February 12th, 2010, 09:27 AM
MGySgt USMC (ret)
This is really hard to answer without having the stocks and trigger mech in hand. Some SAinc. triggerguards lock up tighter than G.I. and some lock up looser due to the way the rounded tabs at top (that lock into the receiver legs) were made. SAinc trigger guards are not nearly as interchangeable as G.I.
Two things may be going on that are causing you the trouble. If they milled the cut outs in the receiver legs higher than on a standard G.I. M14 receiver, that would cause the problem.
I am assuming you are not changing the receiver and stock, but only the trigger mechs, correct? (If not, then the following information won't all apply.)
FIRST THING TO CHECK is try to lock the G.I. trigger mech into the receiver with NO stock in place. There should be almost no tension on the trigger guard when you lock it down that way. If it is difficult to lock the trigger mech down then, you file metal off the top of the trigger housing until the trigger mech will lock down easily. This will allow the trigger mech to go higher and the lugs on the trigger guard can lock into the receiver.
If the trigger guard locks in easily without a stock in place, then the problem is in the distance the bedding surface for the trigger housing pads are from the top of the stock. To fit the G.I. trigger mech, you likely are going to have to file some off the two front bedding surfaces. HOWEVER, if this is what you have to do to fit the G.I. trigger mech, then it is going to be too loose for your SAinc trigger mech.
If you fit the stock so a G.I. trigger mech will fit, then ALL G.I. mechs will fit, BUT some to many SAinc trigger mechs may not fit. Sometimes you can change the SAinc. trigger guard to a G.I. trigger guard in the SAinc housing and both will fit the same stock, BUT sometimes even that won't work because of the difference in the angle of the pads on some SAinc. trigger housings.
Sometimes you can put an SAinc. trigger guard into a G.I. trigger mech to get a G.I. trigger mech to fit a SAinc. stock, BUT sometimes that doesn't work.