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Quote:
Originally Posted by ryanheacox
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I vote for this. Take a look at the size of the drill. You may find it's "big" by a small amount and will, technically, drill beyond the root diameter of the thread.
Threads in the block are 7/16-14NC. Thread root diameter is 0.3499. Pilot drill size for 75% thread (aluminum, brass, plastic) is No. U or 0.3680. Pilot drill size for 50% thread (steel, stainless, iron) is 25/64 or 0.3906.
The reason the tap drill is larger than the thread root diameter is that it is VERY difficult to tap 100 percent to the thread form. Lots of broken taps have occurred until machinists and engineers settled on the "percent threadform" guidelines above.
Using this kit you use the head as your guide. There is some "free play" around each stud so the head technically can move around. However, given all the studs and the machining accuracies involved, the head tends to "center" on an average - and this average is where the bushing is centered to. And hopefully centered above your broken stud.
I would expect the drill size to be the 50% size above. It takes four threads to develop full strength in a bolted connection, and given there is typically 11 threads on a head stud into the block, you can give up a little bit of "root" to one side or the other and still develop full strength.
I would not rush into driving a 7/16-14 tap into that drilled hole. It is very easy to "cross thread" where the thread you're cutting is cast iron - which is quite soft compared to the remaining bolt thread.
Rather, I would "pick out" as much of the steel spiral as you can to try to establish a "line" for the tap to cut into, and use that.
Another modus I might try would be to use the kit - but instead get a LEFT HANDED drill of the proper size which, by the heat and torque of drilling, might back out the stud without further ado, once a good portion of the hole is made. I like that thought the best although left handed drills can be pricey.
https://www.amazon.com/Left-Hand-Hea.../dp/B002FCK65U
Hope this helps, been there done that.
Joe K