SIG SAUER SIG 551-A1 Rifle - Manual (Page 73)

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4. Install the gas valve:
a. Ensure the stop pin recesses on the back of the gas valve tab face the barrel.
b. Insert the gas valve into the gas tube (the gas valve will seat over the operating rod).
c. Depress the stop pin and rotate the gas valve to the right to Position 1.
d. Release the stop pin. Check to ensure the gas valve indexes in Positions 1 and 2.
5. Install the upper hand guard:
a. Insert the front into the gas block.
b. Push the rear downward until seated.
6. Install the lower hand guard:
a. Press the hand guard upward against the upper hand guard, ensuring the locking tabs are aligned with each other.
b. Press the lower hand guard forward until the front is secured by the gas block and the pivot pin notch is aligned.
7. Assemble the bolt head and bolt carrier:
a. Place the bolt carrier in your hand, upside down, cam to the front.
b. With the notch facing up, slide the bolt head shaft fully into the bolt carrier.
c. Rotate the bolt head to the left until the notch rests in the cam.
d. Press forward on the firing pin until the bolt head is fully forward.
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