Question 1

Will these drill & tap 1/4" thick cast iron?

Accepted Answer

Thanks for your question. 
The max depth of drilling and tapping for this product is 4mm. 1/4" equals to 6.35mm. So it's too thick. 
This kind of combination drill & tap bits are only suitable for thin metal sheets, no matter what the brand is.
In addition, it's suggested that you use M2 HSS bits or M35 cobalt bits to drill cast iron.

Question 2

What’s the three count example 1/4 what 20 - 28 kinda important

Accepted Answer

Thanks for your question. 
Take BSW 1/4-20 as an example. "BSW" means British Standard Whitworth screw thread series. "1/4" is the inch diameter of the bit. "20" means 20 threads per inch.
Try to search the keyword "BSW size" on Google or Bing and you'll learn more.
Actually, we don't quite understand your question. If the answer isn't what you need, please reply to us.

Question 3

Will it go through steal?

Accepted Answer

This set of tap drill bits are suitable for wood, plank, plastic and soft metal like aluminum plate and sheet iron. 
We also have TiAIN Finish HSS-Co M35 Drill and Tap Combination Bits for sale. They can penetrate stainless steel. 
M10 tap bit, http://www.amazon.com/dp/B07D6K27V7 
M8 tap bit, http://www.amazon.com/dp/B07D6JNVQ1 
M6 tap bit, http://www.amazon.com/dp/B07D6K75QW 
M5 tap bit, http://www.amazon.com/dp/B07D6HWXMC 
M4 tap bit, http://www.amazon.com/dp/B07D6JK5G4 
M3 tap bit, http://www.amazon.com/dp/B07D6JTG57

Question 4

Is there 1/2"? its all mm? no SAE?

Accepted Answer

Sorry, there isn't 1/2". 
SAE bits are available here: https://www.amazon.com/gp/product/B077JDZF28
Fractional inch bits are available here: https://www.amazon.com/gp/product/B072LYMS6P

ASIN	B07D6JTG57
Best Sellers Rank	#156,639 in Tools & Home Improvement ( See Top 100 in Tools & Home Improvement ) #149 in Countersink Drill Bits
Brand Name	HYMNORQ
Customer Reviews	4.2 4.2 out of 5 stars (640)
Cutting Angle String	118 Degrees
Cutting Diameter	3 Millimeters
Finish Types	Titanium Aluminum Nitride
Included Components	1 piece of M8 TIAIN coated cobalt combination drill and tap bit
Manufacturer	Hymnorq
Material Type	Cobalt Steel
Number of Flutes	2
Point Style	Split Point
Shank Type	Hex
Surface Recommendation	Stainless steel, angle steel, aluminum, mild steel, brass, copper, plastics, cast iron
Tool Flute Type	Spiral
UPC	666389336378
Unit Count	1 Count

Migiwata Metric M3 x 0.5 Black TiAIN Finish HSS-Co M35 Drill and Tap Combination Bit with 1/4” Hex Shank and Self-centering Split Point for Aluminum Iron and Stainless Steel

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