Bang
08-15-2015, 04:15 PM
12-32 so limits gunsmithing work, such as bedding a Mark II into an aftermarket stock, an alternative is worth considering.
1/4" bolts can be had in 20, 24, 28 and 32 tpi as well as taps and dies for them.
Guns call for finer threads where possible. The 1/4-32 thread is common in diesel glow plugs. I would opt for the 28 tpi for more availability in bolt selections and taps and dies, while retaining the finer threads.
The 12-32 threads measure .221" outside diameter. Remember, most thread relationships are 75% thread engagement. Drilling out existing 12-32 hole with #3 or a 7/32" drill leaves close to perfect metal remaining to tap to 1/4-28. The 1/4" bolt is .250" diameter and anything that it would pass through, such as the bottom plate, would need to be drilled out to accommodate it. .221 from .250 is (call it) .030" or a mere .015" difference on the radius...not a large step up.
Drilling out an existing hole has the advantage of the original bore guiding the over-sized drill. A decent attempt to remain square to the hole is always a plus in final results. For extra assurance of the new hole being correctly done, select drills in ascending order, removing a small amount of the existing threads in each increment.
Two taps may be needed...the standard and a bottoming tap. Standard tap first, bottoming tap finishing the threads to the bottom of the hole.
Just offering an alternative to the extremely proprietary bolts. :rockon:
1/4" bolts can be had in 20, 24, 28 and 32 tpi as well as taps and dies for them.
Guns call for finer threads where possible. The 1/4-32 thread is common in diesel glow plugs. I would opt for the 28 tpi for more availability in bolt selections and taps and dies, while retaining the finer threads.
The 12-32 threads measure .221" outside diameter. Remember, most thread relationships are 75% thread engagement. Drilling out existing 12-32 hole with #3 or a 7/32" drill leaves close to perfect metal remaining to tap to 1/4-28. The 1/4" bolt is .250" diameter and anything that it would pass through, such as the bottom plate, would need to be drilled out to accommodate it. .221 from .250 is (call it) .030" or a mere .015" difference on the radius...not a large step up.
Drilling out an existing hole has the advantage of the original bore guiding the over-sized drill. A decent attempt to remain square to the hole is always a plus in final results. For extra assurance of the new hole being correctly done, select drills in ascending order, removing a small amount of the existing threads in each increment.
Two taps may be needed...the standard and a bottoming tap. Standard tap first, bottoming tap finishing the threads to the bottom of the hole.
Just offering an alternative to the extremely proprietary bolts. :rockon: