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I've seen this kind of arm on 5th valve trigger setups.

Rrecently, I converted an old Alexander which had been set up with string linkages to uniball. My choice was to rotate the stop blocks 180 degrees (they had already been so rotated to convert to string linkage) or to operate the stop arms from the side opposite the stop pin. I brazed a short length of 9/16" brass rod to each arm, and drilled and tapped for the ball links. Worked like a charm.

I suppose one could argue that such a setup is better balanced or it's easier on the bearings, but in reality, I suspect it doesn't make a big difference.

I can see a very good advantage to placing the stop pin farther out on the arm.... it gives you better control over the stop point.

TubaTinker wrote:I can see a very good advantage to placing the stop pin farther out on the arm.... it gives you better control over the stop point.

I've wondered why stops aren't placed on the spatula arm rather than on the rotor stop arm. Since the spatula arm has a rather long travel, it would seem to make for a more precise adjustment.

Chuck(G) wrote: I've wondered why stops aren't placed on the spatula arm rather than on the rotor stop arm. Since the spatula arm has a rather long travel, it would seem to make for a more precise adjustment.

Yes, but after you've precisely controlled the paddle, you'd have a minimum of two more joints, each of which might have some play. The overall precision -- or at least the overall consistency -- of valve travel would decrease.

Looking at the drawing, I wonder why the stop pin can't be on the same or longer radius as the link, but rotated a few degrees away from the paddle. You wouldn't be able to use the existing mounting holes for the stop plate, but so what? You could have one lobed stop arm, with the stop arm and link close together (the flex of a longer arm, even one that turns about the center, might be the reason the two-ended stop arm was abandoned). Both stop arm and pin could be at any radius - heck, could even be at ADJUSTABLE radii!
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Joe Baker, who plays pistons....

Joe Baker wrote: Yes, but after you've precisely controlled the paddle, you'd have a minimum of two more joints, each of which might have some play. The overall precision -- or at least the overall consistency -- of valve travel would decrease.

That might have been true in the days of S-arms, but with the uniball joints, I doubt that there is any significant play.

Of course, if one wanted absolute accuracy, we'd have rack-and-pinion linkages...