It's a 1976 or 77' Works Yamaha OW 26 motor. I believe it to be the 400cc. Sand Cast magnesium cases, ignition cover and clutch cover. Motor is in good shape. I haven't ran the motor but it turns over and is not stuck. The ring is pinched and the cylinder has very little wear.
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The cases are stamped OW 26-E-XXX. The head and cylinder is in great shape with no cracks or bent fins. Has titanium cylinder studs and and hollow counter shaft.