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

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.

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

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.

Will it go through steal?

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

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

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

Brand	HYMNORQ
Cutting Angle String	118 Degrees
Cutting Diameter	3 Millimeters
Finish Type	Titanium Aluminum Nitride
Material	Cobalt Steel
Shank Type	Hex

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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