Maintenance

[John Vardanian]

HI Jim, thanks. Would you be able to elaborate on the valve geometry and valve guide side wall scrub issues as you described? Not being skeptical or critical, just trying to learn. Thanks.

john

[Jim Wickstead]

Hi John,

I’ll try to describe this as best I can. Tom may have some additional comments to help this along.

The problem with valve train wear is caused by two things: (1) Wear of the valve tip and adjustment screw resulting from compression loads (i.e.: the hammering of the valve adjustment screw on the tip of the valve and (2) adjustment screw, valve tip and valve guide wear caused by the valve stem being “pulled” inward by the rocker arm/adjustment screw assembly as it moves downward and inward during valve opening. This is a result of this particular valve train design and its geometry (which results from use of a rocker arm).

You can visualize this (and the theoretical “sweet spot” in the valve train geometry) if you draw a large cross on paper. One line faces upward and the other is drawn horizontal (i.e.: 900 from the vertical line). Draw an arc originating from the center of the cross and which goes through the horizontal line. Measure the distance between the centers of the arc to the point where it intersects the horizontal line. Draw 2 horizontal lines equidistant above and below that horizontal line. Now measure the distance from the center vertical line to the point where the new horizontal line intersects the arc. Note that it’s less than the distance measured to the center horizontal line. This difference in horizontal distance is the drag I was referring to.

Think of the rocker arm rotating on that center point. The camshaft presses upward on the rocker arm roller at one end pivoting the other end (with adjustment screw) downward against the tip of the valve. As the adjustment screw presses against the valve tip, its arc of motion causes it to move inward, dragging it across the tip of the valve. The valve rides in a valve guide, and this dragging puts additional side loading on the inside of the valve guide which is softer than the valve shaft) causing the guide to wear. (In addition, this side motion causes added stress on the valve adjustment screw and the tip of the valve).

Looking at the arc again, you notice that if two new horizontal lines are drawn, equally above and below the center horizontal line, the amount of vertical distance (valve lift) is divided equally and the result is minimum side motion. That is the theoretical “sweet spot” that Ferrari engineers tried to obtain.

Gads, this took a lot of words and it’s awfully simplistic….. I should have just drawn a picture, which I will do if this isn’t clear.

Jim

[mark]

Jim:

As one designer to another, drawings have been superior to arm waving and words since the advent of orthographic drawing techniques. Nevertheless, I enjoyed your soliloquy on valve train geometry. In my office I have a saying that my employees probably hate hearing: Looking is not the same as seeing.

Cheers

Mark

[John Vardanian]

Thanks Jim, so if I am getting the picture here, the tip of the screw slides left-right on the head of the valve as the valve is pressed down. The elephant foot by its nature doesn't allow L-R sliding so a greater uneven friction occurs between the valve stem and the guide. Is that it?

john

[Jim Wickstead]

Hi John,

You have nailed it in just one sentence. Next time, I'll take Mark's advice.

The Rolofs system continues to intrigue. I think their use of a smaller diameter cam roller is actually needed to bring the elephant foot geometry into line. This offsets the extra length of the elephant foot screw assembly.

Having seen the consequences of a spun cam roller bushing, I am receptive to use of needle bearings.

A smaller diameter roller also means a lighter roller which possibly offsets the extra weight of the elephant foot screw assembly. It also brings it closer to balance. I wonder if this reduction in reciprocating mass will increase the RPM limit?

Still, my concern is the uncontrolled lubrication inside of the valve cover and the consequences of an elephant foot ball & socket going without.

Jim

[330GT]

Lowell documented the work he did on his rear suspension and I edited it into a web page. This might be useful as it is more of the step by step process needed to rebuild the rear suspension on an early Ferrari.

See http://www.parrotbyte.com/kbc/ferrari/R ... ension.htm

[John Vardanian]

Great work, Lowell. Thanks.

john