For the rear mount (I have 2 different sizes on hand). Measure the down tube (diameter) and get something as close as it as you can. You can use steel water pipe, conduit, PVC pipe, copper tubing, a broomstick etc. Even a socket from a wrench set. Get a couple of sheets of course emery sandpaper, wrap pipe and sand the mount. You want the mount to tube as perfect as it can be.
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The mount on the left (right photo) is too sloppy. You can see the shim I made for a over sized mount.
As far as the front mount, there are a number of choices. Store bought, or make yourself. When I built my first bike it was before I joined either bike forum, this is the mount I came up with in short order.
..Install rear mount first, determine position of engine on down tube then fit the front mount to the frame.
Here are different types of mount shims that can be very easily made. They go between the mount and engine.
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Not my idea.
