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Can cast rocker arms be "REBENT"

chuck hahn

chuck hahn

Forum LongTimer
Past Site Supporter
I dont have the head right here to take any pics, but basically heres what we got. 1973 Yamaha TX750. ALL the holes for the adjuster screws line up perfectly with the center of the corresponding valve stems ..except the left cylinder intake rocker and valve stem. The hole in the rocker arm that the adjuster goes in is offset to the left of center ( in relation to the center of the valve stem ) by about 2 1/2 MM. So what is happening is that the tip of the adjustment screw, instead of striking dead center on the valve stem, is actually hitting on the left 1/3 portion of the stem..follow me? The valve stems are 8MM in diameter and the "witness marks" left from the old adjuster striking it , are at 2MM in from the edge of the stem.

So my question is this..can the rocker arm be heated and tweeked to the right so that the hole will then align directly over center of the valve stem? And to answer the question is it bent now..No it is exactly the same shape and angle as all the others. It litterally looks like that cylinder valve seat is just put in the wrong place.


Second question is this..can the hole be welded shut on the adjuster end and that foot area be enlarged and then just drill another hole and thread it for the adjuster screw? OR, can the end of that cylinders adjustment screw be welded and enlarged and still be hard enough to take on the constant pressing of the valve?? They have what are called "elephant foot adjusters" but they have the allen head setup and not like the ones on the TX with the square top that a little wrench will fit on.
 
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Chuck,

I'd look for a new/used rocker arm. Cast iron is brittle so you are at risk of cracking if you attempt to bend it. Not sure about welding and retapping, but doesn't seem likely.
 
2 1/2mm off centre seems a lot if all the others are dead centre. without pics its hard to invisage. are there any shims or springs on the rocker shaft that centres the rocker arm or is it possible that the arm could actually be bent?
 
I dont have the head right here..its at the shop Randy works at on his bench. I probably wouldnt be able to get pics for a day or so. Bothm myself and Randy installed the rockers and even switched the left rocker ( which was dead center to the valve stem ) for the left one ( not centered) and got the same results.. The left rocker..now on them right side..lined up pefectely and the right one..now on the left was the same distance off center. Its NOT the rockers being bent or anything els. The valve guide for the left cylinder is inboard farther than the right one is and the 2 simply do not line up correctly. I will try to get up and take the head and get some pics of whats happening soon as i can.

Dale..they look like cast metal rockers to me. Kinda rough bumpy exterior finish to them..not semi smooth like one would expect from a steel forged application.
 
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Agemax...In the above post i answered that we swithed the left and right side rocker arms around and duplicated the same results..the "bad"left one now was perfect on the right side and vice versa. There are no spacers missing or used in the application. The cam shaft towers are milled just wide enough to accept the hinge of the rocker on the shaft, but you question sparked an idea.

Wayne, the shop owner, has a Bridgeport which i am skilled in some basic machining skills with one. Suppose i took an end mill and removed the needed amount of aluminum from the rocker arm holder on that side to effectively shift the rocker to the right 2MM...then we machine an aluminum spacer to put on the left side of the rocker arm to take up the induced slop from machining the carrier a bit? Mill, shift, and reshim it on the left side of the rocker arm itself?
 
Possibly thats the case Tom. What do you think of my above idea of machining the rocker carrier so we can shift it over and making a shim for the left side of the rocker body to take up the difference???
 
chuck hahn said:
Suppose i took an end mill and removed the needed amount of aluminum from the rocker arm holder on that side to effectively shift the rocker to the right 2MM...then we machine an aluminum spacer to put on the left side of the rocker arm to take up the induced slop from machining the carrier a bit? Mill, shift, and reshim it on the left side of the rocker arm itself?
Click to expand...

You are onto something, but I would use steel for the spacer.
 
These are the only pics I took of the beat up stem and the ruined adjuster as compared to an undamaged one. Note the witness mark from the beat up adjuster hitting so far off center of the valve itself. I will search photobucket to see if i have a pic of the cam / rocker towers.



002-22.jpg

006-15.jpg
 
In this picture, you can see where the cam is setting in the center and the raised parts of the cam blocks with the slots in them is where the rocker sets in on the shaft. These are machined to JUST allow the rocker to slip in, insert the shafts, and thus they are "supposed to" allign the adjuster screws obove the valve stem..This area is where i had the idea to machine it wider so the rocker can be repositioned and add a spacer to reshim it in place once relocated.


007-11.jpg
 
The rocker won't be cast iron for a start - it'll be forged steel.
Don't bend it though, an easier way is to simply machine the required amount off the side it has go toward and shim the opposite end.
the loss of bearing area is negligible and will make absolutely no difference to longevity.
It's much easier to machine the rocker than the head. Simply push it onto a mandrel - about two thou interference fit on a piece of mild steel - and machine in a lathe.
And yes, a steel shim is preferred - doesn't have to be hardened either.

This sort of correction to rockers is quite often encountered when building SOHC car motors.
 
You 'can' heat and bend forged steel. We do it all the time to pitman arms when to adjust bump steer.
 
I will have to sit down with Randy and Wayne and brain think this possibilty thru with the cam towers and valve inserted in the head so we can mock up a "shifting" of the rocker to the right just a tad. I am gonna bet this problem effectively resolved because it make absoultely no sense to get the new valves and adjusters so that they will be subjected to the same conditions that caused this failure. Its apart now, so it has to be resolved somehow.
 
Jeep..its not a matter of needing any parts..its a matter of getting things aligned properly. I can buy one of those cam blocks and it may or may not be right. If I can machine and shim ot over myself then I can be 100% sure and satisfied that it is gonne mesh things the way they are supposed to be. The left intake was the only real damaged thing in the top end because of this misaligned adjuster.
 
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