rocker offset from valve

Joined
Oct 28, 2014
Messages
1,829
Country flag
I have an Atlas head, just built, and I notice the left exhaust rocker strikes the valve off center. It is possible that the valve guide hole was bored off center but I would like to compensate and get it to hit right in the middle. I ground a bit off the rocker so it would sit a bit more outboard and shimmed to suit. All is well until I tried to install the head and found the pushrod was not going to engage. Apparently the pushrod has very little room for variation. So I chucked the rocker and installed one of my spares. While doing so I noticed the rather clumsy drilling of the rockers for the adjuster. It looks like it was either drilled with a bias towards inboard or the casting has extra metal on the outboard side. What I am wishing for is that if I had 10 rockers sitting on the bench, no two would be drilled in exactly the same position...sort of like finding identical snowflakes. At this point I would find one that was biased towards the outboard side and my problem would be solved. Am I dreaming?
 
I would be surprised if the rocker machining was off by that amount. I would have thought the relationship between the holes and faces would be accurate, and what you’re seeing is excess casting metal or material ‘shift’.

When a casting or forging is completely blank there are no datum points. The first machining op is called ‘spotting’ where some datum points are machined into the raw casting and these become the datum points for subsequent machining Ops. Until this op, the part is just some random material in ‘space’ with no datums.

So, you’ve kinda got two things in play, a series of machining ops corresponding to each other, and the location of the base material ‘in space’. And the two things have to be overlaid onto each other. Thus you can have a situation where all machining is accurate, but the base material is slightly biased in one direction or the other.

Hope that waffle helps, it’s a difficult thing to explain in written word alone !
 
I agree that it is very unlikely that the drilling is that far off. I think the casting was made with generous room so the machinist couldn't miss. I guess these were all handmade in the 1960s.....the parts would be set up in a jig and drilled one by one.
 
by the way, two of my spindles came loose when heated SLIGHTLY. One drifted inward and was turning. I cut a piece of aluminum rod .080 and put it in the inboard spindle pocket. Next, I took the spindle plate and with a very small punch pushed the two little tabs further out.....stock, they barely clear the gasket. Now the rocker spindle is positively located will not turn.
 
I would be surprised if the rocker machining was off by that amount. I would have thought the relationship between the holes and faces would be accurate, and what you’re seeing is excess casting metal or material ‘shift’.

When a casting or forging is completely blank there are no datum points. The first machining op is called ‘spotting’ where some datum points are machined into the raw casting and these become the datum points for subsequent machining Ops. Until this op, the part is just some random material in ‘space’ with no datums.

So, you’ve kinda got two things in play, a series of machining ops corresponding to each other, and the location of the base material ‘in space’. And the two things have to be overlaid onto each other. Thus you can have a situation where all machining is accurate, but the base material is slightly biased in one direction or the other.

Hope that waffle helps, it’s a difficult thing to explain in written word alone !
Realtime Victoria engineering!
 
Does the "new" rocker hit in the valve stem center? Push rod fit ok?

I've got a rocker or two with pretty eccentric adjuster drilling. Gives one confidence when slimming them down!
 
The pushrod fits as normal now. The rocker is inboard of the valve stem center. I have to live with it. Either the datum points for the valve guide are off or the datum points for the rocker are off. When this head needs service again it will be sidelined and one of my spares will be used.
 
Back
Top