what size are the TB screws that go into the bottom of the TB, that hold on the bottom plate.

 

The cheese head Torx screws holding the water passage plate to the bottom of the TB are M4-0.75 x 22mm long. The stock screws had a T-20 Torx recess, but any kind of replacement is acceptable if you install them properly.

I must have removed about a hundred of these that have been broken off. A multiple applications of good penetrating oil (such as Kroil or B'Laster-PB) and a little localized heat might help you if you have any stuck screws. I've actually had good luck by using a high-wattage solder gun (Weller 260W) to heat the screw heads thoroughly without damaging anything else, but a small-tipped propane torch or very fine (000) brazing tip and smallest stable flame possible can be used.

Due to the dissimilarity of metals, and considering that the temperature varies widely and that coolant may seep into the screw threads, it is imperative that you apply anti-seize compound to the screws before threading them in. It is a VERY good idea to run the screws into and out of the holes several times and re-apply compound before final tightening. You only get one chance at assembly and any hopes of removing them in the future are helped greatly by the compound.

 

No coolant running through my TB.

It's a new Holley TB and a new bottom plate. thanks for the specs on the screws.