M14/M1A Glass bedding tutorial and additional info **Updated 08/03/09**
Folks, allow me to begin by saying I'm not convinced a single torque screw rifle is better and can wind up worse than having a rear lug with no torque screw. Having said that, I've also gotten questions about how to do a pillar bedding job with a single torque screw. If you do it this way, you will avoid many of the pitfalls common to using a single torque screw.
First, you must bed the whole upper receiver and rear lug together at the same time (or at least skim glass both together after you glass them separately and that's with the receiver and lug glass areas loose enough they will balance after skim glassing). This balances the whole upper glass job so you don't wind up putting unequal stress either on the receiver or rear lug. Unequal tension on the receiver and rear lug will definitely screw up the possible accuracy of this system.
We don't care what the pillar going through the stock will look like as no one is every going to see it unless you saw the stock in half. I submit you want as large of a pillar as you can get and don't mind if it isn't perfectly the same shape all the way through the stock.
After I have inlet the front of the receiver and the rear lug, I drill from BOTH the top and the bottom of the stock to get room for the pillar for the torque screw. What I do is mark the position of the torque screw hole with masking tape on the side of the stock. That allows me to better align the drill bits to bore into the stock from both the bottom and top of the stock. I then lay the largest size drill bit I can get inside the inlet for the rear of the trigger housing on the outside of the stock and going down towards the bedding area for the rear of the trigger housing. That gives me a good idea on where to begin drilling from the bottom and the angle that has to be drilled. I start with a 1/4" drill bit from both the top and the bottom and only drill a little more than half way into the stock. from either the top or bottom. As you go up in sizes of drill bits, the holes from the bottom of the stock will eventually break through into the top. When that happens, I ONLY drill from the bottom from that point on until I drill through to the top with a 1/2" bit.
You can use a drill as large as close to 5/8" from the bottom, but you risk damaging the wood on the sides of the stock around the inlet for the rear of the trigger housing. Ask me how I know......Grin. So instead of using that large of a drill bit, I carefully open the hole to the max width of the channel inlet for the rear of the trigger guard by using a dremel cutter and/or a round rasp. You need this clearance for the escutcheon that goes around the head of the screw. I prefer to leave the forward 1/8" of wood on the inlet for the rear of the trigger housing more as a guide as anything else, but don't care if I cut out all the rest of the wood going back from there inside the inlet. I will fill the hole in with glass and that's a lot more stable and stronger than the wood around it.
The next step is to position the receiver in the stock where it will go when glass bedded. Turn the stock upside down and see if the torque screw and escutcheon will go in without binding. Check with just the torque screw first. If the screw binds, you have to open the hole a bit larger as we want fiberglass all around the screw. You don't need to leave a huge space around the escutcheon, but it also can not bind.
When you clay up the receiver, I strongly suggest you take a Q tip dipped in mold release and thoroughly coat the threaded hole in the rear lug for the torque screw. Then PACK clay into the hole. Then take the screw and turn it in about four turns. Then remove the screw and coat the shorter hole with mold release again. This allows you to "find" the hole to begin screwing the torque screw down when you bed it.
Now, if you have ever glass bedded a rifle and thought it was messy, wait until you try it with a single or double torque screw gun. This is one time you want to mix up PLENTY of extra glass bedding material. I would suggest the first time you do it, whatever the amount you figure you need - add half again as much. Otherwise, you may well wind up with not enough glass to do it properly. As you do more of them, you will get to the point you can mix up just enough bedding material with only a little left over.
When I pack the glass in, the first thing I do is fill that hole for the torque screw. You know you have enough when you force glass from either the top or bottom of the stock and you see it pushed through the hole on the other end. Then pack glass in the rest of the stock and on the receiver. Then turn the stock upside down and align and push the stock down onto the barreled receiver. The glass is going to squoosh through the hole for the torque screw and that's GOOD because it tells you that you have enough glass in the hole. Remove the excess glass with a putty knife from the bottom. Take the screw and escutcheon and screw them in down to the point the top of the escutcheon and screw goes in to the point it is even with or preferably a bit further down (because the stock is upside down) than the old bedding was for the rear of the trigger housing. If we consider the stock as being right side up, the escutheon and torque screw will go up inside the stock a bit further than the original bedding surface for the rear of the trigger housing. (IOW, we don't want the escutcheon or head of the torque screw keeping the rear of the trigger housing too far down where it would cause the hammer hooks not to engage properly.) More glass is going to SQUOOSH up as you get the screw and escutheon screwed in place and that is also a GOOD thing as it shows you won't have voids in the pillar around the screw. Then you can spread that excess glass around to fill in the rest of the bedding for the rear of the trigger housing. It's better to leave too much than too little as we also want to see it SQUOOSH out when we lock the trigger guard down on the trigger housing. We are NOT concerned with how much tension there will be on the triggerguard. You do that in a separate bedding operation after the glass hardens around the torque screw and escutcheon.
Only after clamping down the trigger housing do we turn the rifle right side up and pack more glass into voids or clean off where the glass has SQUOOSHED out around the receiver.
In case you didn't know, SQUOOSH is a techical term. Grin.