Dennis sent a question:
Perhaps you can help me with my question. We used to road race our modified RD-350s down in Florida back in the 70s and 80s. All of the Yamaha ring sets for the RD included a thin, flexible octagonal shaped “ring expander” to fit behind the bottom ring. The hot tip was to omit the expander for racing. I’m sure I used to know Yamaha’s explanation as to the function of this “ring expander” for the street bike, but over the years I’ve forgotten. Can you help me?
My answer:
Hi Dennis,
Great question.
They went out of style about the time I got started racing. My best guess is that as metallurgy improved, expanders were no longer needed. Other factors may included better machining for better fitment between the ring and the groove and remember that as the pistons got better they collapsed the ring grooves less.
If you have more info I’d like to hear it. Maybe a good RD story too.
Kent Riches, owner of Air Tech, says on my FB page:Â “The top rings had combustion loading… the 2nd ring chattered and skipped on the cylinder wall often breaking from vibration harmonics.. the expander forced the ring against the wall and dampened vibration…”
I think Kent is more right than I.