This is an interesting read. Im in WA and my SB45 14" has one of these and so does my fathers 17". Both are pre 1980 models and I have always thought it was a standard part. I know my local mower shop sells them because a few years ago I lost it when putting a new clutch pad in.

I haven't removed it to see if it causes a vibration.

Maybe WA was supplied all the dodgy out of balanced ones. lol


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