Bringing back memories kirrbby. Had to go and dig thru my pile O'parts. Back in 71 I don't think many "performance" parts were available and if they were I didn't have money for them. If I did have money I'd just get a bigger bike anyway! A friend gave me a S65 motor with the intent of putting in the Z50. Existing manifold was quite different shape so I figured out to make a spacer much the same as yours except we did not have the experience,tools or materials you have today So we made it out of wood :9: and coated the inside with some sort of epoxy so the fuel would not soak in. Honda had the same problem with carb icing so they plugged the oil feed at the case to cylinder junction ran an oil feed line from a boss on the side case thru the carb and then back to a boss drilled into the cylinder so the oil could continue up to the head.In these pic's you can see the oil feed connection and chamber cover on the carb which by the way is a 19mm unit. You can use a S50 (very rare) or S65 clutch cover on a Z or CT cases by just plugging the oil outlet.
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Here is the plugged oil outlet on the clutch cover-
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Here is the return from the carb to feed the upper end-
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Now you know why every early cylinder has that boss cast into it,the S50/65's were the first of the long line of the OHC engines we know today and were first offered here in 1965
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Keep in mind the year but my Z with this engine,which was rated 6.22ps @10,000 was pretty quick. In this time frame most other kids had lawn mower engined bikes,here I show up with my bike which still to the untrained kid eye has not changed. Unfortunately I have no pictures of it being on.
Sorry for the lecture,class dismissed