Screws

• Material
  • Screw Selction
  • Tool Selection
    • Drill Sizes
    • Tap Sizes
• Preparation
  • Drilling
  • Starting the Threads
• Tapping
• Finishing
• Trigger Guard

The screw assembly method will require threading the existing rivet holes and trimming the screws to the correct depth.  This is much more shallow than you would expect and probably only 1/4" deep at the most shallow point.

Material

Screw Selection

You can use either 10/32" or 1/4" machine screws.  The typical machine screw has an allen hex-head and is fastened with an allen wrench.  If you choose 1/4" screw you can select either coarse thread -20 or fine thread -28.  Fine thread is preferred since there is more thread surface in the same amount of space compared to the coarse thread.  In the long-run it may not matter all that much.

I found the screws I needed at a local hardware store.  The holes will be shallow... recommend getting 3/8" long screws so you'll have room to work.  You'll probably still have to dremel them down to make them shorter. 

Tool Selection 

You'll need a hand drill, a set of drills, and an appropriate tap set for the screw holes.  It is recommended to get "bottoming" taps in addition to a standard "taper" tap.  Thread size depends on the hole size, so you *should* need only 10-32 machine screws, hex head, but then again you may need 1/4" screws.  *IF* you strip out the 10-32 threads (or if you break the 10-32 tap in the hole) you'll need to go to the next size, 1/4"-28 (or -20).

Drill Sizes

Since the trunion holes are already drilled, should not have to follow-up with a drill.  If you find the hole is out-of-round due to the drilling from the rivet removal, and decide to follow-up with a drill, the mcmaster.com website indicates the following drill sizes are needed for the taps:

Tap Sizes

If you don't get bottoming taps, and you get all the holes started, you could grind off the top of the taper taps to make them act like bottoming taps. Keep in mind that tap won't be very useful after you do that, so proceed with caution... sucks to grind it down then find you still needed it... Ask me how I know.  :)

Standard "taper" tap:

Bottoming tap:

A taper tap is required for starting the threads.  You should use the bottoming tap to finish.  There is a third kind of tap, a "plug" tap, that is in-between a taper and bottoming tap.  A plug tap may be easier to use for such short holes.  I used a taper and bottoming tap.  My plug tap had a point, like a center punch, that I found I had to remove with a dremel to get it to go into the hole correctly.

Preparation

To prepare the trunions for use with screws, remove the existing rivet material (if using a parts kit as the "donor") and tap threads into the trunions.  

Drilling

You can leave the barrel in the receiver, and CAREFULLY drill out the rivet material using the step-cut method to drill the rivet from the center-out.  Take care to keep the drill centered so as not to cut into the trunion which will require a larger tap sized, and to not drill too deeply.  If you drill into the barrel, determine how deeply you have drilled, and if it was drilled more than surface deep, say a few millimeters, you should consider replacing the barrel.   Note the barrel pin puts a relatively deep notch into the barrel, but don't press your luck and drill too deeply into this critical section of the barrel.

Once drilled out, you will find the bottom of the trunion rivets won't be fully removable.  You may simply leave the rivet material alone, because it won't hurt anything.  If it is going to keep you awake at night, you can try to remove the rivet material by making a "chisel" from a small screwdriver by sharpening the tip, then "chipping" out the material.  This is not necessary since the rivet material will not impede the screws.

Starting the Threads

Beginning the thread is the most difficult part.  If you don't have a tap wrench, you can align the tap by chucking it into a drill press, and clamp the trunion onto the table, thereby allowing you to center and square the hole with the tap.  Hand-crank the tap down until it starts to bite, then remove from the chuck and complete with a tap wrench.  Tapping without a tap wrench is difficult at best, since using vise grips or a normal wrench will almost always result in off-center holes.  Be careful, go slow, and you may be able to do it without a tap wrench... good luck!

Tapping

Take care to tap the holes SLOWLY.  Trunions are made of very hard metal, and may even dull your taps!  Use liberal amounts of cutting oil to avoid seizing the tap and breaking it off in the hole.  The secret to good tapping is to remember the tap is not a screw!  

The best tapping method I have found is to go SLOWLY.  Tap 1/4 turn, back out 1/4 turn to break off the chips, tap another 1/4 turn, etc.  Repeat this process until the hole is completed.  Yes, this will take a LONG time!  Have patience, because if you do it correctly then you will have to do this only ONCE.  Your taps should be OK so long as you treat them like TAPS and not like SCREWS. Use lots of lube, go slow, and back out the tap every turn to break off the chips. If you try cranking down on it and don't let off, it could bind and break.

When you get the hole completed with the normal (taper) tap, the trunion holes are so shallow that you will probably want to follow-up with the BOTTOMING tap.  This will ensure that the hole is threaded to the very bottom (thus the name), giving the most "bite" to the threads.  This is important since the holes are so shallow.  

Finishing

Don't over-tighten and round out the end of the hex head, otherwise you won't get good leverage on the remaining bolts, and will have to grind the allen wrench down some to get good "bite" in the head.

Red Loctite is recommended to finsih.  Degrease the holes with something like brake cleaner before using loctite to assure the loctite properly adheres.  Oil will penetrate over time, and this area is not subject to corrosion so it really shouldn't matter.

Trigger Guard

The heads on 10-32 machine screws are too wide to clear the trigger guard. You can fix this by chucking the screw in a drill and spinning it against a file, or just buy 8-32 screws.  Here's a pic of my 10-32 screws after being filed down:

Tap the bottom of the receiver to finish assembly.  You can add a nut to the lower rear locations on the trunnion if you don't trust the 3 or 4 threads that you will wind up with. If the nut sticks out into the magwell, you may have to Dremel a little to thin it down.  Some make a small threaded plate to go inside the receiver for the threads to screw on to, thereby eliminating the need to use nuts for the screws.  I managed to thread the safety stop and receiver to get enough threads for the screws to connect:

Note that if the screws are too long they will interfere with the hammer, so you may have to trim them down some after fitting the hammer.  As with the trunion screws, apply loctite after completion.