http://www.planetarypinball.com/mm5/merchant.mvc?Screen=online-parts-books Check here.

LTG : )

The parts list (aka Bill of Materials) is readily available for WPC-era Bally/Williams games on www.ipdb.org; unfortunately, no such creature seems to exist for the System 11 (and earlier) games.

As to your question, there will be two different lengths of unslotted hex-head sheet metal screws used to secure the majority of parts to the bottom of the playfield: #6x1/2" for thicker metals such as flipper assemblies, drop targets, etc., and #6x3/8" for lamp sockets having metal brackets that are screwed to the bottom of the playfield. Williams used a yellow zinc plating on the #6x3/8" variety for the WPC-era games to minimize the chance of someone using the wrong length screw (and puncturing the playfield surface). The #6x1/2" variety are also commonly used to fasten items (such as metal ball guides) to the top of the playfield.

Links:
1/2": http://www.pinballlife.com/index.php?p=product&id=225
3/8": http://www.pinballlife.com/index.php?p=product&id=224

Note that there *are* a few other variety of screws used; short Phillips head screws are sometimes used to mount the switch actuator wire at the end of the ball trough, and sometimes were also used to mount solder lugs under the playfield.

You're in a bit of a tight spot. The Pinbot manual does have some parts assembly breakdowns, the BLUE Williams parts catalog (look here: http://www.planetarypinball.com/mm5/merchant.mvc?Screen=BOOK) has more information (but unfortunately the Web link isn't working), but what you really want is a detailed assembly drawing of the playfield - and that's not available in any public form I've encountered. Obviously Williams had those since they were used to actually build the game, but I've never seen one that made it outside of the factory.

Your best bet in these cases is to post a picture of the specific area and highlight the hardware you need in the picture - usually someone can get you information from their game, or the really crazy folks with a deep knowledge of the hardware commonly used can dig that out of the parts catalog. If you have the hardware removed from the playfield include a picture alongside a ruler.

FYI, "#6-32" is an English designation for screw diameter (#6) and threads per inch (32); e.g., a #6 screw is 0.13" in diameter.

One additional point: Williams used a thread-locking compound (LocTite is one brand) on the threads of the the machine screw posts that secure the slingshot kicker posts; this can make it very difficult to remove the screws. Heating the T-nut on the underside of the playfield where that post is threaded can make it a lot easier to remove the post.

Several System 11 BOMs are available - BBBB, Radical!, and Dr. Dude.