Yes, like that one although that is much better than the one I was thinking of. The one from JP appears to use 10-32’s which is a big upgrade and of course it has six, not four, of them. On that one a guy could safety wire the screw heads, just sayin’… but in the end it is still a two-piece clamp-on.
In class yesterday we had a lot of set-screwed on low-pro gas blocks combined with handguards whose vent holes did not line up such that a person could check / retighten the set screws. We also had several Daniel Defense rifles that come from DD with pinned GB’s.
One thing about “pinch bolts”, and why they are not the best idea.
The bore passes through the slit, when the bolt(s) are torqued down, the slit will attempt to collapse. But the top of the slit is supported by the barrel, and the two sides will angle towards each other at the bottom. The two holes are perpendicular to the sides of the slit, so now the holes are at an angle to one another.
Some people will tell you that the holes have enough radial tolerance so as not to bend the bolt, but the head will always only bear at the top, so there must be a bending moment on the bolt.
Pinch type locking is only appropriate for extremely close tolerance fits, that do not need extreme clamping loads.
By “split pin” are you referring to “tubular, slotted, spring pins”? The recommended method for installing these pins as a cross pin connection, the slot should be at 90 degrees to the locked element, see below.
This maximizes the shear area and reduces the deflection from axial loading. If you are really worried about the pin shearing or working loose, use two spring pins. A 1/8" tubular slotted spring pin can have a 5/64" pin driven in the center. In this case the slots should be 180 degrees from each other.
NOTE: The term “split pin” normally refers to a “cotter pin”.
The Mini 14 does not use the clamping force from the bolts to prevent movement like AR gas blocks. In the Mini 14 the gas port bushing locks the “gas block” to the barrel in the axial and radial directions.
For a good press fit you want a hole 0.0010" to 0.0005" under the actual pin diameter.
A little care, a 0.002" undersized hole, and a chucking reamer should give you an 1/8 inch hole with -0.0000/+0.0002 tolerances. Hardened ground dowel pins are toleranced -0.0002/+0.0000.
The problem with dowel pins is they do not “draw” the parts together like a taper pin does.
If you are going through the trouble of reaming a precision hole, make it tapered.
On top of everything else that is barely secure about this one is that 6-32’s are prone both to breakage and to coming loose, 6-32 being actually a very coarse thread. Screws 101 here, the helix angle on a 6-32 is about 4.4°. For comparison, a larger screw with fewer threads per inch, you might think that’s a coarser thread. But let’s take 1/2-28, our muzzle thread, it has a very low helix angle of about 1.3°, and so, it is much less prone to coming loose.
Any time the diameter get larger the helix angle drops.
#6-32 - 4.12°
#8-32 - 3.47°
#10-32 - 3.00°
1/2"-28 - 1.30°
1-3/16"-16 - 0.96°
4"-8 - 0.57°
As long as the angle is less than 5° to 6° the friction will lock the screw in place under static loads . . .
The big problem with 6-32 UNC screws is people over-torque them, so they stretch and loose torque.
What torque values do you recommend in inch-pounds, using loctite or rocksett?
Depends on what material you are screwing into and the material of the screw.
Generally, for a #6-32:
steel (M)/steel (F): 6-12 in-lbs
steel (M)/aluminum (F): 4-6 in-lbs
However, if a torque value is given by the manufacturer, use that.
Increasing the torque on a #6-32 screw from 10 in-lbs to 20 in-lbs doubles the tensile stress in the bolt. Going to 30 in-lbs triples it and starts to approach the yield stress of your typical hex socket, cap head screw.
Lysander, maybe you can answer this. Lubed threads vs/ dry, according to the Machinery’s Handbook, up to 30% more “clamp” can be realized if threads are lubed. Conversely it can take 30% more tq to realize the same clamp if threads are dry. I’ll check, I may have a percentage wrong but not by much.
My questions to Loctite: does LocTite constitute a “lube” when the fastener is being tq’d…? Or some fraction thereof? They would not answer that question for me. It seems to me there must be at least a slight lube quality to it before it sets. And that they would have studied this in depth and might have advice for their customers, but… I was wrong at least about the advice part.
Some excellent testing here. https://youtu.be/V1Y3aB94lwU?t=1356
Summary: new thread locker doesn’t have much effect on bolt tension but old expired stuff may result in low tension.
In an earlier test slathering anti-seize made for an eye-popping 100% increase in bolt tension. https://youtu.be/-hSmtLVESSM?t=1002
Good info. Thanks. I thought loctite had some lubricity. Wet drag vs dry friction. It is actually neutral. Beyond expiration, some polymerization may have happened so that there are micro chunks of solid suspended in the liquid, which jam the threads going in.
-TL
Lysander, I’ve been carrying around that “split pin” misnomer for decades, thanks for the correction.
Sorry, Disagree, I won’t have an AR where the Gas block is not pinned. Pinned is the most fool proof way to guarantee you AR from becoming a Single shot.