how thick

[Big Kahunaa]

how thick are stuffer plates

[Duane HTP]

If set up properly, they will be a different thickness for about each pump.

[Big Kahunaa]

the plate is a 1/4 thick now so i may have to turn it down

[flat broke]

To elaborate on Duane's post, they are all different, because the stuffer is meant to take up the space between the back of the impeller and the front of the bowl where the vanes start. Because of different mfg tolerances on impellers and bowls, this distance will vary, and as such you'll need to custom fit the stuffer for your specific bag of parts.
hope that helps,
Chris

[Squirtin Thunder]

You can come by and look at mine this week !!!