Here is how I converted a set of 33 rods to accept ARP 3/8 UNF bolts.
Im not going into details of basic workshop practice here, if you don't have any experience doing this kind of thing I wouldn't recommend trying this.
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10mm Chucking Reamer
"Q" Chucking Reamer
3/8 UNF tap set (Only the starting and bottoming tap are needed)
Tap with a ground point for locating taps. (or anything with a ground point)
Set of ARP 2000 1.5" 3/8 UNF Rod bolts, 1.75" might be better but Ill get into that later.
Cut down original rod bolt for locating rod. Uncut bolt for reference.
Milling machine, if you have a quality drill press and vice you could use that if you were careful.
Screw in the cut down bolt
Tighten the bolt into the chuck, making sure its on center.
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Wind the rod into the vice making sure it isn't being pushed forward or backwards, clamp down on the rod, I used a dial gauge to check the rod isn't being pushed in either direction when tightening the vice.
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Tighten the beds on the mill, undo the the chuck and remove the cut down original bolt.
Tighten the "Q" reamer in the chuck and ream the original thread out. A small amount of thread will be left.
Remove the reamer and tighten the pointed tap into the chuck. Use the point to keep the tap vertical as you cut the tread. You "chase" the tap as the thread is cut, repeat for the bottoming tap. I found there was no need to use the intermediate tap.
More to follow...