On what car or truck? And do you mean the 1/4" square one between the rear axle and the wheel hub? Mine is 1/4"
I go down to the hardware store or parts store and buy a piece of 1/4" square key stock and cut it to length with a hack saw, actually I prefer to use a Dremel tool with a cut-off wheel in my old age.
Hope this helps and saves you some frogskins.
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Lone Star Region
Chat Group Chapter member http://www.lsrclub.orgLife's a long winding trail, ride a good horse!