The answer to the question I ended the last post with is yes, there were boardtrack sidecar motorcycles:

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Not-great camera work (who am I to talk?), but interesting footage regardless – looks like there’s at least one leaning-wheel rig:

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I’m including this for the drilled cylinder action. From a thread that led me to the first YouTube video, “Since the engines were four stroke anyway, they would often drill holes in the cylinders just above the top of the piston when it was at the bottom of its stroke. This improved exhaust scavenging and gave a higher top speed at the expense of hot oily legs for the rider.”

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And from the same thread, this beautiful machine (notice the brakes – that’s right – there aren’t any!!!):