you would lose lift or at the very least duration.with the same amount of lash. But still how do you know the correct window of height?
 
Not going into full detail of rocker geometry, the skimming or milling of the cylinder head “or” block does not change the lift “so to speak”.
What it does do is effect the ideal contact of the rocker face to valve stem end. The rocker is adjustable via the screw used to set valve clearances for a range of manufacturing variances.

If enough has been removed between the block and or the head that the adjustment screw has no more adjustment then a lot has been removed via a past life.
You would need to question a number of things at this point if it’s just a street engine.
A fix is to shim the pedestals if the rockers were not “as” close to level, sitting with both the valves closed and the proper amount of valve to rocker clearance set. A desire to keep the rocker as level as possible so that an even and centered amount of energy is kept at the center of the valve.
Now all that said, it does effect the lift in some ways but I’m staying away from that part of the geometry aspect. There are many arguments in that area and finite reasons.
"dug"
 
so in a perfect world the rocker should be "level" when the valve is closed thus keeping side loading on the stem to a minimum. correct? level from..pedestal mounting surface?
 
Keeping it simple?
Yes, if that means making mods to get it there? Yes
The end of the rocker that contacts the end of the valve is radiused at the mid point of it’s area, this is the desired point where the rocker is setting at a neutral angle to the valve when starting it’s swing. The highest point is as close to first contact as possible.
Now all this is assuming all the valves are resting as close to even (installed height) to each other as possible.
Meaning if you took a head with just the valves installed and rocker assembly not in place, a straight edge placed on top all the valves would reveal them even to each other in height. At least within 0.015 per say.
Sorry to start down this road but one thing leads to another and I’m not being very complete in description.
“dug”
 
Ok last entry on the subject.
With the info you have related to me unless there was an exteme amount of surface removed from the head and or the block there should be no concern on the pedestal heights.
but if there were, probley the the valve guides would surfer either premature wear and or freezing a valve in the guide (bending a stem)galling the guide.
 
When I saw "Dug" from Oregon... I knew immediately who that was. How are you doing? Long time no hear.

Texas, regarding your last comment, when you start shimming the pedestals... that's when you will change the geometry of the rockers relative to the tips of the valve stems. You won't introduce unusual or changed valve guide wear until you shim the pedestals.
 
Texas,
That is my opinion yes, not gospel in any way. Reading Racer Browns book, chapter on valve train geometry was a cool insight to how much I haven’t got a clue but trying to understand as I go!

Doug L.
Yep you pegged it. Busy like you, We are just starting to play with ABB arms now.
"dug"
 
Back
Top