plastic gear in tranny to hook up speedo.?

[BUXMAN66]

not too up on tranny's, but i have been told of a plastic gear of some type that is under a plate where your speedo cable hooks into your tranny. anyone know what i'm talkin' 'bout? this is on a th350.also i unhooked the cable at both ends to make sure i didn't have a dry cable tubing.it looked brand new and had plenty of lubrication.when i first got it,it used to bounce a lot.now it don't do nothin'.where can i get one of those plastic gears(if thats what it is), and how much do they run?speedo itself works fine.

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'68 swb 350/350 w/edelbrock total performance-performer pkg.ceramic headers,2.5" delta-flow 40 series flowmstrs.no rust of the notorious mid-west on this one! see my truck at free photo-i under "'68 chevy truck"