I was at a BMW rally a couple of summers ago and there was a guy there on a Royal Enfield 500CC Diesel. Single cylinder and he said it produced 6HP and got about 90mpg. He said he rode it to Colorado from the east coast over the coarse of several weeks at a cruising speed of 45mph. Sounded like it was a POS. He was constantly having to repair and cobble together broken pieces. It was definitely not an adventure bike (I suppose having to ride cross country on an unreliable bike could be construed as an adventure) but interesting nonetheless. The Enfields are being built again in India using the same tooling as the original British Enfields from the '50s but I've heard that the metallurgy isn't nearly as good (I don't think it was all that good to begin with

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