Can a 3/8" blank be properly tapped for 7/16-14?

[devinsixtyseven]

This is part shop/tool, part gen 4x4, if it's inappropriate here please feel free to move it :).

7/16-14 normally takes a 23/64 bit, 7/16-20 lists 25/64.

The 3/8" hole measures out damn near 0.375"...maybe 0.380 on the dial calipers, hard to tell exactly on the dial but it barely passes the shank of a 3/8" bolt. The hole was machine cut, not hand cut, no wallowing. It's a very clean hole.

The material to be threaded is 3/8" thick mild steel. If I run a 7/16-14 tap through these 3/8" blanks, will the threads hold properly? They are holding the diff to the housing. Can I run a 7/16-20 tap through, if I'm careful? Is the fine thread bolt even a good idea in this application?

Next problem is even if this works I'll have to enlarge the holes in the 3rd member. Is it even possible to accurately run a 7/16" drill, by hand, through a 3/8" existing hole in 1/2" thick nodular iron, or will the bit grab & shatter if it miraculously doesn't wallow? (I can easily protect the r/p from shavings)

Uhhhh...help :(! I'd rather make this work than send anything back (probably an honest mistake on someone's part, or I'm seriously missing something), and I'm guessing I'm only limited by my ability to drill straight holes by hand.

Is this correct...will this plan work? Leave the holes in the (mild steel) housing as-is, tap to 7/16-20, enlarge the holes in the (nodular) diff to clear 7/16 studs, or bolts if necessary...can it be done by hand or will I be breaking the tap (or losing strength in the threaded interface) and losing necessary accuracy in the hole centers on the diff?

Thanks very much,
Sean

[Gozuki]

Holding the diff to the housing? They aren't the same? Do you mean diff cover? Tap drill size for the fine thread is .390 so your good there. It would be less engagement with the NC. I wouldn't try handheld if the holes are important.

[devinsixtyseven]

*(edit...Actually I should have posted this in Newbie :rolleyes: because I was just doing it plain wrong :shaking:. Glad I called around before I cut anything.)

[CoryHinsz]

I would go with the 7/16-20 because you know you will have enough thread to work with. You might be able to get away with the 7/16-14, but since it does not have a high shear load fine thread should be ok. As far as the drilling and tapping go, just use a lubricant and keep it straight. As you progress into the hole, the previous hole will act as a guide and pull you through. Don't forget to back out your tap every once in a while.